Accepted Manuscript

Paramagnetic Lanthanides as Magnetic Resonance Thermo-Sensors and Probesof Molecular Dynamics: Holmium-DOTA Complex

Sergey P. Babailov, Peter V. Dubovskii, Eugeny N. Zapolotsky

PII: S0277-5387(14)00294-0DOI: http://dx.doi.org/10.1016/j.poly.2014.04.067Reference: POLY 10711

To appear in: Polyhedron

Received Date: 17 February 2014Accepted Date: 29 April 2014

Please cite this article as: S.P. Babailov, P.V. Dubovskii, E.N. Zapolotsky, Paramagnetic Lanthanides as MagneticResonance Thermo-Sensors and Probes of Molecular Dynamics: Holmium-DOTA Complex, Polyhedron (2014),doi: http://dx.doi.org/10.1016/j.poly.2014.04.067

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Paramagnetic Lanthanides as Magnetic Resonance Thermo-Sensors and Probes of

Molecular Dynamics: Holmium-DOTA Complex

Sergey P. Babailova, Peter V. Dubovskiib,c, Eugeny N. Zapolotskya

aA.V. Nikolaev Institute of Inorganic Chemistry, the Siberian Branch of the

Russian Academy of Sciences, Av. Lavrentyev 3, Novosibirsk 630090, Russian

Federation

bShemyakin–Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of

Sciences, 16/10 Miklukho-Maklaya str., Moscow 117997, Russian Federation

cA.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of

Sciences, 29 Leninsky av., Moscow 119991, Russian Federation

Author for correspondence:

S.P. Babailov

A.V.Nikolaev’s Institute of Inorganic Chemistry of

the Siberian Branch of the Russian Academy of Sciences

Av. Lavrentyev, 3

Novosibirsk, 630090

Russia

Tel.: +7(383)-3308957, Fax: +7(3832)-3309489,

e-mail babajlov@niic.nsc.ru

Paramagnetic Lanthanides as Magnetic Resonance Thermo-Sensors and Probes of

Molecular Dynamics: Holmium-DOTA Complex

Sergey P. Babailova, Peter V. Dubovskiib,c, Eugeny N. Zapolotskya

aA.V. Nikolaev Institute of Inorganic Chemistry, the Siberian Branch of the

Russian Academy of Sciences, Av. Lavrentyev 3, Novosibirsk 630090, Russian

Federation

bShemyakin–Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of

Sciences, 16/10 Miklukho-Maklaya str., Moscow 117997, Russian Federation

cA.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of

Sciences, 29 Leninsky av., Moscow 119991, Russian Federation

Abstract

Investigation of lanthanide (Ln) сhelates and their paramagnetic properties with NMR

spectroscopy is an established area of research. However, respective data for coordination compounds

of holmium (Ho) are scarce. To fill this gap, the present work focuses on detailed determination of

intramolecular dynamics and paramagnetic properties for holmium complexes. 1H NMR

measurements are reported for D2O solutions of paramagnetic [Ho(H2O)(DOTA)]- (I) complex in the

273-348 K temperature range. Diamagnetic complex [Lu(H2O)(DOTA)]- (II) was used as a reference

compound. The spectra obtained have been analyzed using band-shape analysis technique in the

framework of dynamic NMR (DNMR). Temperature dependences of lanthanide-induced shifts (LIS)

were taken into consideration, too. Сonformational dynamic process has been found. The dynamics is

caused by an interconversion of square-anti-prismatic (SAP) and twisted- square-anti-prismatic

(TSAP) conformers (the estimated activation free energy ∆G‡(298K) is 65±3 kJ mol-1).

Thermodynamics of equilibrium between SAP and TSAP conformers of I was investigated too. The

results obtained are found to be consistent with those collected for other Ln-DOTA complexes. In

accordance with literature reviewed, the fulfilled experimental study is the first example of

intramolecular dynamics determination for holmium complexes. Taking Ho3+ as an example, the

methodology of paramagnetic 4f-element probe applications for the study of free-energy changes in

chemical exchange processes is discussed. The advantages of this method, compared to DNMR studies

of diamagnetic substances are illustrated. Among them is an extension of the range of NMR-

accessible rate constants for paramagnetic 4f-element complexes, compared to diamagnetic ones. And

last, usage of coordination compounds investigated as a new type of thermometric NMR sensors and

lanthanide paramagnetic probes for in situ temperature control in solutions is demonstrated. The

investigated coordination compounds are suggested to apply as thermo-sensing contrast reagent for

MRI diagnostics of cancer and inflammation.

Key words: Lanthanide complexes, conformational molecular dynamics, kinetics of chemical

exchange, dynamic NMR, DOTA, magnetic resonance thermo-sensor

Abbreviations: CS, chemical shifts; DNMR, dynamic NMR; DNP, dynamic nuclear polarization;

H4DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid; DTPA,

diethylenetriaminepentaacetic acid; EDTA, ethylenediaminetetraacetic acid; EXSY, exchange

spectroscopy; LISs, lanthanide-induced chemical shifts; Ln, lanthanide; MRI, magnetic resonance

imaging; RCA, relaxation contrast reagents; SAP, square-anti-prismatic; TSAP, twisted-SAP.

Introduction

There is growing interest in studies of complexes of lanthanide (Ln) cations, containing O- and/or N-

attached ligands (see refs. [1] {chapter 3.2} and [2-9]). Among them, a special attention is paid to

aminopolycarboxylates [10-18]. This is due to broad implementation of their Gd3+ complexes as

magnetic resonance imaging (MRI) relaxation contrast reagents (RCA) [5-7,9-10]. For diagnostic

purposes, these compounds are injected intravenously to alter the water proton relaxation times in

tissues [5-7,9-10]. As a result, image contrast and information content improve significantly.

Moreover, these compounds (e.g. Ln-EDTA and Ln-DTPA complexes; Ln= Gd, Dy and Ho) were used

for achieving high levels of nuclear spin polarization at experiments on the dynamic nuclear

polarization (DNP) enhancement [17,18]. They could also be of value in the context of a simple low-

cost method of achieving several-hundred-fold improvements in polarization for experiments in which

samples are pre-polarized at low temperatures, then rewarmed and dissolved immediately prior to

analysis.

In particular, the lanthanide complex [Gd(DOTA)]- (H4DOTA = 1,4,7,10-tetraazacyclododecane-

1,4,7,10-tetraacetic acid) is considered as the perfect lead compound of RCA for MRI and prospective

reagent for DNP. This is due to its very high thermodynamic and kinetic stability, combined with the

favorably long electronic relaxation time [1,2,9]. Moreover, because of high stability of Ln complexes

with DOTA and DOTA-like (see detailed description of the term and related molecular structures in

[3,12]) ligands , they are broadly utilized in science and practice as metal chelating low-molecular

weight adducts for protein and other biomolecule conjugates, nanoparticles, biological and medical

sensors. Ln-DOTA-like complexes may also be considered as models for the actinide complexes

formed at chelation therapy (in case of internal human actinide contamination) [3,7,12]. Therefore,

there is definite interest in studies of the relationship between structure, paramagnetic properties and

molecular dynamics in Ln-DOTA complexes. This information brings about detailed understanding of

the processes occurring in the solvated Ln complexes with DOTA and DOTA-like ligands [10-16].

It should be noted that structure, thermodynamics, inter- and intra-molecular dynamics of many

complexes between Ln3+ and aminopolycarboxylate have been studied in very detail [10-20]. Of

mention, 1H-NMR relaxation spectroscopy was used to investigate intermolecular dynamics in

[Gd(H2O)(DOTA)]- complexes, caused by an exchange of water molecules between bound and free

states [19,20]. Previously, intramolecular conformational dynamics in DOTA and DOTA-like

complexes of La3+, Eu3+ and Yb3+ was studied by a combined approach, based on 2D EXSY and

NMR-determination of the coalescence temperatures [11]. For other Ln3+ cations the dynamic properties

of [Ln(H2O)(DOTA)]- complexes are unknown. Efforts to study Ho-DOTA complexes were continuously

undertaken earlier [12]. Nevertheless, conformational dynamics of these complexes have not been

studied comprehensively. Furthermore, from the literature reviewed we did not find a paper where the

holmium complex intramolecular dynamics was presented. The goal of the present work is a detailed study

of the intramolecular conformational dynamics in [Ho(H2O)(DOTA)]- complexes (I) with the use of 1H-

NMR line-shape analysis. Taking into account data on temperature dependence of lanthanide-induced shifts

(LIS), conformational intramolecular dynamics of complexes I is assayed. We also investigated

thermodynamics of the conformer equilibrium for the system I. Diamagnetic complex

[Lu(H2O)(DOTA)]- (II) was used as a reference compound [1,2,9]. Taking Ho3+ as an example, the

methodology of paramagnetic 4f-element probe applications for the study of the free-energy variation

in chemical exchange processes is discussed. Advantages of this method, compared to DNMR studies

of diamagnetic substances, are revealed. In particular, extension of the range of the NMR determined

rate constants for paramagnetic 4f-element complexes, compared to diamagnetic ones, is demonstrated.

And last, for practical applications, the usage of investigated coordination compounds as new type

thermometric NMR sensors and Ln paramagnetic probes for in situ temperature control in solution and

can be recommended. The [Ho(H2O)(DOTA)]- complex is considered to be thermo-sensing contrast

reagent for MRI diagnostics of cancer and inflammation and potential new functional material.

Results and discussion

Qualitative interpretation and signal assignment. DOTA is known to be an 8-dentate ligand,

forming 8-coordinated complex with Ln cations. As many as 4 metal-ligand bonds are formed by the

nitrogen atoms of the macrocycle. The remaining 4 bonds are formed via coordination of iminoacetate

oxygen atoms. A pair of conformational isomers, square-anti-prismatic (SAP) and twisted- square-anti-

prismatic (TSAP) (Scheme 1) exists for the complexes in solution and crystal. They differ in the

mutual orientation of the parallel planes formed by the Ln-bound N- and O-atoms (see Scheme 1). The

twist angles between these planes are about +40 and −24° corresponding to square antiprismatic (SAP,

often labeled M) and twisted square antiprismatic (TSAP, often labeled m) coordination geometries,

respectively (see also discussion and Scheme 1 in ref. [16]). For the [Ln(H2O)(DOTA)]- complexes

relative amounts of the conformers in solution depends on Ln cation type [13]. For solutions of the

heavy Ln complexes, to which Ho belong, SAP form prevails over TSAP one [11,13,14].

Scheme 1

To assign the signals in the 1H NMR spectra, theoretical LIS for Ho-DOTA complexes were

calculated within the framework of the following model. The calculations were based on the LIS data,

available for Yb-DOTA complex [11]. A similar spatial organization of Ho and Yb complexes was

assumed. Fermi-contact contributions to LIS were considered small, compared to pseudocontact one,

and were neglected. In general, theoretical spectra for [Ho(H2O)(DOTA)]- and experimental chemical

shifts are fairly agreed (Table 1).

Table 1

Besides that, we compared assignments obtained with those available for the complex of Ho

with DOTA-like ligand [12]. We found full correspondence between the spectra and assignments made

in the both cases. Thus, the analysis of the CS, made according to Bleaney’s method [9,21], and

comparison to the spectra of the parent compound, allowed us to obtain satisfactory assignments of the

experimental LIS in the Ho-DOTA complex. Scheme 1 depicts an interconversion between SAP and

TSAP conformers in the [Ln(H2O)(DOTA)]- complex. We investigated the band-shapes of the zero-

order 1H NMR spectra of the [Ho(H2O)(DOTA)]- complex over a temperature range from 273 to 348

K. Thus, the activation free energy for the intramolecular dynamics in I was evaluated (see section

Intramolecular dynamics). Also, a comparison was made between thermodynamic parameters of the

intramolecular dynamic processes within a series of relational metal complexes. An extra-sensitivity

was gained via use of a high-field NMR spectrometer (Avance 800, see Material and Methods for

details) equipped with a cryoprobe.

Figure 1.

In Fig. 1 the temperature dependence of 1H NMR spectra of the complex I is presented. There

are four signals in the spectrum obtained at low temperature of 273.2 K. They correspond to the

protons of the acetate groups of SAP (see Scheme 2) and at least two broadened signals of TSAP

protons (Scheme 3). The signals at 190, 102, 63, and -63 ppm correspond to the SAP conformer.

Moreover, the signals with attenuated intensities (at 42 and -39 ppm) are seen due to extra-sensitivity

gained. Probably, they correspond to the minor TSAP conformer of the complex I.

Scheme 2

Scheme 3

The halfwidth of the signals of the SAP conformer decreases with temperature increase in the

273-310 K temperature range. A similar observation can be made for the halfwidths of the signals,

corresponding to the TSAP conformer. It seems, these changes can be partially explained by fast

intermolecular dynamics, caused by an exchange of water molecules between the complex associated

and the free states (see Scheme 4). This type of intermolecular dynamics was previously investigated

by 17O NMR [14,40]. According to information from refs. [14,40], the mechanism of H2O exchange in

[Ln(H2O)(DOTA)]- complexes appeared to be dissociative.

Scheme 4

-22

- DOTA)])(OH(Ln[ OHDOTA)]([Ln →

←+

This chemical exchange (Scheme 4) is accompanied with variation in the coordination

polyhedron of Ho cation. As a result, the parameters of the anisotropy of paramagnetic susceptibility

undergo a change and the chemical shifts of the same protons in the [Ln(DOTA)]- and

[Ln(H2O)(DOTA)]- complexes exhibit substantial variation. On the other hand, these changes can be

rationalized by the Curie contribution to 1/T2. It is directly proportional to the square of the applied

magnetic field. It is clear therefore that this contribution will be important at 18.8 T (800 MHz), as it

has been already shown to be significant at considerably lower magnetic fields [1,23]. This can be

clearly observed by a comparison of the 1H NMR spectra shown in Figs. 1 (800 MHz) and 2 (300

MHz). The resonances in Fig. 1 are clearly broader than those observed at 300 MHz. Apart from the

dependence of the Curie contribution with the magnetic field, it also depends on 1/T2, and therefore

increasing the temperature is expected to decrease this contribution and therefore the linewidths. Thus,

the effect of temperature on the linewidths observed in Fig.1 can be attributed to the Curie contribution

and can be related (in part) with intermolecular dynamic process.

It should be noted that we could not record signals ac2, ax1 and ax2, even with special

cryoprobe. Firstly, it is because of the concentration of TSAP molecular form is less than the

concentration of SAP conformer. Second, signals ac2, ax1 and ax2 are broadened due to valuable the

paramagnetic lanthanide-induced enhancement of relaxation rates [1,2,9].

Figure 2.

Intramolecular dynamics. The signals of SAP and TSAP conformers, being rather narrow at

low temperatures, begins to broaden, when temperature increases in the range from 310 to 348 K (line

broadening progresses with temperature increase; see Fig. 2). As can be seen from Fig. 2, the signals

of SAP protons disappear completely at the temperatures above 314 K (the spectra were acquired at a

300 MHz-spectrometer). Of note, temperature increase results in a change of the lineshape of NMR

signal, corresponding to hydrogen atoms of СН2 groups in the complexes I. The lineshape is

characteristic for CE system (Fig. 2). The observed lineshape variations are quite typical for the

systems, where a low-populated component (in this case, this is TSAP conformer) exchanges with a

high-populated one (SAP conformer). This lineshape variation is likely due to conformation

dynamics, caused by interconversion between SAP and TSAP conformers. Conformational dynamics

of this type, presented in Scheme 1 takes place in aqueous solutions of Ho-DOTA complexes.

Figure 3.

A quantitative analysis of the lineshape variation of these signals enabled us to estimate the rate

constants of the CE process at different temperatures and to determine the activation free energy

(Table 2, Fig.3). The ionic radii for these cations are also presented in Table 2. The data obtained are

presented in Fig. 3. From these data we calculated the activation free energy (see Experimental

section, equation 2) ∆G‡(298K)= 65±3 kJ/mole. As we can see from Table 2, the value of the

activation free energy is comparable to the energy barrier height for conformational transitions in

DOTA-like complexes, encompassing a variety of metal cations, obtained by different teams [11-14].

The rate constants for this dynamic process fall in the range of 18-180 s-1. The activation free energy

of exchange between these conformers falls in the 60-65 kJ/mol range for most of Ln-DOTA

complexes (Table 2). One can see from Table 2 that the magnitude of the activation free energy for

these DOTA complexes increases in the order from La to Yb. According to our viewpoint, this

monotonic increase of the free energy of the intramolecular exchange in the series of lanthanide

[Ln(DOTA)]- complexes is related to decrease of the ionic radii in lanthanides, known as the

“lanthanide contraction” [9,28]. The concentration of I appeared to have no influence on the rate

constant of molecular dynamics (studied between 10−3 and 10−2 mol/dm3), i.e., interconversion of the

isomers is obviously a first-order reaction.

Table 2

Discussing the obtained values of the activation free energy for DOTA molecular dynamics in the

complexes under investigation, we have to note the following. The discovered magnitude of the

activation free energy is commensurable with the values of the energetic barriers of conformational

inversion in 18-crown-6 molecules of [Ln(18-crown-6)(NO3)3] complexes (where ∆G‡(310K)

appeared to be 58 ± 6, 49 ± 6, and 45 ± 5 kJ/mol for Pr, Ce and Nd, respectively [25]). However, in the

[Ln(18-crown-6)(NO3)3] complexes observed decrease in the free energy of a number of metals, rather

than increasing as in [Ln(DOTA)]- complexes.

The intramolecular dynamics in the macrocyclic molecule investigation results may be relevant

for understanding some of their other physicochemical properties, for example, reactivity, reaction

mechanism, etc.

Thermodynamics of the Isomer Equilibrium. Although an estimate of the ratio between

isomers for [Ln(DOTA)]- complexes (where Ln= Er, Tm, Yb and Lu) has already been reported [13],

we obtained it here for holmium complex at different temperature. Table 3 reports thermodynamic

parameters (equilibrium constant, reaction enthalpy and entropy) describing reaction of an equilibrium

exchange between SAP and TSAP isomers for [Ho(H2O)(DOTA)]- complex. The values found are

also comparable to the values for other Ln, reported elsewhere [13]. The magnitude of the reaction

enthalpy (describing reaction of an equilibrium exchange between SAP and TSAP) decreases in the

order from Ho to Lu (Table 3). A decrease of ∆Hо in the series of lanthanide [Ln(DOTA)]- complexes

may be also related to the “lanthanide contraction”.

Table 3

Paramagnetic probes based on 4f elements for DNMR at high magnetic fields. There is a

peculiarity of DNMR use for studies of paramagnetic compounds of 4f elements, deserving of a

special comment to be made on. The range of measurable paramagnetic LISs is known to exceed

above ~100 ppm. Due to paramagnetic chemical shifts (δν) in 4f complexes, the range of measurable

rate constants expands considerably, compared to the respective range in diamagnetic compounds. This

may be illustrated for a system with a degenerate two-site exchange process. For NMR spectrometer,

whose operating frequency for protons is 800 MHz, the highest value of the rate constant, accessible

to measurements is kmax ∼ 1010 Hz (upper limit). Thus, it can be assumed that δν=8×104 Hz, or 100

ppm in paramagnetic compounds. Let us assume that an error in halfwidth determination is about ∼1

Hz. The kmax value in paramagnetic compounds is much larger than the kmax (∼108 Hz) in

diamagnetic compounds (δν ∼ 8×103 Hz). The lower limit for the range of the rate constant becomes

kmin ∼ 1 Hz. Thus, using the paramagnetic probe method for investigation of the intramolecular

dynamics of ligands, coordinated to this metal cation, is equivalent to using of a NMR spectrometer

with an unprecedented high operating frequency of 8 GHz. Therefore, the potential of this method in

studies of molecular dynamics of various ligand complexes with paramagnetic metal cations is

significantly higher than in the case of complexes with diamagnetic cations.

In the present paper, using variable-temperature 1H NMR, conformational dynamics of Ho3+

complex in the 319-348 K temperature range was investigated (Fig. 2). The maximum rate constant of

the conformational dynamics was estimated to be 3.4×103 s-1 at 348 K.

Temperature dependence of paramagnetic LIS and thermo-sensing. The temperature

dependencies of paramagnetic LISs of [Ho(H2O)(DOTA)]- for protons of different groups are

presented in Fig. 4. As seen in Fig. 4, experimental paramagnetic LIS values (δobs – δdia) are well

fitted by linear dependence on 1/T (Curie-Weiss approximation) [9,24-28]:

δobs – δdia = a + b/T (1)

It is known that Fermi contact contribution to LIS is proportional to 1/T (Curie’s law). From theoretical

standpoint [21], the pseudocontact contributions to LIS is proportional to 1/T2 (it was calculated in the

second approximation). Concerning experimental investigations, some of them confirmed the 1/T2

dependence. Nevertheless, in many cases, there was no control of the thermodynamic and kinetic

stability of complexes. In some cases this was confirmed. We support the viewpoint that the

temperature relationship of LIS depends heavily on higher-order states, but for practical purposes, the

experimental dependence in the 200-350 K temperature range is described by the Curie–Weiss

approximation adequately [9,24-28].

Figure 4.

This result correlates well with studies of [Ln(H2O)(EDTA)]- (Ln= Pr, Er, Ho, Tm and Yb)

complexes [26-31,36], [Ln(18-crown-6) (ptfa)2]+ [9,30-33], [Ln(diaza-18-crown-6)(NO3)3] [9],

[Ln(18-crown-6)(NO3)3] (where Ln= Ce, Pr and Nd) [9,23-25,28], and [LnH(oep)(tpp)] (where Ln=

Dy and Yb, ptfa is 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione, tpp is tetraphenylporphyrin, and oep

is octaethylporphyrin) [38]. One can use this linear temperature relationship of LIS in practice.

Recently, we proposed to use this temperature dependence of paramagnetic LISs for in situ

temperature control in both aqueous [26-29,36] and nonaqueous media [24-28,32-35].

We also investigated experimental paramagnetic LISs ∆δex(T) versus T. The derivative of

∆δex(T) was calculated (see Table 2). The signal of the ax1 H atom (-242,6 ppm at 300 K) exhibit

maximum temperature gradient, amounting to 1,0 ррm/К . The value found is much larger than that

for the temperature gradient of pure water (0.01 ppm/K). This temperature sensitivity corresponds to

uncertainty in the estimation of the temperature of ~0.03 K (assuming the error of the chemical shift

determination to be of ~0.03 ppm). It should be noted that maximum d(∆δex)/dT value in I is larger

than those for many other lanthanide chelates and comparable to that for Na[Tm(DOTP)]2- (see Table

2).

Of note, our previous NMR investigations of lanthanide complexes were carried out mainly in

organic solvents [9,26-34]. In this paper the results are presented for aqueous solutions. The

temperature dependencies of LISs revealed for Ln complexes in aqueous solutions, might have

practical importance for biological and medical applications. In particular, [Ho(H2O)(DOTA)]-

complex might be used as a subnanoscale NMR spectroscopic probe for local temperature

determination in aqueous media. It can be done (for example for the ac2 signal) as follows. After

determining the value of the paramagnetic contribution to chemical shift at some point, one can find

the local temperature using the ac2 dependence of LIS shown in Fig.4. Favorably, this complex can be

used as thermometric NMR sensors either in reaction media directly, or in situ studies of exothermic or

endothermic processes. Also, I can be applied for control of local temperature in medical magnetic

resonance imaging for in vivo three-dimensional mapping of the body-temperature distribution and

diagnostics of different body parts in diseases, including those related to cancer.

Experimental Section

The [Ho(H2O)(DOTA)]- complexes were prepared, according to ref [12]. Solutions in D2O had

the concentration of complexes C= 10-2 M (at pD=7.0). The pD values were measured in a galvanic

cell by a microprocessor-equipped pH-meter/ionmer Anion-410 of Infraspak-Analyt (Novosibirsk) and

pocket-sized pH-meter with replaceable electrode of HANNA Instruments. The glass electrode,

combined with a silver/silver chloride reference electrode, filled with a saturated potassium chloride

solution, was used. The cell volume was of 2 ml. The pD measurement precision was 0.05. The

combined electrode was calibrated, using a set of reference solutions in D2O: 1) 0.05 M potassium

citrate, 2) 0.025 M KH2PO4 and 0.025 M Na2HPO4 mixture of phosphates, and also 3) 0.025

vNaHCO3 and 0.025 M Na2CO3 mixture of carbonates. The volume of solutions was 10—20 ml. For

these solutions the pD values (25o C) of 4.29; 7.43, and 10.74, respectively, were assumed [28-29].

The 1Н NMR spectra were measured with MSL-300 and Avance-800 spectrometers (all

produced by Bruker, Germany). The operating frequency was 300.31 and 800.13 MHz, respectively.

The high-field spectrometer was equipped with a cryoprobe. The residual proton signal in D2O (4.75

ppm at 303 K) was used for chemical shift referencing. Through all the measurements the magnetic

field was stabilized, using deuterium lock of the solvent. The spectra were measured in the tubes with

an outer diameter of 5 mm. A B-VT-1000 temperature unit with the accuracy of 1 K and stability of

0.2 Kh–1 was used for temperature control. The unit was calibrated, using standard samples with

known temperature dependence of chemical shifts. Studies of exchange-broadened NMR spectra were

carried out by line-shape analysis within the framework of DNMR [9,24-28].

Paramagnetic LISs were determined relative to diamagnetic complex II, which served as a

reference (as in refs. [9-10]). As it can be seen in Fig. 4, experimental LIS values are well fitted,

assuming their linear dependence on 1/T. As required for NMR line-shape analysis, LIS values of

protons signals in the absence of either intermediate, or fast exchange on the NMR time-scale [9,24-

28], were calculated, according to the following equation:

∆δi(T) = ∆δi(T0)(1/T + A)/ (1/T0 + A) (2)

Parameter А was defined by analysis of average LIS value for ac2 proton, T0= 273 К. Eq.2 serves to determine the chemical shift (for different signals) depending on the temperature.

The free energy of activation, ∆G‡, of molecular dynamics in [Ho(H2O)(DOTA)]- was

calculated according to the Eyring equation [9,24-27],

k = (K kB T/h)exp(-∆G‡/RT), (3)

where k is the rate constant of chemical exchange reaction, T is the absolute temperature, R is the gas

constant, kB is Boltzmann’s constant, h is Planck’s constant and K is the coefficient, dependent on unit

measures.

1H NMR Spectral assignments.

[Ho(H2O)(DOTA)]- (SAP ). δH (ppm, D2O, pD = 7, T= 273.2 K): 190.57 (N-CH2-COO, ac2,

s); 102.26 (N-CH2-CH2-N, ax2, s); 63.62 (N-CH2-COO, ac1, s); -64.35 (N-CH2-CH2-N, e1, s); -

280.48 (N-CH2-COO, ax1, s).

[Ho(H2O)(DOTA)]- (TSAP). δH (ppm, D2O, pD = 7, T= 273.2 K): 41.98 (N-CH2-COO, ac1,

s); -40.17 (N-CH2-CH2-N, e1, s); -40.64 (N-CH2-CH2-N, e2, s).

Conclusion

The present work is the first-ever study, reporting the parameters of intramolecular dynamics of

paramagnetic holmium complex with DOTA4- in aqueous media, elucidated by 1Н NMR-spectroscopy.

With holmium complexes as an example, this method proves to be effective for studies of molecular

dynamics in paramagnetic lanthanide chelates. Conformational dynamic process was investigated (it is

caused by an interconversion of SAP and TSAP conformers of the complex). The activation free

energy was estimated to be ∆G‡(298K)= 65±3 kJ mol-1. The fulfilled experimental study is the first

example of intramolecular dynamics determination for holmium complexes. The results obtained are

consistent with those collected for other Ln-DOTA complexes. For practical purposes, the complexes

of DOTA4- with Ho3+ might be used as a nanoscale NMR spectroscopic probes to determine

temperature in aqueous media and thermo-sensing contrast reagent for MRI diagnostics of cancer and

inflammation.

Author information

Corresponding Author

*E-mail: babajlov@niic.nsc.ru . Tel.: +7(383)- 3308957. Fax: +7(3832)-3309489

Acknowledgment

The authors are grateful to Professors V.P. Fedin and A.S. Arseniev for useful discussions. The

work was fulfilled with the partial financial support of the Russian Foundation for Basic Research

(grant N14-03-00386-a).

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Figure 1. Variable temperature dependence of 800 MHz 1H NMR spectra of

[Ho3+(H2O)(DOTA4-)]-, in D2O; chemical shift values (δ scale) are relative to internal

4,4-dimethyl 4-silapentane sodium sulfonate (DSS); at temperatures (T, K): 273 (1), 288

(2), 300 (3), 310 (4), 318 (5); signal at 4.8 p.p.m. corresponds to HDO.

Figure 2. Variable temperature dependence of 300 MHz 1H NMR spectra of

[Ho(H2O)(DOTA)]-, in D2O; chemical shift values are relative to DSS (δ scale); at

temperatures (T, K): 314 (1), 319 (2), 328 (3), 338 (4), 348 (5).

Figure 3. Dependence of ln(k/T) on 1/T, where k is the rate constant of the

intramolecular dynamic process in [Ho(H2O)(DOTA)]-, T is the temperature, and D2O

as the solvent.

Figure 4. Temperature dependence of the paramagnetic LISs in 800 MHz 1H NMR

spectra for ac2 (♦), ax2 (■), аc1 (▲) protons of the SAP conformer of [Ho(H2O)(DOTA)]-

dynamic system, with D2O as the solvent.

Author Biographies

1. Sergey Pavlovich Babailov, Russian, born in Chita region (Russia). Short CV: Dr. Babailov received his Ph.D. in chemical physics and Full Doctor of Chemical Sciences in physical chemistry from the Nikolayev Institute of Inorganic Chemistry, SB Russian Academy of Sciences. Currently, he is a Principal Research Associate at the Nikolayev Institute of Inorganic Chemistry. He is also an Invited Scientist at the European Center of Magnetic Resonance at the Florence University in Florence, Italy. Previously, Dr. Babailov was Deputy Head of the Laboratory of Optic Methods Investigations and the Temporary Head and Deputy Head of the Radio-spectroscopic Laboratory at the Nikolayev Institute of Inorganic Chemistry. Dr. Babailov’s specialization is in chemical physics, magnetic resonance spectroscopy and imaging, photo-induced chemical exchange, processes of chemical exchange on the data of NMR, and nano-NMR thermal and pH sensors. His current research interests include the application of NMR/MRI in coordination chemistry, photochemistry and biomedical research. He has 60 refereed papers (including publications in the Progress in Nuclear Magnetic Resonance Spectroscopy, the Inorganic Chemistry and the European Journal of Inorganic Chemistry) and 95 communications to scientific meetings. 2. Peter V. Dubovskii, Russian, born in Moscow (Russia).

Dr. Dubovskii received his Ph.D. in biological chemistry from the Shemyakin–Ovchinnikov

Institute of Bioorganic Chemistry, Russian Academy of Sciences. Currently, he is a Research Associate at the Shemyakin–Ovchinnikov Institute of Bioorganic Chemistry. His current research interests include the application of NMR/MRI in coordination chemistry and biomedical research. He has 30 refereed papers and 40 communications to scientific meetings.

3. Eugeny N. Zapolotsky, Russian, born in Altai region (Russia).

Short Curriculum Vitae: Mr. Zapolotsky finished Novosibirsk State University. Currently, he

is a postgraduate student at the Nikolayev Institute of Inorganic Chemistry. His current research interests include the application of NMR/MRI in coordination chemistry and biomedical research. He has 2 refereed papers (including publications in the Inorganic Chemistry) and 10 communications to scientific meetings.

Table 1. Experimental (∆δex, ppm) and Calculated (∆δcalc, ppm) 1H NMR Lanthanide-induced Shifts in Ln-DOTA Complexes; Hydrogen Atoms of the DOTA Ligand Designation in SAP*** and TSAP* Conformers.

Designation of H atoms** in [Yb(DOTA)]-

∆δex** in [Yb(DOTA)]- (Т=298К)

∆δcalc in [Ho(DOTA)]- (Т=300K)

∆δex for a mixture of [Ho(H2O)(DOTA)]- and [Ho(DOTA)]- (Т=300K)

Designation of H atoms in [Ho(H2O)(DOTA)]-

SAP SAP

ax1 133 -236 -242,64 ax1 ax2 -47 83 86,85 ax2 e1 24 -43 -55,41 e1 e2 20 -35 - e2 ac1 -38 67 53,55 ac1 ac2 -82 145 160,99 ac2 TSAP TSAP

ax1* 80 -142 -124 ax1* ax2* -32 57 53,54 ax2* e1* 15 -27 -35,62 e1* e2* 10 -18 -34,7 e2* ac1* -25 44 39,33 ac1* ac2* -54 96 104,49 ac2*

* See Scheme 2. ** See reference [11].

*** See Scheme 3.

Table 2. Activation Free Energies (∆G‡298, kJ mol-1), Derivative of Experimental

Lanthanide-induced Shifts (d(∆δex)/dT, ppm/K) and Methods Used for Investigation

the Intramolecular Dynamics in DOTA-like Lanthanide Complexes.

Complex Radius/ Å**

Activation

Free Energy/

(kJ mol-1),

∆G‡

298

Rate Constants of CE k298, s-1

Method d(∆δex)/dT, ppm/K

Reference

[La(DOTA)]- 1.16 60.7±1.2 - 1H and 13C BSA#

0.0 Desreux at al., 1994 [11]

[Nd(DOTA)]- 1.11 - - 13C 0.1& Aime at al., 1992 [10]

[Eu(DOTA)]- 1.07 63 - 1H BSA# - Aime at al., 1997 [13]

[Eu(DOTAM)]- 1.07 54.6 k250=68±5 1H BSA# - Dunand at al., 2000 [14]

[Eu(DOTA-like)]- 1.07 - k293=45±15 1H 2D EXSY 0.19& Woods at al., 2000 [40]

[Ho(OH2)(DOTA)]- 1.02 65±3 k319=180±20 1H BSATD-LIS*

1.0 This paper

Na[Tm(DOTP)]- 0.99 - - 1H 1.17& Trubel at al., 2003 [39]

[Yb(DOTA)]- 0.99 65.6 k298=33±3 1H 2D EXSY 0.22& Desreux at al., 1994 [11]

[Lu(DOTA)]- 0.98 65.9±1,2 k298=18 1H and 13C BSA#

0.0 Aime at al., 1996 [12]

#BSA means band shape analysis technique within the framework of the dynamic NMR. *BSATD-LIS means band shape analysis taking into account temperature dependence of LIS within the framework of the dynamic NMR. &Our calculations.

** Data for complexes with coordination number 8 (see ref. [41]).

Table 3. Reaction Enthalpies and Entropies for the Isomer Equilibrium Reaction SAP =

TSAP of [Ho(OH2)(DOTA)]- Complexes (KT = [TSAP]/[SAP]).

Ln K298 ∆Hо, kJ mol-1

∆Sо, J mol-1 K-1

Reference

Ho 0.04±0.01 20±1.1 39±5 this paper Er 0.03 - - [13] Tm 0.08 16.2±0.9 33±3 [13] Yb 0.20 17.5±0.7 45±2 [13] Lu 0.18 10.1±0.1 18±1 [13] [TSAP] and [SAP] are concentration of TSAP and SAP, respectively.

Synopsis

Conformational dynamics processes in the holmium-DOTA complex was identified by 1H

NMR. It is caused by an interconversion of SAP and TSAP conformers in the complex (∆G‡(298K) =

65±3 kJ mol-1). Coordination compound investigated in the paper represents new type of

thermometric NMR sensors.

Figure 1. Variable temperature dependence of 800 MHz 1H NMR spectra of

[Ho3+(H2O)(DOTA4-)]-, in D2O; chemical shift values (δ scale) are relative to internal

4,4-dimethyl 4-silapentane sodium sulfonate (DSS); at temperatures (T, K): 273 (1),

288 (2), 300 (3), 310 (4), 318 (5); signal at 4.8 p.p.m. corresponds to HDO.

Figure 2. Variable temperature dependence of 300 MHz 1H NMR spectra of

[Ho(H2O)(DOTA)]-, in D2O; chemical shift values are relative to DSS (δ scale); at

temperatures (T, K): 314 (1), 319 (2), 328 (3), 338 (4), 348 (5).

Figure 3. Dependence of ln(k/T) on 1/T, where k is the rate constant of the

intramolecular dynamic process in [Ho(H2O)(DOTA)]-, T is the temperature, and D2O

as the solvent.

Figure 4. Temperature dependence of the paramagnetic LISs in 800 MHz 1H NMR

spectra for ac2 (♦), ax2 (■), аc1 (▲) protons of the SAP conformer of

[Ho(H2O)(DOTA)]- dynamic system, with D2O as the solvent.

Scheme 1

Scheme 2

Scheme 3

Scheme 4

-22

- DOTA)])(OH(Ln[ OHDOTA)]([Ln →

←+