organometallic complexes: catalysis and application in...

Indian Journal of Chemistry

Vol. 52A, Aug-Sept 2013, pp. 992-1003

Organometallic complexes: Catalysis and application in protein modification

Maheshwerreddy Chilamari & Vishal Rai*

Department of Chemistry, Indian Institute of Science Education and Research (IISER) Bhopal, Bhopal 462 023, MP, India

Email: [email protected]

Received 31 March 2013; accepted 16 May 2013

Organometallic catalysis has witnessed phenomenal growth with time and methodologies have been developed to

facilitate a variety of chemo-, regio- and stereoselective reactions. The potential of these transformations has been examined

to differentiate the reactivity of amino acid residues in a peptide or protein in recent years. However, these

biomacromolecules extend astringent chemoselectivity challenge to the classical methodologies. The efforts en route to

develop new catalytic approaches in this area has been delineated in this review.

Keywords: Organometallic catalysis, Metallopeptides, Peptide-peptide interactions, Protein-protein interactions,

Post-translational protein modification

Metals offer Lewis acidic sites to coordinate with

amino acid polymers owing to the abundance of

Lewis base centers. Meaningful complexation can

be achieved by engineering partners that show

bioorthogonal interaction. Such coordination

complexes have been found to stabilize secondary

structures such as α-helix, β-sheets and various loops.1

Serving as an inspiration for design of ion sensors,2

these complexes have also enabled rapid detection

of tagged proteins and fluorescent markers.3 A

chemoselective metal coordination with poly-histidine4

peptide fused to the protein of interest has been

instrumental in purification of recombinant proteins.

The potential of metal centers for selective

coordination in a pool of functionalities make

them a probable candidate for their application in

post-translational modification (PTM) of proteins.

Apart from controlling several intra- and extracellular

events, these modifications can regulate the structure

and function of proteins.1b,c

Chemical reactivity of

amino acid side chains offer a range of PTM's that

includes transfer of phosphate, farnesyl, acyl,

ubiquitin, formyl, glycosyl and several other

fragments to the protein backbone. This approach is

driven by the establishment of unique side chain

functionality that can react orthogonally with respect

to the residues in natural amino acids. One of the

established route to answer this chemoselectivity

challenge is enabled by enzyme catalysis.5 Another

classical approach involves incorporation of unnatural

amino acid into the protein sequences,6-8

assisted by

engineered bacterial stains.9 Such mutation would

depend on the modified tRNA that suppresses the stop

codon. These stop codons introduced into mRNA can

in turn define the analogs. However, access to

modified suppressor tRNA's encounters a whole set of

new challenges.10

In a conceptually infant approach, efforts have

been directed towards transition metal catalyzed

chemoselective bioconjugation for labeling

unmodified natural amino acid polymers. The

selective catalytic system involves metallopeptide

complexes inspired by metalloenzymes.11

Here, the

first coordination sphere involves side chain amino

acid residues that bind to the metal. This regulates the

electronic properties of metal and in turn defines the

activity of metal-peptide complex. The catalytic fine

tuning and selectivity is governed by the second

coordination sphere that involves hydrogen bonds,

electrostatic, hydrophobic and steric interations.12

The

peptide sequence constituting the second coordination

sphere does not bind with the metal directly but

provides a channel for substrates/products and an

appropriate microenvironment in the cavity around

the active site. In this review, we have highlighted

various constraints and methodologies that has

directly or indirectly enabled the evolution of this

approach.

CHILAMARI & RAI: ORGANOMETALLIC COMPLEXES IN CATALYSIS & PROTEIN MODIFICATION

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Organometallic Complexes in Protein Modification

Organometallic catalysis has a profound effect on

the C-C and C-X bond formation.13

Metal center,

ligand, additive and solvent are crucial parameters

for emergence of an efficient catalytic method.

Recent reports have established the approach in

differentiating functional groups of amino acid side

chain (1a-m, Fig. 1) by exploiting subtle differences

in their reactivity. Functional group modification on

the surface of proteins is dictated by a new set of

parameters defined by the protein itself. This

exponentially drops the number of metal catalyzed

methodologies that would qualify for their application

in this area. A focused approach with collective

optimization of parameters is desired to understand

the behavior and mode of activation of these catalysts.

Efforts for expanding native parameters for

organometallic catalysis, i.e., organic solvents, high

substrate concentrations (100 mM to 1 M) and high

temperature have led to a few successful realization of

organometallic milieu that shows high functional

group tolerance in the conditions desired for reaction

of proteins. The protein modification should be

realized in aqueous buffer, low concentration,

ambient temperature, and physiological pH. Adhering

to these conditions and achieving high rate of reaction

is a considerable challenge.

A unique opportunity for bio-orthogonal

transformation14

is offered by unnatural amino acids

utilizing “tag and modify” technology.15

Some of the

representative examples include genetic incorporation

of azide, unsaturated bonds, and coupling conjugates

into proteins that offer the possibility of Cu catalyzed

click chemistry,16

Ru catalyzed olefin metathesis17

and

Pd catalyzed cross coupling reactions.18

Continuing

efforts in this area have established the proof of

principle with successful validations utilizing S-allyl

cysteine (2), p-iodophenyl alanine (4) and

homopropargylglycine (6) outlined in Scheme 1.17b,19

Allylation

Electrophilic π-allyl complexes are suitable

candidates for labeling of nucleophilic residues on

proteins. The required allylic acetate (8), carbonate or

carbamate precursors are biologically inert and

become reactive only after activation by

palladium catalyst. It’s utility has been established in

allylation of phenolic groups,20

for chemoselective

functionalization of tyrosine (Scheme 2). The excess

allylic acetate (8) forms diene (10) that can be

Fig. 1 – Pool of natural amino acids (1a-m) with reactive side chains.

INDIAN J CHEM, SEC A, AUG-SEPT 2013

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easily purified due to considerable difference in

mass and polarity.

These catalytic systems have the capacity to modify

proteins in aqueous medium with high functional

group tolerance preserving the protein structure.21

Palladium-catalyzed cross-coupling can also modify

aryl halides22

and boronic acids23

within peptide

fragments. Difference in access to solvent and

microenvironment of amino acid residue can also assist

in achieving selective modifications. Solvent-accessible

tyrosine residue on chymotrypsinogen A can undergo

Mannich-type alkylation24

with rhodamine-labeled

allylic acetate (8), Pd(OAc)2, and triphenylphosphine

tris-sulfonate (TPPTS).25

The transformation results in

double alkylation where Y171 in T14-15 fragment gets

modified predominantly. The catalyst was found to be

inert in presence of 19 lysine residues of horse heart

myoglobin and cysteine thiolate of H-Ras.

Reductive amination

An iridium hydride complex has been reported to

reduce imines chemoselectively over their aldehyde

precursors. The water-stable [Cp*Ir-(bipy)(H2O)]SO426

complex and its derivatives (13a-c) achieved

reductive alkylation of lysine residues of lysozyme

at room temperature and neutral pH (Scheme 3).

The reaction has been shown to be very

successful with structurally diverse proteins including

cytochrome c (19 lysines), R-chymotrypsinogen

A (14 lysines), myoglobin (19 lysines), ribonuclease

A (10 lysines), and an intact virus (bacteriophage

MS2, 6 lysines per monomer). Reactivity is dependent

on the structure of these biomacromolecules

and solvent accessibility of lysine residues. The

reductive alkylation adducts are amenable to

further modification with alkoxyamines to form

oximes.21e,27


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Oxidative cross coupling

Oxidative cross-coupling is one of the important

techniques for protein modification that utilizes

transition metal catalysis.28

The methodology relies

on the formation of tyrosyl radical (19) by oxidation

of tyrosine, donating an electron to high-valent

transition metal (Ni, Mn, Fe).29

The tyrosyl radical

(19) can couple with another tyrosine linking the two

proteins (21) (Scheme 4A).30

However, tryptophan

and cysteine side chain oxidation or their coupling

with tyrosyl radical are major side reactions.31

High

reactivity of tyrosyl radical poses enormous challenge

and makes it vulnerable for reaction with various

residues of nearby proteins.32

To control the

reactivity, a number of catalytic systems have been

developed. Ni-GGH (22a),33

Ni-His6 (22b),34

and


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Ni-ribonuclease A35

complex along with an oxidant

were reported to promote protein cross-linking

(Scheme 4B). Ni2+

-peptide-salen complex offers

oxidative cross-linking of DNA to the peptide

fragment of metallopeptide complex.36

Metallocarbenoid mediated conjugation

Metallocarbenoids37

react with 3-methylindole

(26)38

in presence of water39

and a large number of

polar residues (Scheme 5A). In this transformation,

both N-alkylation and C-alkylation products

[(27):(28), 1:1.4, 51% yield] are formed. The residual

diazo compound (25) leads to formation of two other

side products, alcohol (29) and pyrazole (30). In

absence of water soluble substrates, an organic

co-solvent was considered necessary to improve the

reactivity. Concentration below 100 µM is critical to

have control over selectivity. Solvent exposed

tryptophan residues (W7 and W14) in myoglobin

(10-100 µM) lead to a mixture of mono- and

di-labeled products with 60% conversion.21d,40

The

starting materials were found to be inert in the

absence of rhodium catalyst. Single tryptophan

residue in subtilisin Carlsberg required lower pH (1.5)

for conversion to the monolabeled W113 product.

Metallopeptides in Catalysis

Metal-peptide complexes have gained attention for

their success in asymmetric C-C bond formation.41

Peptides constitute an attractive set of modular chiral

pool due to well established synthetic protocols.

However, understanding the secondary structure of

catalyst is not trivial. Dipeptides [(35-37), Fig. 2]

have served as efficient ligands to generate chiral

Schiff base for enantioselective catalysis.42

The

phenol-based Schiff bases (e.g. 35) are better partners

for early transition metals (Ti, Zr), whereas,

P-containing ligands (e.g. 36) are apt for the late

transition metals (Cu, Zn). Some of the successful

metallopeptide catalyzed transformations include

Fig. 2 – Structures of dipeptide ligands.


997

Ti-catalyzed addition of TMSCN to epoxides43

and

imines (Scheme 6),44

copper catalysed conjugate

addition (Scheme 7),45

Zr-catalyzed addition of

dialkylzinc to imines (Scheme 8)46

and Al catalysed

addition of cyanide to ketones (Scheme 9).47

Proper spatial alignment of Schiff base and amide

carbonyls is critical to achieve high selectivity. The

chirality of the second amino acid also plays an

important role in achieving stereodifferentiation.

For example, bifunctional Ti-Schiff base catalyst

coordinates with the substrate (38) and carbonyl of

peptide (39) to direct the cyanide to activated imine

(Scheme 6). Dipeptide serves an ideal length while

additional amino acids were found to be detrimental

to the selectivity.48

A common approach for screening

peptide based ligands is hinged on combinatorial

screening of library.49

However, similar results can

also be achieved by an alternative approach where

rational screening leads to identification of reactive

substrate and metal, Schiff base, ligand-metal

combination and peptide. Utilization of this approach

led to an efficient Cu catalyzed asymmetric allylic

substitution utilizing pyridinyl peptides (44) or (45)

(Scheme 7).


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In another report, a combinatorial library of

peptides was used for enantioselective addition of

alkylmetals to C=N bonds (49) (Scheme 8).46

Commercially available Et2Zn was used as the

alkylating reagent. Both early and late transition

metals were screened with phenol (50), (54)

and phosphine-based Schiff bases (36) and

[Zr(OiPr)4].HO

iPr was identified as the most effective

catalyst. Al-peptide complex was also found to be an

efficient catalyst to render asymmetric cyanation of

ketones (56) and (59) (Scheme 9). A systematic

screening supported by modular peptide-based

chiral ligands led to the identification of suitable

catalyst.47,50

Acetophenone (56) and TMSCN react in

presence of chiral peptide ligand (57) and Al(OiPr)3.

Stereochemistry of amino acid at C-terminus was

found to be critical for enantio-differentiation.

Phosphine, a well-established ligand for the

transition metal catalysis, along with peptide

ligands have been explored with palladium, ruthenium

and rhodium.51

Secondary structure of the peptide

plays a key role for bidentate metal chelation. For

example, a C2-symmetric cyclic peptide gramincidin S,

(Pro-Val-Orn-Leu-dPhe)2, adopts a rigid cyclic

β-hairpin structure and predisposes the Orn

residues for facile binding with transition metals.

Phosphine-gramicidin S conjugate (63) complexed

with Rh can serve as an efficient catalyst for


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asymmetric hydrogenation (Scheme 10).52

When

complexed with Pd, this conjugate renders

efficient allylic substitution, albeit with low

stereoselectivity.

The contributions to foldamers53

and small peptide

helices54

offer another secondary structure where two

binding sites are positioned at i and i+4 position for

metal binding. With the hypothesis that such

structurally defined peptide sequences can coordinate

with metal, the nonapeptide KADAALDAK with two

carboxylate side chains positioned at i and i+4 was

screened.55

In a stereoselective carbenoid insertion to

Si-H bond, Rh2(OAc)2-monopeptide complex (69)

was found to be modest with respect to bis-peptide

complex Rh2(70a)2 or Rh2(70b)2 (Scheme 11). It is

important to note that the bis-peptide complexes

are formed as separable mixture of parallel (70a)

and antiparallel isomers (70b).56

These two isomers of

bis-peptide complexes follow independent pathways

for enantio-differentiation. Other crucial parameter

includes nature and spacing of the carboxylates.

Increase in spacing between metal and peptide

by one methylene (Asp to Glu) compromises

the catalysis.

Metallopeptides in Protein Modification

The dirhodium monopeptide complex (72) has

been successfully utilized in modification of

peptides.57

The key to selectivity was unique

interaction between two peptide strands inspired by

the role of coiled coils in protein-protein interaction.58

This proximity-driven approach can gain the rate

acceleration up to >103. Coiled-coil sequences with

heptad repeats [(75), abcdefg, Scheme 12] position


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1001

hydrophobic residues (a and d) on one side of R-helix.

Complementary charge of E3/K3 pair (e and g)

can be utilized to trigger formation of heterodimeric

pairs.59

Tryptophan, tyrosine and phenylalanine were

included in the reactivity profile of the catalytic

modification of peptides (Scheme 12B). A control

experiment demonstrated that sequence recognition

can allow preferred transformation of less reactive

amino acid side chain (77) suppressing the inherent

reactivity order (Scheme 13). The methodology also

witnessed partial success with bZip domain of c-Fos.

In this structurally defined protein domain,60

glutamine (Q19, position e) of Jun-Fos dimer was

selected for the modification (Fig. 3). The crude

dirhodium-Jun complex catalysed the chemoselective

modification of glutamine, asparagine or phenylalanine

side chains. Recognition based peptide sequence

guided the dirhodium catalyst for bioconjugation.

Conclusions

Organometallic catalysis have seen remarkable

growth in past several decades and metallopeptides

comprise a relatively under-explored but unique

section of this area. It offers a modular catalyst library

where peptides can perform two functions: (a) stereo-

differentiation and (b) serve as recognition sequence

to interact with peptides and proteins. Chemical

methodologies for modification of proteins has also

gained attention, albeit not in conjugation with

organometallic catalysis. This is primarily due to the

considerable difference in parameters that govern

both the areas. Solubility, limitations with

temperature and pH range, and selectivity of metal

coordination in presence of numerous potential

binding sites pose considerable challenge. On the

other hand, proteins require low concentration to

work efficiently. Possibility of aggregation of

peptides and proteins cannot be ignored at higher

concentrations. Examples discussed in this review

highlights the efforts that have increased the overlap

between these marginally intersecting areas. Catalysis

by metal complex of one amino acid and dipeptides

are being intensively investigated. For longer

peptides, secondary structure plays a crucial role for

rendering reactivity and selectivity. β-turns and

helices have been shown to predispose binding sites

for the coordination with metal. However,

multidisciplinary investigation for the role of

secondary coordination sphere to regulate the

microenvironment of the substrate and thus efficiency

of the catalyst is necessary.

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