AcknowledgementsThis work was supported by an NSF-RUI grant (#1213532) to Drs. Fereshteh

Billiot and Kevin Morris, a Robert A. Welch Chemistry Departmental Grant to

the Chemistry Program at Texas A&M University-Corpus Christi, an NSF

CAREER grant (#0449742) to Dr. Eugene Billiot, and Office of Naval

Research (#N00014-17-1-2105) and HUMAA Endowed Founder’s Chair in

Basic Science awards to Dr. Yayin Fang. We also acknowledge the generocity

of the Ralph E. Klingenmeyer Family.

Figure 4. ROESY spectrum of an und-Phe, L-Arg mixture. Red

assignments indicate an intermolecular cross peak and green and black

assignments indicate intramolecular cross peaks.

Figure 6. und-Phe and counter-ion fraction bound values and und-Phe

micelle radii versus pH for solutions containing (a) 50 mM und-Phe

and 50 mM L-arginine. (b) 50 mM und-Phe and 50 mM L-

homoarginine, (c) 50 mM und-phe and 50 mM L-lysine, and (d) 50 mM

und-Phe and 50 mM L-ornithine.

Figure 5. Model of (a) arginine and (b) lysine binding to the surface

of the und-Phe micelle. The arrows indicate intermolecular cross

peaks observed in the ROESY spectra.

HN

O

CO2-

NH

NH2+H2N

HN

O

CO2-

CO2-+H3N

+H3N

-O2C

HN

O

CO2-

HN

O

CO2-

NH3+ +H3N

CO2-

NH3+

= ROESY Cross Peak

Micelle

Micelle

(a) Arginine Binding (b) Lysine Binding

Abstract

NMR spectroscopy was used to study micelle formation by the amino acid-based surfactant L-Undecyl

Phenylalaninate (und-Phe). Amino acid-based surfactants like und-Phe are used in the pharmaceutical, food,

and cosmetic industries because they are biodegradable, biocompatible, have a low toxicity, and can be

produced using renewable materials. NMR measurements of the surfactant’s critical micelle concentration

(CMC) showed that und-Phe micelles formed at approximately 6.0 mM in solutions containing the anionic

surfactant and sodium counter-ions. In contrast, micelles formed in the 3.5-4.0 mM range in solutions

containing either L-arginine or L-lysine counter-ions. NMR diffusion experiments showed that L-arginine,

L-lysine, L-ornithine, and L-homoarginine counter-ions bound to the micelles below pH 9.0, but dissociated

from the micelle surface at high pH. Finally, the mechanism of L-arginine/L-homoarginine and L-lysine/L-

ornithine binding to und-Phe micelles was found to be different and the und-Phe aromatic rings were found

to rotate toward the hydrocarbon core of the und-Phe micelles.

Figure 1. Structures of (a) L-Undecyl Phenylalaninate (und-

Phe), (b) L-arginine (Arg), (c) L-homoarginine, (d) L-

lysine, and (e) L-ornithine. Atom labels are used to assign

the two-dimensional NMR spectra.

O-

O

NH

NH3+

NH2

+H2N

O-

O

NH3+

+H3Nd

d

e

g

b

a

g

0

5

10

15

20

25

0

0.2

0.4

0.6

0.8

1

7 8 9 10 11 12

Rad

ius

(Å)

Fra

ctio

n B

ou

nd

pH

Fb Arg

Fb und-phe

Rh und-phe

ba

Figure 2. (a) Stack plot of NMR spectra for und-

Phe: L-arginine mixtures at pH 9.0 showing changes

in the surfactant chemical shift with concentration.

(b) Representative plot of chemical shift versus

reciprocal concentration used to determine the CMC.

0

5

10

15

20

25

0

0.2

0.4

0.6

0.8

1

7 8 9 10 11 12

Rad

ius

(Å)

Fra

ctio

n B

ou

nd

pH

Fb Arg

Fb und-phe

Rh und-phe

(c)

0

5

10

15

20

25

0

0.2

0.4

0.6

0.8

1

7 8 9 10 11 12

Rad

ius

(Å)

Fra

ctio

n B

ou

nd

pH

Fb und-phe

Fb Ornithine

Rh und-phe

(d)

(a)

(b) (c)

(d)(e)

Table 1: CMC of und-phe micelles vs. pH in solutions containing

NaHCO3, L-Arginine and L-Lysine counter ions.

a

b

g

g

d b

a

d

e

0

5

10

15

20

25

0

0.2

0.4

0.6

0.8

1

7 8 9 10 11 12

Rad

ius

(Å)

Fra

ctio

n B

ou

nd

pH

Fb Lys

Fb und-phe

Rh und-phe

(a)

(b)

(a) Arginine binding

(b) Lysine Binding

Figure 3. : (a) Proposed model of L-arginine binding to and

dissociating from und-Phe micelles. Dissociation of

L-arginine from the micelle surface is accompanied by a

change in the hydrodynamic radius. (b) Proposed model of

L-lysine binding to and dissociating from und-Phe micelles.

Dissociation of L-lysine from the micelle surface is

accompanied by little change in the hydrodynamic radius.

Conclusions

1.In und-phe micelles, the Phe aromatic ring is rotated toward

the micelle core.

2.L-arginine and L-homoarginine bind perpendicular to the

und-Phe micelle surface, while L-lysine and L-ornithine bind

parallel to micelle surface.

3.The surfactant’s CMC increased with solution pH in all

mixtures investigated.

Experimental Details•CMC measurements were made with NMR spectroscopy by monitoring the change in the surfactant chemical shift

with solution concentration. Chemical shift was plotted versus inverse concentration (Figure 2) to determine the

CMC.

•Micelle radii and the fraction of counter ions bound to the micelles were measured with pulsed gradient NMR

diffusion experiments. Fraction bound values were calculated with Equation (1) and micelle radii were calculated

with the Stokes-Einstein equation (Equation 2).

•Structures of micelle-counter ion complexes were studied with two-dimensional ROESY experiments (Figure 4)

Equation (1):

Dobs = observed counter ion diffusion coefficient

fb = fraction of counter ions bound to micelle

Dmicelle = micelle diffusion coefficient

Dfree = free solution counter ion diffusion coefficient

Dobs = fb ∙ Dmicelle + (1 – fb) ∙ Dfree

D = micelle diffusion coefficient

kB = Boltzmann constant

T = Kelvin temperature

h = viscosity

Rh = hydrodynamic micelle radius

D =kB

×T

6 ×p ×h ×Rh

Equation (2):

NMR Investigation of the Effect of pH on Micelle Formation by an

Amino Acid-based Surfactant Gabriel Rothbauer1, Elisabeth Rutter1, Fereshteh Billiot2, Eugene Billiot2, Yayin Fang3, and Kevin Morris1

1Department of Chemistry, Carthage College, 2001 Alford Park Drive, Kenosha, WI, 2Department of Physical and Environmental Sciences, Texas A&M

University-Corpus Christi, 6300 Ocean Drive, Corpus Christi, TX, 3Department of Biochemistry and Molecular Biology, Howard University College of

Medicine, 520 W Street NW, Washington, DC.