Experimental Details •  Surfactant synthesis was done in the laboratory of Dr. Feri Billiot. •  Small angle neutron scattering measurements were made by Dr. Eugene Billiot at Oak Ridge National Laboratory. •  NMR experiments were performed with a Bruker 400 MHz spectrometer. •  Hydrodynamic radii were calculated from diffusion coefficients with the Stokes-Einstein equation (Equation 1). •  The fraction of bound surfactant molecules and/or counterions was calculated with Equation (2). Equation (1) Equation (2)

Conclusions 1.  NMR measurements showed the radii of the UND-LV micelles were ∼21 Å. Micelle radii did

not change with pH and were in good agreement with small angle neutron scattering experiments.

2.  Arginine and Lysine cations bound to the micelle surface. Zwitterionic amino acids had very weak micelle interactions. L-Arginine and DArginine association with the micelles were almost identical.

3.  Two-dimensional ROESY experiments suggested that the surfactant headgroup turned toward the micelle core.

4.  ROESY experiments also confirmed L-arginine binding and suggested that arginine binds to the micelles primarily through its charged amino acid side chain.

Acknowledgements: This work was supported by NSF-RUI Grant #1213532. We also acknowledge the generosity of the Ralph E. Klingenmeyer Family.

Abstract

Results

Figure 1: Chemical structure of (a) Sodium Undecyl-(L,L)-leucine-valine(b) (L)-arginine, (c) (L)-lysine

Molecules Investigated

D = micelle diffusion coefficient kB = Boltzmann constant T = Kelvin temperature η = viscosity Rh = hydrodynamic micelle radius

Dobs = surfactant diffusion coefficient Dmicelle = micelle diffusion coefficient fb = fraction of micelle-bound surfactant molecules or counterions Dfree = free solution surfactant diffusion coefficient

h

B

RTkD··6·ηπ

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

NMR spectroscopy was used to investigate micelle formation by a chiral surfactant. The surfactant contained a hydrocarbon tail attached to a leucine-valine dipeptide. Micelles formed by this surfactant have been used to separate the enantiomers in racemic mixtures. NMR diffusion experiments showed that in the pH range 7.0 to 11.5, the micelles had radii of approximately 22 Angstroms. These radii agreed well with measurements from small angle neutron scattering. Below pH nine, lysine and arginine amino acids were found to bind to the anionic micelles. At higher pH’s, though, both amino acid dissociated from the micelles. Finally, the dipeptide NH protons were observed to exchange with solvent protons. The rate of this exchange reaction was investigated along with the conformation of the surfactant’s dipeptide headgroup. .

Arg+ + Arg+

pH < 7 pH > 10

Figure 2: (a) Hydrodynamic radii of micelles (with Na+ counterion) vs. pH from NMR and Small angle neutron scattering. (b) Comparison of elliptical and spherical radii.

Figure 3: Association equilibria of free and micelle-bound arginine.

Figure 4: (a) Fraction bound of L-arginine and L-lysine and fraction bound surfactant vs. pH (b) Comparison of L-arginine and D-arginine binding to the micelles. (a)

(b)

(b)

(c)

Arginine/Lysine-Micelle Association

Two-Dimensional NMR Spectra

Figure 5: Protonation states of arginine

Valine

Leucine

0

10

20

30

40

7 8 9 10 11 12

Radius(A

ngstroms)

pH

UND-LVNMRRadius

UND-LVSANSMajorAxis

UND-LVSANSMinorRadius

(a) (b)

0

0.2

0.4

0.6

0.8

1

7 8 9 10 11 12

Frac?o

nBo

und

pH

Fbound,LysineFbound,LVFbound,ArginineFbond,LV(Ar)

0

0.2

0.4

0.6

0.8

1

7 8 9 10 11 12

Frac

iton

Bou

nd

pH

Fbound, (L)-Arginine

Fbound, (D)-Arginine

Figure 6: (a) ROESY spectrum of UND-LV micelles. (b) ROESY cross peak between the Valine g protons and hydrocarbon chain suggest the micelle headgroup rotates toward the micelle hydrocarbon core.

(b)

ROESY interaction Observed.

Figure 7: (a) ROESY spectrum of a UND-LV-L-arginine mixture. (b) ROESY cross suggest that arginine cations bind to the micelles primarily through the charged side chain.

Val γ – LV hydrocarbon chain

NMR Investigation of Micelle Formation by a Chiral Dipeptide Surfactant

Tyler Witzleb1, Fereshteh Billiot2, Eugene Billiot2, 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, Texas