Molecular dynamics (MD) simulations were used to compare the structures of the chiral molecular micelles (MM) poly-(sodium undecyl-(L,L)-leucine-valine) (poly(SULV)) and poly-(sodium undecyl-(L,L)-valine-leucine) (poly (SUVL)). Both MM contained polymerized surfactant monomers terminated by chiral dipeptide headgroups. The study was undertaken to investigate why poly(SULV) is generally a better chiral selector compared to poly(SUVL) in electrokinetic chromatography separations. When comparing poly(SULV) to poly(SUVL), poly(SULV) had the more conformational flexible dipeptide headgroup and hydrogen bond analyses revealed that the poly(SULV) headgroup conformation allowed a larger number of intramolecular hydrogen bonds to form between monomer chains. In addition, a larger number of water molecules surrounded the chiral centers of the poly(SULV) molecular micelle. Poly(SULV) was also found to have a larger solvent accessible surface area (SASA) than poly(SUVL) and fluctuations in the poly(SULV) SASA during the MD simulation allowed dynamic monomer chain motions expected to be important in chiral recognition to be identified. Finally, approximately 50% of the Na+ counterions were found in the first three solvation shells surrounding both MM, with the remainder located in the bulk. Overall the MD simulations point to both greater headgroup flexibility and solvent and analyte access to the chiral centers of the dipeptide headgroup as factors contributing to the enhanced chiral selectivity observed with poly(SULV).
The enantiomers of chiral drugs often have different pharmacological potencies and toxicities. As a result, FDA guidelines issued in 1992 required the activities of drug enantiomers to be investigated separately and for only the therapeutic enantiomer to be brought to market [
In EKC-based chiral separations, enantiomeric resolution results from differential analyte interactions with a chiral selector as both are pulled through a capillary by an electric field [
The roles played by MM headgroup amino acid order [7-9], steric factors [7,8,10-12], hydrogen bonding [12, 13], electrostatic interactions [15,16], and the number and position of the headgroup chiral centers [8,15] in governing chiral selectivity in EKC separations have been reported. Research has also addressed how chiral selectivity is affected by the concentration at which the surfactant monomers were polymerized [
This project is part of an ongoing effort to characterize more fully dipeptide terminated MM structure and the analyte:MM intermolecular interactions that lead to chiral recognition in EKC separations. Here we have used molecular dynamics (MD) simulations to investigate the structures of two dipeptide terminated MM. The rationale for choosing these MM is based upon the experimental observation that chiral selectivity is generally higher when the larger of the two amino acids is in the N-terminal position [8,20]. For example, the MM poly- (sodium undecyl-(L,L)-leucine-valine) (poly-SULV) is a relatively effective chiral selector [
A previous MD simulation and NMR study of poly- (SULV) and poly(SUVL) provided insights into differences between the two MM structures and offered clues as to why poly(SULV) is the better chiral selector [
investigate intramolecular hydrogen bonding between the MM monomer chains and intermolecular hydrogen bonding between the MM molecules and water. Hydrogen bonding would be expected to affect both the local structures and overall shapes of the MM. Also, in an effort to probe the access that poly(SULV) and poly(SUVL) provide to the solvent, and thus by extension to potential chiral analytes, MD simulations were used to compare the solvent accessible surface areas (SASA) of the two MM. The surface area analyses also allowed a number of monomer chain motions that open and close chiral grooves or pockets near the MM headgroups to be identified. The final two MM properties investigated with the MD simulations were the number of water molecules surrounding the two headgroup chiral centers and the distribution of Na+ counterions around the MM carboxylate groups.
The details of the poly(SULV) and poly(SUVL) MD simulations have been previously reported [
Distributions of selected headgroup dihedral angles were used to assess the conformational flexibilities of the poly(SULV) and poly(SUVL) headgroups.
values are relatively small. In contrast, angle ω1 is very flexible with smaller maxima at 94˚ and 274˚ but with significant dihedral angle population ranging from 0 to 360˚. This flexibility may contribute to the high chiral selectivity observed for poly(SULV). For example, rotation of the headgroup about angle ω1 would allow the poly(SULV) headgroup to open and close thus giving chiral molecules an opportunity to insert into chiral pockets near the dipeptide headgroup and experience stereo selective interactions with both the leucine and valine chiral centers. Access to the N-terminal leucine may be important because NMR and EKC studies of analyte association with poly(SULV) have shown that binaphthyl analytes such as 1,1’-binaphthyl-2,2’-diyl hydrogen phosphate and 1,1-’bi-2-naphthol interact predominately with the leucine chiral center [4,18].
The distributions of dihedral angles ω1, ω2, and ω3 in poly(SUVL) are shown in
Analyses of the intraand intermolecular hydrogen bonds
formed during the MD simulations give further insight into the relative abilities of the two MM investigated to act as chiral selectors. The intra-molecular hydrogen bonds formed by poly(SULV) and poly(SUVL) are shown in
mer chains facilitates H-bond formation between the donor and acceptor atoms.
Results from the poly(SUVL) hydrogen bond analysis are also shown in
The observations that in poly(SUVL) there are fewer overall intra-molecular H-bonds and relatively fewer intra-molecular H-bonds with the N-terminal amino acid are consistent with a large fraction of the poly(SUVL) headgroups forming folded conformations that point the valine amino acid side chain toward the hydrocarbon core. This conformation shields potential donor-acceptor atoms on adjacent chains from one another and reduces the potential for inter-chain hydrogen bond formation.
The number of intermolecular hydrogen bonds between water molecules and the poly(SULV) and poly- (SUVL) headgroup atoms were also analyzed. It has been previously reported that of the two MM investigated here, poly(SULV) had more water molecules in the micelle headgroup region [
A similar trend was observed for intermolecular hydrogen bonds between water hydrogens and MM headgroup oxygen atoms. In poly(SULV), 56.4% of these intermolecular H-bonds formed between water and the C-terminal carboxylate oxygens. The remaining 43.6% formed with the oxygen atomsO1 and O2 in
Finally, hydrogen bond analyses showed that while the total number of H-bonds formed by poly(SULV) and poly(SUVL) were nearly identical, poly(SUVL) had a larger number of intermolecular hydrogen bonds with percent occupancies greater than 10%. In poly(SUVL), five H-bonds formed between water and the headgroup amide hydrogens with occupancies ranging from 16.13% to 10.30%. Three H-bonds with occupancies of 11.94%, 11.85%, and 10.90% were formed between water and the poly(SUVL) headgroup oxygen atoms. In contrast, only three H-bonds with occupancies greater than 10% were found to form between water and the poly(SULV) amide hydrogens and all H-bonds between water and the poly(SULV) oxygen atoms had occupancies less than 10%. These results suggest that the intermolecular hydrogen bonds formed between water and the poly(SULV) headgroup atoms are shorter-lived or that the H-bonding environment around the poly(SULV) headgroup is relatively dynamic. This environment could provide more opportunities for chiral analytes to access and interact with the poly(SULV) headgroup atoms. In poly(SUVL), though, the H-bond analysis suggested that there is a more fixed, less dynamic solvation shell around the poly- (SUVL) headgroups as evidenced by the larger number of hydrogen bonds with high percent occupancies.
A plot of the MM solvent accessible surface area (SASA) versus simulation time for both poly(SULV) and poly- (SUVL) is shown in
these MM showed that poly(SULV) had a more open structure than poly(SUVL) that allowed greater water penetration into the hydrocarbon core and didpeptide headgroup regions [
Insight into differences between the MM investigated here and conventional micelles formed from surfactants such as sodium dodecyl sulfate (SDS) can be gained by calculating the SASA per monomer chain. Dividing the average SASA for each MM by the twenty chains making up each macromolecule yields values of 409 Å2 and 380 Å2 per monomer chain for poly(SULV) and poly- (SUVL), respectively. In contrast, MD simulations by Bruce, et al. yielded a SASA per monomer residue of only 176 Å2 for SDS micelles [
The results plotted in
In order to test this hypothesis and to identify the monomer chain motions associated with the observed changes in the SASA, MM structures with high and low SASA values were identified. These structures were then overlaid to show the structural changes leading to the fluctuations in the SASA observed in
leading to the changes observed in the SASA is an opening and closing of cavities or pockets near the monomer headgroups. This motion could allow movement of chiral analytes into the cavities created between the monomer headgroups where they could then bind to the MM in a stereo selective fashion. The motion depicted in
Other chain motions, though, identified by the superimpositions discussed above produced changes in the SASA that were likely not important in chiral recognition. For example, the superimposed structures in
As discussed above, poly(SULV) has been found experimentally to be a relatively effective chiral selector, while chiral selectivity with poly(SUVL) is generally poor [8,21]. Therefore, it seems reasonable to expect that the motions identified above leading to fluctuations in the SASA should occur to a greater extent in poly(SULV) than in poly(SUVL). This should especially be true for those motions opening pockets near the dipeptide headgroup that give chiral analytes access to the MM chiral centers. To test this hypothesis, the variability in the positions of selected poly(SULV) and poly(SUVL)atoms during the MD simulations were compared using the following method. Four carbon atoms were chosen that spanned the midpoint of the monomer hydrocarbon chain to the end of the dipeptide headgroup. Moving up toward the headgroup, these atoms are labeled C6, C10, N-terminal Ca, and C-terminal Ca in
bon atoms C6, C10, and the Nand C-terminal Ca atoms, respectively. The dashed line at 4.0 Å is included to facilitate comparison between the four graphs.
In
For chiral recognition to take place chiral pockets must open as described above, but analytes must also have access to the chiral centers of the dipeptide headgroup where stereo selective interactions take place. Thereforeto compare the access that each MM provides to the dipeptide headgroup chiral centers, the number of water molecules surrounding the chiral carbons in both MM was investigated. The rationale for this comparison was that greater water access to the chiral centers should also allow greater access for chiral analytes.
The final MM property investigated with the MD simulations was the distribution of sodium counterions around each macromolecule. The counterion distribution was studied because the charge and the distribution of counterions around each monomer headgroup would be expected to influence the electrostatic repulsion between monomer chains and thus the overall size and shape of the macromolecule. The first step of this analysis was to calculate the radial distribution functions (RDF) between sodium ions and the carboxylate oxygen atoms. The RDF plot for poly(SULV) is shown in
the MM carboxylate anions, thus producing the first peak in the RDF in the 2.0 - 3.3 Å range. Other sodium ions were found to have moved farther from the carboxylate headgroups yielding the second peak in the RDF between 3.3 and 5.2 Å. Finally, the remaining sodium ions are located farther than 5.2 Å from the MM carboxylate headgroups.
To better understand the distributions of sodium ions around the MM, the system was divided into three Na+ shells extending, respectively, 3.3 Å, 5.2 Å, and 10.0 Å from the carboxylate oxygens. The populations of sodium ions in these shells for LV are plotted in Figures 9(b)-(d). Figures 9(b) and (c) show that relatively few sodium occupy the first two shells closest to the carboxylate oxygens. The distribution in
MD simulations were used to investigate the chiral MM poly(SULV) and poly(SUVL). The more effective chiral selector poly(SULV) was found to have a more conformationally flexible headgroup and a larger SASA. Poly- (SULV) also allowed more intraand intermolecular H-bonds to form with atoms on the N-terminal amino acid of the dipeptide headgroup. In addition, the MD simulations suggested that large-scale monomer chain motions occur in both MM. These motions may open cavities or pockets that could be used by chiral analytes to access the chiral centers of the MM dipeptide headgroups. Finally, water molecules were found to have greater access to the poly(SULV) chiral centers and an analysis of the distribution of sodium ions showed that approximately 50% of the sodium counterions are found within 10.0 Å of the MM carboxylate groups.
This work was supported by grant # 8G12 MD007597 from NIMHD, NIH to the RCMI program at Howard University, a NSF CAREER grant to Dr. Eugene Billiot (No. 0449742), and a Robert A. Welch Chemistry Departmental Grant to the Chemistry Program at Texas A&M University-Corpus Christi, a NSF grant to Drs Kevin F. Morris and Fereshteh H. Billiot (No. CHE- 1213532), Howard University College of Medicine Bridge Funds and Pilot Study Awards program (BFPSAP) to Dr. Yayin Fang (No. U400040). We also acknowledge the Donors of the American Chemical Society Petroleum Research Fund (46707-B4) and the generosity of the Ralph E. Klingenmeyer family.