Role of Charge and Solvation in the Structure and Dynamics of Alanine-Rich Peptide AKA2 in AOT Reverse Micelles.

The propensity of peptides to form α-helices has been intensely studied using theory, computation, and experiment. Important model peptides for the study of the coil-to-helix transition have been alanine-lysine (AKA) peptides in which the lysine residues are placed on opposite sides of the helix avoiding charge repulsion while enhancing solubility. In this study, the effects of capped versus zwitterionic peptide termini on the secondary structure of alanine-rich peptides in reverse micelles are explored. The reverse micelles are found to undergo substantial shape fluctuations, a property observed in previous studies of AOT reverse micelles in the absence of solvated peptide. The peptides are observed to interact with water, as well as the AOT surfactant, including interactions between the nonpolar residues and the aliphatic surfactant tails. Computation of IR spectra for the amide I band of the peptide allows for direct comparison with experimental spectra. The results demonstrate that capped AKA2 peptides form more stable α helices than zwitterionic AKA2 peptides in reverse micelles. The rotational anisotropy decay of water is found to be distinctly different in the presence or absence of peptide within the reverse micelle, suggesting that the introduction of peptide significantly alters the number of free waters within the reverse micelle nanopool. However, neither the nature of the peptide termini (capped or charged) nor the degree of peptide helicity is found to significantly alter the balance of interactions between the peptides and the environment. Observed changes in the degree of helicity in AKA2 peptides in bulk solution and in reverse micelle environments result from changes in peptide confinement and hydration as well as direct nonpolar and polar interactions with the water-surfactant interface.

Exploring the role of hydration and confinement in the aggregation of amyloidogenic peptides Aß(16-22) and Sup35(7-13) in AOT reverse micelles.

Knowledge of how intermolecular interactions of amyloid-forming proteins cause protein aggregation and how those interactions are affected by sequence and solution conditions is essential to our understanding of the onset of many degenerative diseases. Of particular interest is the aggregation of the amyloid-ß (Aß) peptide, linked to Alzheimer's disease, and the aggregation of the Sup35 yeast prion peptide, which resembles the mammalian prion protein linked to spongiform encephalopathies. To facilitate the study of these important peptides, experimentalists have identified small peptide congeners of the full-length proteins that exhibit amyloidogenic behavior, including the KLVFFAE sub-sequence, Aß16-22, and the GNNQQNY subsequence, Sup357-13. In this study, molecular dynamics simulations were used to examine these peptide fragments encapsulated in reverse micelles (RMs) in order to identify the fundamental principles that govern how sequence and solution environment influence peptide aggregation. Aß16-22 and Sup357-13 are observed to organize into anti-parallel and parallel ß-sheet arrangements. Confinement in the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles is shown to stabilize extended peptide conformations and enhance peptide aggregation. Substantial fluctuations in the reverse micelle shape are observed, in agreement with earlier studies. Shape fluctuations are found to facilitate peptide solvation through interactions between the peptide and AOT surfactant, including direct interaction between non-polar peptide residues and the aliphatic surfactant tails. Computed amide I IR spectra are compared with experimental spectra and found to reflect changes in the peptide structures induced by confinement in the RM environment. Furthermore, examination of the rotational anisotropy decay of water in the RM demonstrates that the water dynamics are sensitive to the presence of peptide as well as the peptide sequence. Overall, our results demonstrate that the RM is a complex confining environment where substantial direct interaction between the surfactant and peptides plays an important role in determining the resulting ensemble of peptide conformations. By extension the results suggest that similarly complex sequence-dependent interactions may determine conformational ensembles of amyloid-forming peptides in a cellular environment.

Probing the structure and dynamics of confined water in AOT reverse micelles.

Reverse micelles are attractive nanoscale systems used for the confinement of molecules in studies of structure and chemical reactions, including protein folding, and aggregation. The simulation of reverse micelles, in which a water "pool" is separated from a nonpolar bulk phase by a surfactant layer, poses significant challenges to empirical force fields due to the diversity of interactions between nonpolar, polar, and charged groups. We have explored the dependence of system density, reverse micelle structure, and water configurational relaxation times as a function of reverse micelle composition, including water:surfactant ratio, absolute number of water molecules, and force field using molecular dynamics simulations. The resulting structures and dynamics are found to depend more on the force field used than on varying interpretations of the water:surfactant ratio in terms of absolute size of the reverse micelle. Substantial deviations from spherical reverse micelle geometries are observed in all unrestrained simulations. Rotational anisotropy decay times and water residence times show a strong dependence on force field and water model used, but power-law relaxation in time is observed independent of the force field. Our results suggest the need for further experimental study of reverse micelles that can provide insight into the distribution and dynamics of shape fluctuations in these complex systems.

Protein folding in a reverse micelle environment: the role of confinement and dehydration.

Characterization of the molecular interactions that stabilize the folded state of proteins including hydrogen bond formation, solvation, molecular crowding, and interaction with membrane environments is a fundamental goal of theoretical biophysics. Inspired by recent experimental studies by Gai and co-workers, we have used molecular dynamics simulations to explore the structure and dynamics of the alanine-rich AKA(2) peptide in bulk solution and in a reverse micelle environment. The simulated structure of the reverse micelle shows substantial deviations from a spherical geometry. The AKA(2) peptide is observed to (1) remain in a helical conformation within a spherically constrained reverse micelle and (2) partially unfold when simulated in an unconstrained reverse micelle environment, in agreement with experiment. While aqueous solvation is found to stabilize the N- and C-termini random coil portions of the peptide, the helical core region is stabilized by significant interaction between the nonpolar surface of the helix and the aliphatic chains of the AOT surfactant. The results suggest an important role for nonpolar peptide-surfactant and peptide-lipid interactions in stabilizing helical geometries of peptides in reverse micelle environments.