Spectra and Simulation of Model Beta-Sheets and Hairpins : Impact of Turn Sequences and Aromatic Contacts on Equilibria and Dynamics

Abstract

b-sheets can be modeled as antiparallel strands coupled by turns. Isolated model systems are less stable, due to amide solvation), but can be induced to fold into sheet-like structures by restricting the conformational space of turn residues, such as with DPro-Gly sequences, or by inclusion of crossstrand hydrophobic interactions, especially aromatic contacts, which tend to desolvate parts of the structure. Expanding to multistrand structures can provide different characteristics for study of physical properties. Larger structures have strand differentiation, since the center strand of a threestrand sheet has a different H-bonds than the outer ones. In larger structures, integral hairpin-like sequences of either the first two or the second two strands might have different stabilities and folding mechanisms, if it is overall a multistate process. To explore these factors, we have prepared a series of three-stranded sheet structures that are stabilized by either DPro-Gly or by Aib-Gly turns and contain various Trp-Tyr side-chain contacts. To assess the impact of the third strand we also separately synthesized the component hairpins and studied their relative properties with temperature dependent circular dichroism, fluorescence, infrared (IR) and vibrational circular dichroism for evaluation of global thermodynamic equilibria as well as IR–detected temperature jump relaxation dynamics of selected components (strands, turns). Where possible, NMR structures were obtained for comparison and MD simulations of unfolding were used to aid interpretations. While all the sequences showed evidence of b-structure formation, the extent and stability varied markedly. In terms of equilibrium properties, the turn residues (especially DPro-Gly) seemed to have the strongest influence, while the aromatic contacts had minor effects. However in terms of dynamics, the aromatic effects differentiated the structures more, suggesting a variation in the folding mechanisms.