Stereopositional Outcome in the Packing of Dissimilar Aromatics in Designed β-Hairpins.

A popular strategy in the de novo design of stable β-sheet structures for various biomedical applications is the incorporation of aromatic pairs at the non-hydrogen-bonding (NHB) position. However, it is important to explicitly understand how aryl pair packing at the NHB region is coordinated with backbone structural rearrangements, and to delineate the benefits and drawbacks associated with stereopositional choice of dissimilar aromatic pairs. Here, we probe the consequences of flipped Trp/Tyr pairs by using engineered permutants at the NHB position of dodecapeptide β-hairpins, proximal and distal to the turn. Extensive conformational analysis of these peptides using NMR and CD spectroscopy reveal that a classic Edge-to-Face and Face-to-Edge geometry at the proximal and distal aromatic pairs, respectively, in YW-WY, is the most stabilizing. Such a preferred packing geometry in YW-WY results in a highly twisted β-sheet backbone, with Trp always providing a 'Face' orientation to its dissimilar aromatic partner Tyr. Flipping the proximal and/or distal aromatic pair distorts the ideal T-shaped geometry, and results in alternate aryl arrangements that can adversely affect strand twist and β-sheet stability. Our study reveals the existence of a strong stereopositional influence on the packing of dissimilar aromatic pairs. Our findings highlight the importance of modeling physical interaction forces while designing protein and peptide structures for functional applications.