Microsolvation Effect on the Twist of β-sheets

Under certain circumstances β-sheets prefer to be twisted instead of flat. To get insight into the reasons of such preference, bare and microsolvated parallel and antiparallel two-strand polyalanine β-sheets are investigated using density functional theory. Full geometry optimizations show that microsolvation increases interstrand twisting and promotes a flat to twist transition. It is found that the latter behavior is connected to compressive strain resulting from microsolvation. Residues in flat β-sheets adjust the sense of its local intrastrand twist, which leads to the appearance of interstrand twist, to release strain and to favor water-water hydrogen bonding. The predicted microsolvation effect is corroborated analyzing the geometry of residues forming β-sheet motifs in protein crystals.