N-(tert-butoxycarbonyl)-L-valyl-L-valine methyl ester: a twisted parallel β-sheet in the crystal structure of a protected dipeptide.

The title compound, C(16)H(30)N(2)O(5), crystallizes with three molecules in the asymmetric unit, each adopting a β-strand/polyproline II backbone conformation. The main-chain functional groups are hydrogen bonded into tapes having the characteristics of parallel β-sheets. Each tape has a left-handed twist and thus forms a helix, with six peptide molecules needed to complete a full 360° rotation. A comparison of hydrogen-bond lengths and twisting modes is made with other related structures of protected dipeptides and with a hexapeptide derived from amyloid-β containing the Val-Val segment. Additionally, a comparison of the backbone conformation is made with that of the Val141-Val142 segment of the water channel aquaporin-4 (AQP4).