Crystallographic characterisation of novel β-turn like folds in a model peptide: stabilisation by main-chain to side-chain interactions

X-Ray diffraction analysis of the model peptide Boc-Thr-Thr-OCH3 reveals the existence of distinctive conformational characteristics: semi-extended (φ=−62.1°; ψ=137.1°) and semi-folded (φ=−130.3°; ψT=5.7°) of the N- and C-terminus Thr residues, respectively. Surprisingly, an overall significantly ‘flat’ conformation is stabilised by a number of novel main-chain to side-chain intramolecular hydrogen bonds, i.e. two non-conventional: Cγi+1H⋯OCi−1 and CγiH⋯Oi+1 types and two conventional: NiH⋯Oγi and OγiH⋯Ni types of interactions.