α-Proton chemical shifts and secondary structure in proteins

Statistical analyses have been performed on 1H chemical-shift distributions in 32 polypeptides and proteins (ranging from 27 to 129 amino acid residues in length) for which reliable assignments (primarily from analyses of their spectra by 2D NMR methods) and secondary structure information were both available. On the basis of two independent statistical tests, significant differences were found between aH chemical shifts of aliphatic amino acid residues, Ala, Ile, Leu, Lys, and Val, in helical and β-sheet structures. Residues in regular helices have their αH atoms shifted upfield by −0.4 ppm, on average, and those in regular β-sheet structures are shifted downfield by +0.4 ppm with respect to the random coil value.