A sup 1 H NMR study of human calcitonin in solution

Human calcitonin (hCT) has been investigated by NMR at 400 MHz in DMSO{sub d6} and in an 85% DMSO{sub d6}-15% {sup 1}H{sub 2}O (v/v) cryoprotective mixture. All backbone and side-chain resonances have been assigned and the secondary structure has been determined in both solvents. In DMSO{sub d6}, the simultaneous presence of d{sub {alpha}N}, d{sub NN}, and some specific weak medium-range nuclear Overhauser effects, together with the amide temperature coefficients and the analysis of the NH-{alpha}CH spin-spin coupling constants, indicates that hCT is highly flexible but with three domains (comprising segments Asn{sup 3}-Gly{sup 10}, Gln{sup 14}-Thr{sup 21}, and Thr{sup 25}-Ala{sup 31}) in extended conformations which dynamically transform into isolated {beta} turns in the N- and C-terminal regions and into adjacent tight turns, resembling a 3{sub 10} helix structure, in the central part. The DMSO-water mixture rigidifies the polypeptide chain, favoring and ordered, extended conformation. NOESY data indicate the presence of a short double-stranded antiparallel {beta} sheet in the central region made by residues 16-21 and connected by a two-residue hairpin loop formed by residues 18 and 19. Two tight turns, formed by residues 3-6 and 28-31, were also identified. The central {beta} sheet does not favor an amphipathic distribution of the residues as found for salmon calcitonin. This is in agreement with the smaller tendency of hCT to form the amphipathic {alpha} helix, postulated to be responsible for the interaction of hCT with lipids. The possible role of the cis-trans isomerism of Pro is discussed.