Comparative study of human and salmon calcitonin secondary structure in solutions with low dielectric constants

Molecular conformations of salmon (sCT) and human (hCT) calcitonin in media with different concentrations of methanol/water and trifluoroethanol/water have been investigated by fluorescence, circular dichroism (CD) and infrared spectroscopy techniques. In these media, sCT and hCT adopt an alpha-helical structure comprising up to 40-48% of the amino acids. CD experiments reveal that for both peptides, the ordering of the Cys1-Cys7 disulfide link and the alpha-helix formation can be distinguished. Disulfide bond ordering is similar in both calcitonins. sCT adopts the alpha-helical structure more readily than hCT, as solvent polarity is reduced. In 2,2,2-trifluoroethanol (TFE)/water mixtures the alpha-helix formation for both hCT and sCT is a two-step process. The first alpha-helix formation step occurs at lower TFE concentrations (less than 11 mol% TFE for hCT and 6 mol% TFE for sCT). The second alpha-helix formation step represents 50 and 23% of the whole conformational change for hCT and sCT, respectively. Tyrosine fluorescence measurements correlate with the far ultraviolet CD changes associated with the peptide backbone. hCT is seen to adopt a left-handed, extended conformation in aqueous media below -50 degrees C.