Fourier-transform infrared conformational study of bovine insulin in surfactant solutions.

To obtain conformational data on the monomeric form of insulin, which is believed to be the physiologically active form of the hormone, insulin in sodium dodecyl sulfate solution was studied by Fourier-transform infrared (FTIR) spectroscopy and circular dichroism, and results were compared with those obtained with des(B23-30) octapeptide insulin (DOI) and dimeric insulin in buffer. The FTIR amide I band (1600-1700 cm-1) was examined, and a quantitative evaluation of the secondary structure fractions of the various conformations showed less of a beta-sheet component for both insulin in SDS and DOI in buffer than for insulin in buffer, corresponding to a lack of monomers binding to form dimers. At the concentrations used for FTIR (> or = 2 mg/mL), the CD spectra of insulin in SDS and DOI in buffer were qualitatively identical but different from that of insulin in buffer, which is associated at these concentrations. The CD spectrum pattern of insulin in very dilute solution (80 nM), where it is prevalently monomeric, is very similar to that of monomeric insulin in SDS, which suggests that the conformation of the hormone in the two cases is very similar.