Fourier transform IR attenuated total reflectance spectroscopy studies of cysteine‐induced changes in secondary conformations of bovine serum albumin after UV‐B irradiation

Abstract Fourier transform IR spectroscopy equipped with attenuated total reflection was used to investigate the cysteine‐induced alteration of the protein secondary structure of bovine serum albumin (BSA) in aqueous solution before and after UV‐B irradiation. Several amino acids were also studied. The results indicate the unchanged IR spectra of BSA coincubated with amino acids, except cysteine, did not change after 72‐h UV‐B irradiation. There was no difference in the IR spectrum of the unirradiated BSA coincubated with cysteine. A shoulder at 1620 cm −1 attributed to the intermolecular β‐sheet structure was observed for the IR spectrum of BSA coincubated with cysteine after 72‐h UV‐B irradiation. Moreover, the peak intensity at 1303 cm −1 that is due the α‐helix structure was reduced, but the peak intensity at 1247 cm −1 corresponding to β‐sheet structures was increased. Longer UV‐B exposure for a BSA solution coincubated with cysteine changed the BSA solution from clear to viscous to gel form in which a transparent gel and another white gel were simultaneously observed. A gradual IR spectral alteration was found for BSA coincubated with cysteine and subjected to increased UV‐B irradiation. The longer UV‐B irradiation yielded increased intensity at 1620 cm −1 . The second‐derivative IR peaks at 1655, 1631, and 1548 cm −1 were shifted to 1650, 1620, and 1544 cm −1 , respectively, by the increase of UV‐B irradiation, suggesting a progressive transformation from an α‐helix to an intermolecular β‐sheet structure for BSA coincubated with cysteine. This strongly implies that longer UV‐B exposure time for the BSA solution in the presence of cysteine did alter the protein secondary structures of BSA more, thus inducing gel formation by protein aggregation. © 2003 Wiley Periodicals, Inc. Biopolymers (Biospectroscopy) 72: 345–351, 2003