The effects of pH and ultraviolet-B (UV-B) irradiation on the secondary structure of human serum albumin (HSA) in the absence or presence of captopril were investigated by an attenuated total reflection (ATR)/Fourier transform infrared (FTIR) spectroscopy. The UV-B exposure affecting the stability of captopril before and after captopril–HSA interaction was also examined by using confocal Raman microspectroscopy. The results indicate that the transparent pale-yellow solution for captopril–HSA mixture in all pH buffer solutions, except pH 5.0∼7.0, changed into a viscous form then a gel form with UV-B exposure time. The secondary structural transformation of HSA in the captopril–HSA mixture with or without UV-B irradiation was found to shift the maxima amide I peak in IR spectra from 1652 cm−1 assigned to α-helix structure to 1622 cm−1 because of a β-sheet structure, which was more evident in pH 3.0, 8.0 or 9.0 buffer solutions. The Raman shift from 1653 cm−1 (α-helix) to 1670 cm−1 (β-sheet) also confirmed this result. Captopril dissolved in distilled water with or without UV-B irradiation was determined to form a captopril disulfide observed from the Raman spectra of 512 cm−1, which was exacerbated by UV-B irradiation. There was little disulfide formation in the captopril–HSA mixture even with long-term UV-B exposure, but captopril might interact with HSA to change the protein secondary structure of HSA whether there was UV-B irradiation or not. The pH of the buffer solution and captopril–HSA interaction may play more important roles in transforming the secondary structure of HSA from α-helix to β-sheet in the corresponding captopril–HSA mixture than UV-B exposure. The present study also implies that HSA has the capability to protect the instability of captopril in the course of UV-B irradiation. In addition, a partial unfolding of HSA induced by pH or captopril-HSA interaction under UV-B exposure is proposed.
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Vol. 81 • No. 6