Ethanol or/and captopril-induced precipitation and secondary conformational changes of human serum albumin.

We determined the secondary structure of solid-state native human serum albumin (HSA) and its precipitates induced by ethanol, captopril, or a captopril/ethanol mixture. A transmission Fourier transform infrared (FT-IR) microspectroscopy equipped with a thermal analyzer was used. The secondary structural composition of solid-state native HSA was 54% alpha-helices (1655 cm(-1)), 22% beta-turns (1679 cm(-1)), and 23% beta-sheets (1633 cm(-1)). After ethanol treatment, a new peak was observed at 1690 cm(-1), and the peak at 1633 cm(-1) was more apparent in the HSA precipitates. The corresponding compositions consisted of 59% alpha-helices, 17% beta-turns, and 24% beta-sheets. After treatment with captopril with or without ethanol, the percentage of alpha-helices and beta-turns decreased in both HSA precipitates, but the percentage of beta-sheets increased. The temperature-dependent structural transformation from alpha-helices/random coils to beta-sheets for the solid-state HSA samples occurred at markedly different onset temperatures. The onset temperature for native HSA was 85 degrees C, and that for HSA precipitates obtained from ethanol, captopril, or captopril/ethanol was 100, 48 or 57 degrees C, respectively. The thermal-induced structural transformation from alpha-helices/random coils to beta-sheets implies a partial unfolding structure in these HSA samples.