title: Vibrational spectroscopic studies on the disulfide formation and secondary conformational changes of captopril-HSA mixture after UV-B irradiation.
abstract: 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 approximately 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 alpha-helix structure to 1622 cm(-1) because of a beta-sheet structure, which was more evident in pH 3.0, 8.0 or 9.0 buffer solutions. The Raman shift from 1653 cm(-1) (alpha-helix) to 1670 cm(-1) (beta-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 alpha-helix to beta-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.
authors.affiliation: Biopharmaceutics Laboratory, Department of Medical Research and Education, Taipei Veterans General Hospital, Shih-Pai, Taipei, Taiwan, Republic of China.
authors.forename: Mei-Jane
authors.initials: MJ
authors.lastname: Li
authors.forename: Shan-Yang
authors.initials: SY
authors.lastname: Lin
mesh_terms: D002216:Captopril / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects; D004220:Disulfides / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D017433:Protein Structure, Secondary / Q000528:radiation effects; D012709:Serum Albumin / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects; D017550:Spectroscopy, Fourier Transform Infrared; D013059:Spectrum Analysis, Raman; D014466:Ultraviolet Rays / Q000145:classification
chemical_list: D004220:Disulfides; D012709:Serum Albumin; D002216:Captopril