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.

Thermal micro‐Raman spectroscopic study of polymorphic transformation of famotidine under different compression pressures

“…26,27 (2) Confocal Raman microspectroscopy A powder of each famotidine polymorph was placed on a glass slide and then the spectra recorded by a confocal micro-Raman spectrometer (Ventuno, Jasco Co., Tokyo, Japan) equipped with a 30 mW green (532 nm) solid-state laser as an excitation source.…”

Application of ATR‐FTIR spectroscopy to the study of thermally induced changes in secondary structure of protein molecules in solid state

“…One of the most used techniques for the characterization of the secondary structure of proteins is infrared spectroscopy (FTIR). This accessible technique is a valuable tool in the examination protein conformation in aqueous solution and in the solid state . Monitoring protein conformation with IR spectroscopy is based on the fact that proteins have nine characteristics absorption bands in the infrared region: Amide A, B and I to VII.…”

“…The use of FTIR in combination with ATR has been extensively applied to examine the structure of proteins in solution; however, only a few articles have reported the use of this spectroscopy and sampling technique to study biomolecules in dry state . All of these ATR‐FTIR studies performed directly on solid state have been carried out with the purpose of studying structural changes due to dehydration but there are no previous works on thermally induced conformational changes and their relationship with denaturation on solid samples.…”