Some Optical Properties of S-β-(4-Pyridylethyl)-L-Cysteine and its Wheat Gluten and Serum Albumin Derivatives

Abstract

The optical properties of S-β-(4-pyridylethyl)-L-cysteine (PEC) and its bovine serum albumin (BSA) and wheat gluten derivatives were studied in 0.01–0.1 N HCl. Optical rotatory dispersions (O.R.D.), circular dichroism (C.D.), ultraviolet (U.V.), and infrared (I.R.) spectra were obtained of PEC, BSA, BSA reduced with mercaptoethanol and alkylated with 4-vinylpyridine (PE-BSA), BSA reduced with mercaptoethanol and alkylated with acrylonitrile (CN-BSA), gluten, gluten reduced with mercaptoethanol and alkylated with 4-vinylpyridine (PE-gluten), and gluten reduced with mercaptoethanol and alkylated with acrylonitrile (CN-gluten). I.R. spectra of the proteins showed the presence of α-helical and unordered conformations. The U.V. absorption at 254 mμ of PEC and PE-BSA depends on pH; a pK value near 6 for the pyridyl group was obtained by spectrophotometric titration. The possible use of PEC as a conformation probe was explored. The O.R.D. of PEC depends on concentration, solvent, and pH but not on ionic strength. The O.R.D. data of proteins were analyzed by the Moffitt–Yang method to get α-helical contents. These helical contents indicate that there is not enough interaction left to support α-helix in gluten with disulfide bonds broken at pH 2.2. However, approximately one-third of the helical content of BSA persists without the aid of disulfide bonds.