MODIFICATION OF THE LOW-MOLECULAR WEIGHT BASIC ALBUMIN FRACTION FROM RAPESEED (BRASSICA NAPUS L.) BY ACETYLATION PART 1. CHEMICAL AND PHYSICOCHEMICAL ASPECTS

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Abstract
The chemical and physicochemical changes of the low-molecular weight basic albumin fraction from rapeseed, as a function of degree of acetylation, were studied using amino and ester groups analyses, PAGE electrophoresis, isoelectric focusing, viscometry, circular dichroism and fluorescence spectroscopy. The surface hydrophobicity was evaluated by means of the ANS fluorescence probe technique. The protein was readily acetylated at the amino groups by addition of acetic anhydride. Acetylation of amino acid hydroxyl groups was significantly slower and proceeded in the presence of an excess of the reagent after the amino groups had already been blocked. Acetylation resulted in protein species with isoelectric points at pH 7.6, 6.6, 5.95 and 5.4. The intrinsic viscosity of the native protein fraction dropped from 0.159 dlg-1 to 0.038 dlg-1 at a moderate degree of modification. The secondary structure of the protein, characterized by a content of 40-45% helix conformation, was not significantly influenced by acetylation. Modification did not result in wavelength shifts of the peaks in the near ultraviolet CD and fluorescence spectra. However, the negative ellipticities in the 250-270 nm region of the CD spectrum increased markedly with increasing degree of acetylation. The surface hydrophobicity increased linearly with the amount of acetyl groups introduced into the protein.

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