Alkaline treatment, amino acid modification

Reactions Analytical Methods. Alkaline treatments are used in the food and feeds technology for solubilization and purification of proteins, to destroy toxic contaminants, and to obtain functional properties. These alkaline treatments induce many chemical modifications on the side chains of the amino acid residues, which have been described by many workers. 

Improvement in functionality of the major soy globulins can be obtained by structural modification of the proteins. Frequently this is done by chemical modification of the lysine epsilon-amino groups or by alkaline or heat treatment. We have described a procedure for preparation of a superior soy product without alteration of the amino acids (1). 

Two major chemical modifications of proteins that occur during alkaline treatment are crosslinking and racemlzation. Lysine, ornithine (via arginine), cystine and 0-substituted serine can participate in base-catalyzed reactions forming the crossllnked amino acids lysinoalanine, ornithinoalanine and lanthionine Under the same conditions, inversion can occur when the or hydrogen of an amino acid residue is abstracted by the base, resulting in a planar, optically inactive carbanion ( ), as illustrated in Figure 1. The carbanion may be reprotonated from either face of the plane, which causes inversion when this occurs from the opposite face. 

Acetic anhydride is the only monocarboxylic acid anhydride that is important in modification reactions. The acetylation of the amino groups of proteins can be made relatively specific if the reaction is done in saturated sodium acetate, since the o-acetyltyrosine derivative is unstable to an excess of acetate ions (Fraenkel-Conrat, 1959). The tyrosine derivative rapidly hydrolyzes in alkaline reaction conditions, even in the absence of added acetate buffer (Uraki et al., 1957 Smyth, 1967). Treatment with hydroxylamine also cleaves any o-acetyltyrosine modifications, forming acetylhydroxamate, which can be followed by its purple complex with Fe3+ at 540 nm... 

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