45] Reductive alkylation of amino groups

Abstract

Many simple aldehydes and ketones react rapidly and reversibly with amino groups of proteins. Neither the initial adduct or the Schiff base formed upon dehydration is very stable in dilute aqueous solution, but extensive modification of protein amino groups can be obtained by the reduction of the Schiff base to a stable secondary amine. Reductive alkylation of protein amino groups can, thus, be accomplished with many different aldehydes and ketones under very mild conditions by using sodium borohydride as the reductant. Under the conditions described in the chapter, both α- and ɛ-amino groups are readily modified, but other common protein groups are not affected. Because the modified groups experience only a small change in basicity and, at neutral pH, retain their normal cationic charges, the overall charge and the relative distribution of charged groups in most proteins are not greatly changed by reductive alkylation. By using a large variety of readily available aldehydes and ketones, the size, shape, and hydrophobicity of added substituents can be easily varied. Reductive alkylation procedure is applicable to most proteins, except those containing readily reducible components or prosthetic groups, such as pyridoxal phosphate and rhodopsin, or those which are not stable at the required alkaline pH values.