Amino acid derivatives formed by haloketones

Amino acid derivatives formed by haloketones The identification of the amino acid residue modified by haloketones often represents a challenging problem. In fact, one inherent limitation in the use of most affinity labels is that authentic samples of the various amino acid derivatives formed are not readily available since the syntheses from the free amino acids and the affinity label are not simple. 

The derivatives of histidine formed by haloketones are probably generally stable to acid hydrolysis. At the very least, histidine is not regenerated following hydrolysis in 6 N HCl for 20 hr. Stevenson and Smillie (1965) in their studies of the modification of chymotrypsin by the chloromethyl ketone of N-tosyl phenylalanine or compound II have shown that oxidation by vapors of performic acid for 2 hr of peptides derivatized by the above reagents yields 3-carboxymethyl 

The derivatives of methionine formed by haloketones are not stable to the usual conditions of acid hydrolysis. These sulfonium salts are degraded in three different ways. Some methionine is regenerated, some homoserine and homoserine lactone is formed and possibly the homocysteine derivative of the general structure indicated below is produced where X represents the rest of the affinity label. 

The degradation product of the sulfonimn salt formed in the greatest yield after acid hydrolysis is undoubtedly a function of the structure of the affinity label. Generally, if the total amino acid composition of the modified protein was determined, low yields of methionine and increased yield of homoserine would be indicative, but certainly not proof of, the modification of a methionine residue. Sulfonium salts of methionine are generally not oxidized by hydrogen peroxide (Sigman and Blout 1967). 

The derivatives of serine formed by haloketones are stable to acid hydrolysis. The resulting ether linkage is apparently not cleaved by the usual hydrolytic conditions so that the loss of a serine residue, if it can be accurately determined, can be used as evidence for the modification of a serine residue (Schroeder and Shaw 1971). 

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