作者:Victoria L. Boyd、MeriLisa Bozzini、Piotr J. Guga、Robert J. DeFranco、Pau-Miau Yuan、G. Marc Loudon、Duy Nguyen
DOI:10.1021/jo00113a042
日期:1995.4
Previously, we reported [Anal. Biochem. 1992, 206, 344-352] a new method of sequencing proteins from the carboxy terminus (C-terminus). The carboxyl group at the C-terminus is activated and derivatized into a thiohydantoin (TH). We reported that, by alkylating the TH formed at the C-terminus, the TH is converted into a readily displaced leaving group. Reaction with NCS}(-) under acidic conditions cleaves the alkylated thiohydantoin (ATH) and derivatizes the freshly exposed C-terminus into a new proteinyl-TH. The efficiency of the initial activation of the carboxy group at the C-terminus is critical to the initial yield of the first ATH residue. In order to directly observe the intermediates that form during activation of the C-terminus, a model tripeptide, acetylalanine-alanine-alanine-OH (Ac-Ala-Ala-Ala-OH) was subjected to the reagents used to form the peptidyl-TH, Ac-Ala-Ala-Ala-TH. The reaction was monitored by nuclear magnetic resonance spectroscopy. An oxazolone was observed to form immediately at the C-terminus during the reaction with diphenyl chlorophosphate (DPCP), tetraphenyl pyrophosphate (TPPP), or tetramethylchlorouronium chloride (TMU-Cl). The oxazolone was observed to react with an excess of the carboxy group-activating reagents while under basic conditions. Diketopiperazine formation at the C-terminus was also observed. These side reactions prevent or retard the reaction of (NCS)- to form a peptidyl-TH and correlate with a reduced initial yield observed during automated C-terminal protein sequencing. The carboxylic acid-reactive reagents react with the side-chain carboxylic acid groups of aspartic and glutamic acid residues as well as the C-terminus. We found that the sidechain carboxylic acid groups in a protein could be selectively amidated in the presence of the proteinyl-oxazolone at the C-terminus.