Hussain A A, Awad R, Crooks P A, Dittert L W
College of Pharmacy, University of Kentucky, Lexington 40536.
Anal Biochem. 1993 Nov 1;214(2):495-9. doi: 10.1006/abio.1993.1528.
The determination of the rate and pH dependency of chlorination of amino acids by chloramine-T (CAT) using the model compounds beta-alanine, L-alanine, and L-alanine ethyl ester was investigated. The reactions were studied under pseudo-first-order conditions where the amino acid concentration was in large excess over CAT. The rates of the reactions were determined spectrophotometrically by following the disappearance of CAT. The overall reaction was found to obey second-order kinetics. The effect of pH was studied over the range 6.1 to 12.0. The reaction rate was found to be independent of pH in the range 6.1 to 8.5, and to decrease with increasing pH above 8.5. The pH dependency was rationalized by assuming that un-ionized CAT reacts with the un-ionized amino group of the amino acid. The value of the resulting second-order rate constant (1.9 x 10(7) M-1 min-1) was of the same order of magnitude as rate constants reported for similar N-chlorination reactions. A cyclic transition state involving a water molecule is proposed.
利用β-丙氨酸、L-丙氨酸和L-丙氨酸乙酯等模型化合物，研究了氯胺-T（CAT）对氨基酸氯化反应的速率和pH依赖性。反应在准一级条件下进行，其中氨基酸浓度大大超过CAT。通过跟踪CAT的消失，用分光光度法测定反应速率。发现总反应服从二级动力学。研究了pH在6.1至12.0范围内的影响。发现在6.1至8.5范围内反应速率与pH无关，而在pH高于8.5时随pH升高而降低。通过假设未电离的CAT与氨基酸的未电离氨基反应来解释pH依赖性。所得二级速率常数的值（1.9×10⁷ M⁻¹ min⁻¹）与报道的类似N-氯化反应的速率常数处于同一数量级。提出了一种涉及水分子的环状过渡态。
