Martínez J H, Navarro P G, Garcia A A, de las Parras P J
Department of Physical Chemistry, Faculty of Pharmacy, University of Granada, Spain.
Int J Biol Macromol. 1999 Aug;25(4):337-43. doi: 10.1016/s0141-8130(99)00052-5.
Kinetic schemes are established for degradation catalysed by Cd2+ ions in methanolic medium for penicillin G, penicillin V and cephalothin, a cephalosporin. Methanolysis of penicillin V and cephalothin occurs with the formation of a single substrate-metal ion intermediate complex, SM, while degradation of penicillin G occurs with the initial formation of two complexes with different stoichiometry, SM and S2M. In each case. degradation is of first order with respect to SM with rate constant values equal to 0.079 min(-1), 0.120 min(-1) and 0.166 min(-1) at 20, 25 and 30 degrees C, respectively, for penicillin G; 0.061 min(-1) at 20 degrees C for penicillin V; and 2.0 x 10(-3) min(-1) at 20 degrees C for cephalothin. Activation energy for the decomposition process of the SM intermediate for penicillin G was calculated to be about 5.5 x 10(4) J/mol. Equilibrium constant values between SM compound and S2M at 20 degrees C (77.1 l/mol), 25 degrees C (45.3 l/mol) and at 30 degrees C (25.7 l/mol) were also calculated as well as the normal enthalpy of this equilibrium. With respect to the reaction products there is evidence that Cd2+ becomes part of their structure, forming complexes between Cd2+ and the product resulting from antibiotic methanolysis (L). Some characteristics of these complexes are discussed.
建立了在甲醇介质中由Cd2+离子催化青霉素G、青霉素V和头孢菌素头孢噻吩降解的动力学方案。青霉素V和头孢噻吩的甲醇解反应生成单一的底物-金属离子中间体络合物SM，而青霉素G的降解最初生成两种化学计量不同的络合物SM和S2M。在每种情况下，降解对SM均为一级反应，在20、25和30℃时，青霉素G的速率常数分别为0.079 min(-1)、0.120 min(-1)和0.166 min(-1)；青霉素V在20℃时为0.061 min(-1)；头孢噻吩在20℃时为2.0×10(-3) min(-1)。计算得出青霉素G的SM中间体分解过程的活化能约为5.5×10(4) J/mol。还计算了20℃（77.1 l/mol）、25℃（45.3 l/mol）和30℃（25.7 l/mol）时SM化合物与S2M之间的平衡常数以及该平衡的标准焓。关于反应产物，有证据表明Cd2+成为其结构的一部分，在Cd2+与抗生素甲醇解产生的产物（L）之间形成络合物。讨论了这些络合物的一些特性。
