Kaminskaia N V, Spingler B, Lippard S J
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
J Am Chem Soc. 2001 Jul 11;123(27):6555-63. doi: 10.1021/ja002699e.
Inactivation of beta-lactam antibiotics by metallo-beta-lactamase enzymes is a well-recognized pathway of antibiotic resistance in bacteria. As part of extensive mechanistic studies, the hydrolysis of a beta-lactam substrate nitrocefin (1) catalyzed by dinuclear zinc(II) model complexes was investigated in nonaqueous solutions. The initial step involves monodentate coordination of the nitrocefin carboxylate group to the dizinc center. The coordinated substrate is then attacked intramolecularly by the bridging hydroxide to give a novel intermediate (2') characterized by its prominent absorbance maximum at 640 nm, which affords a blue color. The NMR and IR spectroscopic data of 2' are consistent with it being zinc(II)-bound N-deprotonated hydrolyzed nitrocefin that forms from the tetrahedral intermediate upon C-N bond cleavage. Protonation of the leaving group is the rate-limiting step in DMSO solution and occurs after the C-N bond-breaking step. Addition of strong acids results in rapid conversion of 2' into hydrolyzed nitrocefin (3). The latter can be converted back to the blue species (2') upon addition of base. The low pK(a) value for the amino group in hydrolyzed nitrocefin is explained by its involvement in extended conjugation and by coordination to zinc(II). The blue intermediate (2') in the model system resembles well that in the enzymatic system, judging by its optical properties. The greater stability of the intermediate in the model, however, allowed its characterization by (13)C NMR and infrared, as well as electronic, spectroscopy.
金属β-内酰胺酶使β-内酰胺抗生素失活是细菌抗生素耐药性中一种广为人知的途径。作为广泛机理研究的一部分，在非水溶液中研究了双核锌（II）模型配合物催化的β-内酰胺底物硝噻吩（1）的水解反应。第一步涉及硝噻吩羧酸根基团与二锌中心的单齿配位。然后，配位的底物被桥连的氢氧根进行分子内攻击，生成一种新型中间体（2'），其在640nm处有突出的最大吸收峰，呈现蓝色。2'的核磁共振（NMR）和红外（IR）光谱数据与它是锌（II）结合的N-去质子化水解硝噻吩一致（该硝噻吩由四面体中间体在C-N键断裂后形成）。离去基团的质子化是二甲基亚砜（DMSO）溶液中的限速步骤，且发生在C-N键断裂步骤之后。加入强酸会导致2'迅速转化为水解硝噻吩（3）。加入碱后，后者又可转化回蓝色物种（2'）。水解硝噻吩中氨基的低pK（a）值可通过其参与扩展共轭以及与锌（II）配位来解释。从其光学性质判断，模型体系中的蓝色中间体（2'）与酶促体系中的蓝色中间体非常相似。然而，模型中中间体的稳定性更高，这使得可以通过碳-13（¹³C）核磁共振、红外光谱以及电子光谱对其进行表征。
