Cleland W W
Institute for Enzyme Research, University of Wisconsin, Madison 53705.
FASEB J. 1990 Aug;4(11):2899-905. doi: 10.1096/fasebj.4.11.2199287.
Secondary 18O isotope effects have been developed as a tool for determining transition state structures in enzymatic and nonenzymatic phosphoryl transfer reactions. 18O substitution in the nonbridge oxygens of a phosphoryl group makes the reaction go faster when the bond order is higher to these oxygens in the transition state than in the reactant, whereas the reaction goes slower if the bond order is less. The isotope effects are measured by the remote label method, using an isotope ratio mass spectrometer for analysis. The bond order to p-nitrophenolate ion when it is the leaving group is indicated by the secondary 15N isotope effect in the nitro group, with a value of 1.0028 representing nearly complete bond cleavage. It appears that the transition states for phosphoryl transfer have no more than one negative charge on the nonbridge oxygens, so that reactions of monoesters are dissociative, reactions of triesters are associative, and reactions of diesters are SN2 with half bond order to entering and leaving groups.
二级18O同位素效应已被开发成为一种工具，用于确定酶促和非酶促磷酰基转移反应中的过渡态结构。在磷酰基的非桥连氧原子中进行18O取代时，如果过渡态中这些氧原子的键级高于反应物中的键级，反应会进行得更快；而如果键级更低，则反应会进行得更慢。同位素效应通过远程标记法测量，使用同位素比率质谱仪进行分析。当对硝基苯酚离子作为离去基团时，硝基中的二级15N同位素效应表明其键级，值为1.0028表示几乎完全的键断裂。似乎磷酰基转移的过渡态在非桥连氧原子上的负电荷不超过一个，因此单酯的反应是解离性的，三酯的反应是缔合性的，二酯的反应是SN2反应，进入和离去基团的键级为半键级。
