Deprotonation of zinc(II)-water and zinc(II)-alcohol and nucleophilicity of the resultant zinc(II) hydroxide and zinc(II) alkoxide in double-functionalized complexes: theoretical studies on models for hydrolytic zinc enzymes.
The factors governing the deprotonation ability of zinc(II)-water and zinc(II)-alcohol and nucleophilicity of the resultant zinc(II) hydroxide and zinc(II) alkoxide as complex models for zinc enzymes have been investigated through Hartree-Fock and density-functional theory methods with the 6-311++G(d,p) basis set. Our calculations showed that in these double-functionalized complexes (i.e., zinc complexes having both a zinc(II)-alcohol motif and a zinc(II)-water motif) zinc(II)-alcohol is preferred in deprotonation over zinc(II)-water (i.e., zinc(II)-alcohol has a much lower pK(a) than zinc-coordinated water in the same molecule). Natural bond orbital analysis revealed that zinc(II) alkoxides are more nucleophilic than their respective counterparts zinc(II) hydroxides. The analysis of the transition state in the transformation reaction from zinc(II) hydroxide species to zinc(II) alkoxide species indicates that zinc(II) alkoxides are the preferred deprotonated species not only thermodynamically but also kinetically. Further examination of the proposed mechanisms of the zinc(II) alkoxide-promoted transesterification path and the zinc(II) hydroxide-promoted hydrolysis path revealed the structures of the intermediates and energy diagrams in the reactions. These results, entitled double-functionalized complexes, for the first time, put a firm theoretical foundation of why the zinc(II)-alcoholic OH is a better model for hydrolytic zinc enzymes (having both stronger acidity and better nucleophilicity).
Xia
Jiang
J
State Key Laboratory of Coordination Chemistry and Department of Chemistry, Nanjing University, Nanjing 210093, People's Republic of China.
Shi
Yan-bo
YB
Zhang
Yong
Y
Miao
Qiang
Q
Tang
Wen-xia
WX
D000438:Alcohols / Q000737:chemistry; D002384:Catalysis; D004798:Enzymes / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D008962:Models, Theoretical; D008968:Molecular Conformation; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry; D015032:Zinc / Q000737:chemistry*
D000438:Alcohols; D004798:Enzymes; D009942:Organometallic Compounds; D014867:Water; D015032:Zinc