Cobalt and Copper Promoted Amide Hydrolysis. Model Reactions for Carboxypeptidase A.

The mechanism for catalytic activity of several metalloenzymes, such as Carboxypeptidase A, is yet unclear. In order to investigate some of the mechanistic questions about these enzymes, the metal promoted amide hydrolysis of several model systems of known spatial orientation were studied. Specifically, the cobalt (III) complexes of 1-{{6-{(Dimethylamino)methyl}-2-pyridyl}methyl}hexahydro-5H-1,4-diazepin-5-one (1) and {{{6-{Hexahydro-5-oxo-1H-1,4-diazepin-1-yl)methyl}-2-pyridyl}methyl}imino}diacetic acid (2); and the copper (II) complex of 4-Benzyl-1-{{6-{(dimethylamino)methyl}-2-pyridyl}methyl}hexahydro-5H-1,4-diazepin-5-one (3) were prepared. These studies have shown that large rate enhancements ((TURNEQ)10('8)) are observed for the amide hydrolysis of Co('3+)-1 and Co('3+)-2. In these systems, neither the nitrogen nor the carbon of the amide bond are interacting with the metal during the hydrolysis of the amide. Amide hydrolysis in these complexes does occur intramolecularly by a metal-bound nucleophile and , therefore, are examples of model systems where the zinc-hydroxide mechanism proposed for CPA obtains. The pH rate profile is bell-shaped with the metal-bound water species reacting approximately 150 times faster than the unassisted metal-bound hydroxide species. A buffer effect was observed for these complexes only for the amide hydrolysis of the unassisted metal-bound hydroxide species around neutrality. Subsequent to the amide hydrolysis, the cobalt (III) complexes investigated undergo a substitution and a pH dependent isomerization reaction. The presence of a neighboring carboxylate in Co('3+)-2 did result in a slight increase in the rate of amide hydrolysis for the metal-bound hydroxide species. This suggests that the carboxylate is catalyzing the breakdown of the tetrahedral intermediate in the rate determining step for this species. Substitution of the amide nitrogen, as in Cu('2+)-3, resulted in a decrease in the rate of amide hydrolysis. This decrease in rate was attributed to the inability of the complex to obey stereoelectronic control without adopting a sterically hindered conformation.PhDOrganic chemistryUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159891/1/8412096.pd