Isotopic mapping of transition-state structural features associated with enzymic catalysis of methyl transfer
For comparison of the molecular structures of nonenzymic and enzymic sulfur-to-oxygen transmethylation transition states by the use of kinetic isotope effects, a series of isotopic maps is produced. In these, contours of constant isotope effect are displayed vs. the Pauling bond orders B/sub CS/ and B/sub CO/, for the carbon-sulfur and carbon-oxygen bonds, respectively, taken as independent variables to describe the transition states. Maps are calculated by the BEBOVIB approach for k(CH/sub 3/)/k(CD/sub 3/), k(/sup 12/CH/sub 3/)/k(/sup 13/CH/sub 3/), k(/sup 16/O)/k(/sup 18/O), and k(/sup 32/S)/k(/sup 34/S), with two models for the reaction coordinate, two force-field assumptions, and four temperatures. Nonenzymic isotope effects and isotope effects for catechol-O-methyltransferase action are then used to construct figures on the CH/sub 3//CD/sub 3/ and /sup 12/CH/sub 3///sup 13/CH/sub 3/ maps which correspond to allowed spaces of transition-states structures. Superposition of the figures yields the spaces of transition-state structures simultaneously consistent with both hydrogen and carbon isotope effects. It is concluded that the enzyme compresses the S/sub N/2 transition state and that the compression of the C-O and C-S bonds may well be of the order of 0.15 A per bond and could conceivably be more than twice as large.
- Research Organization:
- Univ. of Kansas, Lawrence
- OSTI ID:
- 6948223
- Journal Information:
- J. Am. Chem. Soc.; (United States), Vol. 104:12
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
CARBON 13
ISOTOPE EFFECTS
DEUTERIUM
ENZYMES
METHYLATION
OXYGEN 18
SULFUR 34
EXPERIMENTAL DATA
CARBON ISOTOPES
CHEMICAL REACTIONS
DATA
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
HYDROGEN ISOTOPES
INFORMATION
ISOTOPES
LIGHT NUCLEI
NUCLEI
NUMERICAL DATA
ODD-ODD NUCLEI
OXYGEN ISOTOPES
STABLE ISOTOPES
SULFUR ISOTOPES
400302* - Organic Chemistry- Isotope Effects- (-1987)