Ab initio quantum mechanical characterization of the ground electronic states of uracil, 5-fluorouracil, thymine, 5-trifluoromethyluracil, and their 6-SCH/sub 3/ anion derivates

Journal Article · Thu Feb 21 00:00:00 EST 1980 · J. Phys. Chem.; (United States)

DOI:https://doi.org/10.1021/j100441a018· OSTI ID:5336512

Lin, J ^[1]; LeBreton, P R; Shipman, L L

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1. Univ. of Illinois, Chicago

The molecular fragment floating spherical Gaussian orbital method has been used to investigate the molecular orbital structure and ..pi..-charge distributions of uracil (Ura), 5-F-Ura, thymine (Thy), 5-CF/sub 3/-Ura, 6-SCH/sub 3/-Ura anion, 5-F-6-SCH/sub 3/-Ura anion, 6-SCH/sub 3/-Thy anion, and 5-CF/sub 3/-6-SCH/sub 3/-Ura anion. For each of the eight species, the total ..pi.. populations and bond orders were compared. For each of the four neutral molecules, the populations and bond orders for the lowest unoccupied molecular orbitals (LUMOs) were compared; the results indicate that the experimentally observed site specificity for nucleophilic attack (i.e., the C/sub 6/ position) is electronically controlled in that the LUMO has its highest density at C/sub 6/. For the four anionic species, the populations and bond orders for the highest occupied molecular orbitals (HOMOs) were compared; the results indicate that the site specificity for the enzymatic ..pi..-electrophilic attack on the anion is electronically controlled in that the HOMO has its highest density at C/sub 5/. For the addition of bisulfite in substituted uracils in vitro, the first step of the reaction involves the-nucleophilic attack of the sulfur atom at C/sub 6/. The second and rate-determining step under low general acid concentration involves sigma-electrophilic attack of a proton onto the oxygen (O/sub 2/) bound to C/sub 4/. Calculations show that the number of electrons on O/sub 2/ in the anionic reaction intermediate and the O/sub 2/-protonation energy decreases in the order 5-F-Ura > Ura > Thy, an identical ordering to that found for the overall reaction rates for bisulfite addition under conditions of low general acid concentration. Thus the overall rate for bisulfite addition correlates well with the basicity of O/sub 2/ in the anionic reaction intermediate.

• OSTI ID: 5336512
• Journal Information: J. Phys. Chem.; (United States), Vol. 84:4
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
ORGANIC FLUORINE COMPOUNDS
CHEMICAL REACTIONS
ORGANIC SULFUR COMPOUNDS
THYMINE
URACILS
EXPERIMENTAL DATA
ISOLATED VALUES
AZINES
DATA
DATA FORMS
HETEROCYCLIC COMPOUNDS
HYDROXY COMPOUNDS
INFORMATION
NUMERICAL DATA
ORGANIC COMPOUNDS
ORGANIC HALOGEN COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PYRIMIDINES
550200* - Biochemistry