| Abstract: | Molecular orbital calculations (MINDO /3) using energy minimized molecular geometries were performed on oxidized and reduced lumiflavin and related methylated isoalloxazines, including cationic and anionic species. Close agreement with experimental geometry, photoelectron spectra, and NMR data supports the importance of optimized geometries for these molecular systems and provides the basis for interpretation of chemical and biological properties. Oxidized forms are shown to be most stable in the planar configuration but also highly flexible about the N(5)—N(10) axis; only 1 kcal/mol is required for a 10° bend. N(10) is generally out of the plane slightly; also, C(9)-methyl substitution introduces nonplanarity. The unsubstituted isoalloxazine is computed to be 0.76 kcal/mol (ΔH) less stable than its isomer, alloxazine. Calculations were also performed on enol as well as quinone-methide tautomeric forms. Reduced flavin geometry depends on methyl substitution pattern: N(10) substituted forms are bent with typical fold angles around 155°, whereas the unsubstituted reduced form is planar. Both oxidized and reduced forms are also flexible. Proton affinities were calculated for protonation and deprotonation of oxidized and reduced forms. Protonation of oxidized forms is favored at N(1) by 10–12 kcal/mol and produces somewhat nonplanar isoalloxazinium ions. In addition, ΔH for the two-electron reduction of lumiflavin is estimated to be ?19.7 kcal/mol. In this paper investigations of geometric aspects are presented along with introductory and background material. Orbital structure and electron distribution studies are presented in paper II. |
