MNDO–MOCIC evaluation of the uracil force field: Application to the interpretation of flavin vibrational spectra

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Abstract
A normal mode analysis has been carried out on the in-plane vibrations of uracil and three isotopic molecules, N,N-dideutero, C,C-dideutero, and perdeutero uracil, using a molecular orbital calculation (MNDO) to constrain the ratios of off-diagonal to diagonal elements in the compliance matrix (MOCIC). The 81 observed frequencies are calculated with a mean error of 9.7 cm−1. The force constants and normal mode patterns are discussed. The characteristic up-shift of the 1236 cm−1 mode upon N,N deuteration is reproduced by the calculation, and is ascribable to a change in mode composition from predominantly C=O deformation to predominantly C–N stretching, in agreement with the pattern observed for other cyclic imides. The analogous mode in flavin is variable in different flavoproteins and has been suggested to be sensitive to hydrogen bonding at flavin N3. This effect of hydrogen bonding is modeled in the present calculation by increasing the N–H deformation force constant. A similar perturbation of the C=O deformation constants shows that the 1236 cm−1 mode frequency may also be somewhat sensitive to hydrogen bonding at C4=0, but not at C2=0.