ELECTRONIC-SPECTRA OF CRYSTALLINE GUANOSINE - TRANSITION-MOMENT DIRECTIONS OF THE GUANINE CHROMOPHORE

Polarized absorption spectra of single crystal of guanosine dihydrate have been obtained by Kramers-Kronig analysis of reflection data taken from the (001) and (100) crystal faces. Four strong in-plane polarized transitions can be identified, In addition, three features apparently polarized perpendicularly to the planes of the guanine chromophores are observed. The effects of resonance coupling between molecules in the crystal lattice are considered, and a generally consistent picture emerges for the effects of exciton coupling on the crystal spectra. The calculated change of transition moment directions induced by the crystal field is of the order of 10-15 degrees. Consideration of these effects brings the experimental crystal results into better agreement with the results of semiempirical MO calculations for the excited states of the guanine chromophore.