831
from hydrogen-bond donor solvents t o nonhydrogenbonding solvents. The following explanation is offered to account for the observed blue shift in the R-P* C D bands upon hydrogen bonding of the keto oxygens. The hydrogenbond forming power of the keto oxygen can, to a first approximation, be estimated by the net charge on the keto oxygen. If changes in charge distribution upon T-T* excitation decrease the net negative charge at a keto oxygen which is hydrogen bonded t o water, a blue shift would be expected relative to the reference nonhydrogen-bonding state. Several recent theoretical treatments predict that T-T* excitation reduces the net negative charge on the keto oxygen in uracil and cytosine. Cytosine nucleosides all exhibited K-P* bands that blue shift in water relative to 1,2-dichloroethane
(DCE).29
Finally a recent study of phosphorescence to fluorescence ratios indicated that there are no closely spaced n-a* and P-T* singlets in the 260-mp spectral region.14 Further support for the interpretation that all C D bands in uridine arise from P-P* transitions is found in the observed red shift of all CD bands upon conjugative substitution on the uracil ring.51 According to Goodman and associates5? P--T* bands should undergo red shifts and n-ir* bands blue shifts when conjugative substituents are placed on aromatic chromophores. Acknowledgment. The authors thank Mr. William Miles for his excellent technical help. (51) D. W. Miles, M. J. Robins, R. I
