ELECTRONIC-TRANSITION DIPOLE-MOMENTS OF THE 1,N-6-ETHENOADENINE CHROMOPHORE

The electronic spectra of 1,N-6-ethenoadenine (epsilon-adenine, epsilon-A), 3-methyl-epsilon-adenine (3-me-epsilon-A), and epsilon-adenosine (epsilon-Ado) have been examined with respect to the number of electronic transitions, their intensities, and their transition moments, on the basis of measurements of UV and IR linear dichroism (LD) on samples partially oriented in stretched poly(vinyl alcohol) films, fluorescence anisotropy (FA), and magnetic circular dichroism (MCD). The experimental results are compared with semiempirical molecular orbitaI calculations. Four pi-->pi(*) transitions were found to contribute to the UV spectrum of the epsilon-adenine chromophore with the following transition moment directions (relative to the ''adenine'' pseudosymmetry long axis, counted toward N-1): +27 degrees +/- 10 degrees (I at 32 500 cm(-1)), -47 degrees +/- 5 degrees (II at 37 000 cm(-1)), -30 degrees +/- 10 degrees (III at 32 000 cm(-1)), and -11 degrees +/- 10 degrees or -49 degrees +/- 10 degrees (IV at approximate to 46 000 cm(-1)). The orientation parameters in the stretched film experiments were determined from polarized IR measurements combined with UV LD and FA data. In addition, fluorescence emission and excitation spectra and fluorescence lifetimes of E-A in aqueous solutions indicate that epsilon-A does not behave as a single spectroscopic species. In contrast to 3-me-epsilon-A and epsilon-Ado, epsilon-A displays an emission wavelength dependent excitation spectrum and a biexponential fluorescence decay. This observation is attributed to a prototropic tautomeric equilibrium between the 1-H and 3-H forms of epsilon-A.