ELECTRONIC-TRANSITION MOMENTS OF 2-AMINOPURINE

The electronic spectrum of the unnatural nucleic acid base 2-aminopuri ne is important in order to understand the spectroscopy and photophysi cs of the normal DNA bases. Knowledge of transition moment directions and effective absorption components is also a prerequisite for the int erpretation of emission spectra of 2-aminopurine incorporated into DNA as a probe of nucleic acid base motion or excitation energy transfer processes. Using linear dichroism, fluorescence anisotropy, and ordina ry and magnetic circular dichroism techniques, the near-UV spectrum of 2-aminopurine is resolved into contributions from five electronic tra nsitions. Four moderately strong pi-->pi transitions (I, III, IV, and V) are observed, polarized at +53 degrees (I), -72 degrees (III), -77 degrees (IV), and +57 degrees or -67 degrees (V) relative to the shor t molecular axis. In addition, one weak n --> pi transition (II) pola rized perpendicular to the molecular plane has been identified. Semiem pirical INDO/S calculations support these assignments.