Electronic Transition Moments of 2-Aminopurine

Abstract

The electronic spectrum of the unnatural nucleic acid base 2-aminopurine is important in order to understand the spectroscopy and photophysics of the normal DNA bases. Knowledge of transition moment directions and effective absorption components is also a prerequisite for the interpretation 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 ordinary and magnetic circular dichroism techniques, the near-UV spectrum of 2-aminopurine is resolved into contributions from five electronic transitions. Four moderately strong π → π* transitions (I, III, IV, and V) are observed, polarized at +53° (I), −72° (III), −77° (IV), and +57° or −67° (V) relative to the short molecular axis. In addition, one weak n → π* transition (II) polarized perpendicular to the molecular plane has been identified. Semiempirical INDO/S calculations support these assignments.