Knowledge Bank

The electronic spectrum of the adenine chromophore has been reinvestigated using single crystal polarized reflection spectroscopy. The original assignments of the low energy absorption region (260 nm in solution) were made over twenty years ago and since then have been the basis for almost all the numerous spectroscopic interpretations and calculations of the electronic properties of polynucleotides containing the adenine residue. New spectra obtained from crystals of 9-methyladenine and 6-methylaminopurine suggest that the original assignments are incorrect. The present data lead to an interpretation in which two overlapped transitions of comparable intensity are to be found in the lower energy spectral region. The transition moment directions of the slightly lower energy component is about -6' (positive angles are measured counterclockwise from the central $C4-C5$ bond). The second and somewhat more intense band is polarized at $\sim +48\prime$. Some comments regarding the higher energy bands $(\sim 200nm)$ as well as the mixing effects owed to intermolecular inteactions in the crystal will be presented.