EXCITED STATES OF DNA and ITS COMPONENTS AT ROOM TEMPERATURE–V. SPECTRAL, POLARIZATION and QUANTUM YIELD STUDIES OF CYTIDYLYL‐(3‘,5’)‐ADENOSINE

The corrected normalized emission spectrum, quantum yield, and emission anisotropy are reported for partially‐stacked CpA excited at 266 nm. Utilizing parameters determined separately for cytidine and AMP, the emission spectrum of CpA is quantitatively resolved into components characteristic of the two monomers plus a red shifted emission characteristic of complex formation. The results fit accurately to a simple stacking model in which monomer‐like fluorescence originates only from the unstacked fraction and “complex” fluorescence from the stacked fraction. It is shown that the stacked fraction absorbs in a manner qualitatively distinct from the unstacked fraction and the “complex” emission is really the excited state of a ground state complex. This emission, which peaks at 440 nm and contains at least two species can be resolved into three Gaussian components. It is suggested that the emitting species may originate in distinct stacked conformations.