UV resonance Raman studies of DNA-pyrene interactions: optical decoupling Raman spectroscopy selectively examines external site bound pyrene

Intercalation of pyrene into DNA results in a strong hypochromism of those pyrene electronic absorption bands which overlap the nucleic acid electronic transitions. The resonance Raman cross sections of pyrene excited within these absorption bands are decreased by factors of 10-100-fold. Raman saturation measurements indicate a dramatic increase in the intercalated pyrene nonradiative relaxation rate, which is consistent with fluorescence quenching observations. We demonstrate using the new technique of optical decoupling Raman spectroscopy (ODRS) that the remaining fluorescence derives from pyrenes not intercalated between bases, but most likely bound externally