Electronic states of the indole-acrylamide molecular pair

A model is suggested for tryptophan fluorescence quenching by acrylamide based on the prediction that acrylamide can absorb a photon from the excited indole moiety and then dissipate the optical energy into a sink of fast exchanging conformations. Semiempirical electronic structure calculations of the indole-acrylamide pair indicate little actual intermolecular orbital mixing at van der Waals contact distances. However, the two lowest singlet transitions of the molecular pair, assigned to the acrylamide (pi *)----n(O) line and to the indole 1Lb----1A1 line, respectively, vary significantly in energies and in transition and excited state moments with the geometry of interaction between the two entities. The distribution of optimal quenching coordinations depends separately on the benzene and pyrrole portions and has a distinctly non-spherical shape at these distances.