Excitation and Excited State Properties of Heterocyclic Chromophores. Indole and Purine Derivatives of Biological Significance
Proteins and nucleic acids absorb light in the near-UV due to the presence of aromatic heterocyclic chromophores. In this work linear dichroism (LD) in combination with fluorescence polarization anisotropy (FPA) and magnetic circular dichroism (MCD) is used to investigate electronic excitations and excited states of indole derivatives, purine derivatives, the wye base, 2-phenylindole and 4',6-diamidino-2-phenylindole (DAPI).
The UV spectrum of indole, which is the chromophoric core of tryptophan, is resolved into contributions from the lowest nearly degenerate 1A1 --> 1La and 1A1 --> 1Lb transitions, and the moment directions for the four lowest transitions in indole and eight selected methyl and methoxy derivatives are determined. Substitution does not generally perturb the transition moment directions, with exception for the 1A1 --> 1Lb and 1A1 --> 1Bb transitions whose moment directions are significantly altered by a methoxy group in 4- or 6-position.
The transition moment directions and the resolved UV-spectra were also determined for purine, a model compound for the purine nucleic acid bases, and the tRNA base wye (Y-base). Purine has a clearly resolved n-->p* transition lowest in energy, followed by two nearly degenerate p-->p* transitions with almost perpendicular moment directions. The phosphorescence spectra of purine and three methyl derivatives were investigated and the emitting triplet was found to have effectively pp* character. In addition, the purines with mobile imidazolic protons (purine and 6-methylpurine) are shown to exist, in an organic glass at 80 K, in two tautomeric forms (7H and 9H) as judged from phosphorescence emission and excitation spectra.
A DNA-binding probe 4',6-diamidino-2-phenylindole (DAPI) and its parent chromophore 2-phenylindole were finally investigated using the above mentioned experimental techniques as well as quantum mechanical calculations. Five distinct electronic transitions are found above 200 nm and their moment directions are determined. The theoretical results indicate a weak relation of the two lowest transitions in 2-phenylindole to the 1A1 --> 1Lb and 1A1 --> 1La transitions of indole.
linear dichroism (LD)
fluorescence polarization anisotropy (FPA)
magnetic circular dichroism