TWO-PHOTON STUDIES OF AMINO ACIDS AND NUCLEIC ACIDS

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Two-photon electronic state spectroscopy which offers selection rules complementary to those of ordinary UV-visible spectroscopy is used to study the low lying electronic singlet excited states of aromatic amino acids, DNA bases, nucleotides and DNA polymers. Studies of model aromatic systems are continued for the purpose of forming a firm bridge of understanding between substituted aromatics and the less well-understood indoles, purines and pyrimidine bases. INDO/S calculations, with emphasis on thorough investigation of basis set, parameter and geometry effects, will be applied to each experiment. The long range goal is to attain a firmly based understanding, with the aid of transition density pictures, of the electronic structure of these systems -- one that enjoys a similar degree of trust given to simple substituted benzenes. Some focus is on the specific aim of locating the "La origin" in the tryptophan chromaphore, indole; one-photon laser UV polarized fluorescence experiments on indoles in vapor and hydrocarbon glass are proposed to support parallel two-photon experiments. One-photon fluorescence as well as two-photon experiments on one and two-stranded viral DNA's are proposed, especially to see if intrinsic fluorescence depends on supercoiling and protein complexation. The intrinsic fluorescence of Z-DNA will be observed.