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The study employs several techniques to present a complete analysis of the crystal structures and time-resolved fluorescence of the adenine-specific DNA methyltransferase M. Taql that contains 2-aminopurine flipped into its active site. The analysis reveals and shows that there exists an alternate binding site in the extra-helical target base of the DNA, proving that thymine assists in delivering the target base into the active site.

The fluorescence decay characteristics of 2-aminopurine (2AP) and 2-aminopurine riboside (2APr) were investigated as a function of excitation wavelength in aqueous and ethanolic solutions. Global analysis of the decay data showed that 2AP existed as two emitting species, whereas 2APr existed as a single species.

The use of laser desorption laser photoionization time-of-flight mass spectrometry (L2TOFMS) for directly analyzing polynuclear aromatic hydrocarbons (PAHs) in solid waste matrices was evaluated. The results showed that L2TOFMS is a rapid, highly selective and sensitive method for analyzing PAHs. It will be a useful technique for predicting PAH distributions in soils and other solid environmental matrices.

The fluorescence decay of rigid 2-aminopurine (AP)-labeled duplexes at 77 K are examined in order to investigate the nature of the conformational states that give rise to the heterogeneous decay of AP and the roles of base dynamics in populating these states. The results have shown that the highly stacked geometry that gives rise to the very short decay time of AP-labeled DNA can be attained only through thermal motion of the bases.

The conformational behaviour of tyramine and phenethylamine as well as the number, geometry and electronic structure of stable conformers are described. The conformer dependence of the TDM orientation and the influence of the OH group on these properties are examined.

Investigated here are the aggregation properties of CuPcS4 in aqueous salt solutions using UV-vis spectroscopy and equilibrium statically mechanics. It is shown that the monomer-dimer model (MD) is unable to describe the qualitative behavior of the molar absorption coefficients at CuPcS4 concentrations above 10 (super -4) M and alternative model that takes into account the formation of trimer and all higher aggregates has to be put forward.

We report the crystal structure of the DNA adenine-N6 methyltransferase, M.Taql, complexed with DNA, showing the fluorescent adenine analog, 2-aminopurine, flipped out of the DNA helix and occupying virtually the same position in the active site as the natural target adenine. Time-resolved fluorescence spectroscopy of the crystalline complex faithfully reports this state: base flipping is accompanied by the loss of the very short (∼50 ps) lifetime component associated with fully base-stacked 2-aminopurine in DNA, and 2-aminopurine is subject to considerable quenching by π-stacking interactions with Tyrl 08 in the catalytic motif IV (NPPY). This proves 2-aminopurine to be an excellent probe for studying base flipping by M.Taql and suggests similar quenching in the active sites of DNA and RNA adenine-N6 as well as DNA cytosine-N4 methyltransferases sharing the conserved motif IV. In solution, the same distinctive fluorescence response confirms complete destacking from DNA and is also observed when the proposed key residue for base flipping by M.Taql, the target base partner thymine, is substituted by an abasic site analog. The corresponding cocrystal structure shows 2-aminopurine in the active site of M.Taql, demonstrating that the partner thymine is not essential for base flipping. However, in this structure, a shift of the 3' neighbor of the target base into the vacancy left after base flipping is observed, apparently replicating a stabilizing role of the missing partner thymine. Time-resolved fluorescence and acrylamide quenching measurements of M.Taql complexes in solution provide evidence for an alternative binding site for the extra-helical target base within M.Taql and suggest that the partner thymine assists in delivering the target base into the active site. [ABSTRACT FROM AUTHOR]

To develop luminescent molecular materials with predictable and stimuli-responsive emission, it is necessary to correlate changes in their geometries, packing structures, and noncovalent interactions with the associated changes in their optical properties. Here, we demonstrate that high-pressure single-crystal X-ray diffraction can be combined with high-pressure UV-visible absorption and fluorescence emission spectroscopies to elucidate how subtle changes in structure influence optical outputs. A piezochromic aggregation-induced emitter, sym -heptaphenylcycloheptatriene ( Ph 7 C 7 H ), displays bathochromic shifts in its absorption and emission spectra at high pressure. Parallel X-ray measurements identify the pressure-induced changes in specific phenyl-phenyl interactions responsible for the piezochromism. Pairs of phenyl rings from neighboring molecules approach the geometry of a stable benzene dimer, while conformational changes alter intramolecular phenyl-phenyl interactions correlated with a relaxed excited state. This tandem crystallographic and spectroscopic analysis provides insights into how subtle structural changes relate to the photophysical properties of Ph 7 C 7 H and could be applied to a library of similar compounds to provide general structure-property relationships in fluorescent organic molecules with rotor-like geometries.

The ability to routinely detect fluorescent nucleobase analogues at the single-molecule level would create a wealth of opportunities to study nucleic acids. We report the multiphoton-induced fluorescence and single-molecule detection of a dimethylamine-substituted extended-6-aza-uridine ( DMA th aU ). We show that DMA th aU can exist in a highly fluorescent form, emitting strongly in the visible region (470-560 nm). Using pulse-shaped broadband Ti:sapphire laser excitation, DMA th aU undergoes two-photon (2P) absorption at low excitation powers, switching to three-photon (3P) absorption at high incident intensity. The assignment of a 3P process is supported by cubic response calculations. Under both 2P and 3P excitation, the single-molecule brightness was over an order of magnitude higher than reported previously for any fluorescent base analogue, which facilitated the first single-molecule detection of an emissive nucleoside with multiphoton excitation.