Spontaneous mutations

A systematic study of 1-azacarbazole (1AZC) dissolved in 2-methylbutane (2MB) at gradually decreasing temperatures from room temperature to 77 K revealed the chromophore to exhibit four fluorescence emissions: a structured fluorescence in the UV region that is due to the 1-azacarbazole monomer, a structureless emission centered at 500 nm and assigned to the centrosymmetric dimer formed by double hydrogen bonding, an also structureless emission centered at ca. 400 nm and due to a noncentrosymmetric doubly hydrogen bonded dimer, and a fourth, structured emission at 357 and 375 nm due to a card-pack dimer. Evidence obtained from dilute solutions of 1-azacarbazole is for the first time assigned to a card-pack dimer, consistent with the photophysical behavior of carbazole in the same medium. Previously established photophysical evidence for such an interesting compound, which has been used as a model for studying light-induced double proton transfer mutational mechanisms, is completed or discussed here. The evidence obtained in this work reveals that 1AZC at a 10-4 M solution in 2MB does not exhibit doubly hydrogen bonded centrosymmetric dimer emission as the temperature decreases from room temperature up to 113 K (with a corresponding exponential increase of the solvent viscosity). At this temperature and below, however, the doubly hydrogen bonded centrosymmetric dimer emission appears. This evidence and others implemented in this work contradict the assumption of Waluk et al. that the appearance of the doubly hydrogen bonded centrosymmetric dimer is hindered by an increased viscosity of the medium.

Section: Introductionmentioning

confidence: 99%

Photophysics of 1-Azacarbazole Dimers: A Reappraisal

Catalán

2007

“…In the Watson−Crick model for DNA, the genetic code is related to the situation of the protons that yield the linkage of base pairs via hydrogen bonds. Such a code can be altered either spontaneously or via external perturbations. , The molecular basis of mutations related to alteration of the DNA code has been the focus of extensive studies since the Watson and Crick suggestion 1 of anomalous pairing via formation of tautomers of some bases. Hydrogen-bond tunneling, isomerization of bases, pyrimidine dimers, effect of ionizing radiation and UV irradiation, ,− formation of eximers − have all been addressed as contributing to mutations.…”

Section: Introductionmentioning

confidence: 99%

Photophysics of 7-Azaindole, Its Doubly-H-Bonded Base-Pair, and Corresponding Proton-Transfer-Tautomer Dimeric Species, via Defining Experimental and Theoretical Results

and

Kasha

2000

Photophysical research on the experimental and theoretical bases for the biprotonic transfer in the doubly-H-bonded dimer of 7-azaindole (7AI) is reported. The spectroscopic properties of the 7AI monomer, the normal-tautomer dimer, and the proton-transfer tautomer dimer are delineated by absorption, fluorescence and excitation spectra. The monomeric 7AI molecule is shown to exist at 10-6 M in hydrocarbon solution at 298 K, and the pure dimer at temperatures below 227 K in 10-4 M solution in 2-methylbutane (2MB). Deuteration of the pyrrolic sites yields dimers which exhibit only normal-tautomer fluorescence, whereas the nondeuterated 7AI dimer yields unique proton-transfer tautomer fluorescence, indicating quantum-mechanical tunneling as the principal mechanism for the biprotonic transfer. Density functional theory (B3LYP/6-31G** framework) calculations on the ground-state potential energy curves conform to the experimental results.

“…Because the double-proton transfers in 7AI and 1AZC dimers have been proposed as models − for mutational mechanisms triggered by electromagnetic radiation on DNA, − we thought it of interest to match available photophysical evidence of these compounds against that for TH1AZC dimer to determine whether its photophysical behavior was consistent with that of 7AI and 1AZC dimers.…”

Section: Introductionmentioning

confidence: 99%

Proton Phototransfers in Doubly Hydrogen Bonded Dimers: The Photophysics of 6,7,8,9-Tetrahydro-5H-pyrido[2,3-b]indole Dimers

Catalán

2009

A photophysical study of 6,7,8,9-tetrahydro-5H-pyrido[2,3-b]indole dissolved in 2-methylbutane at decreasing temperatures from 293 to 93 K revealed the presence of two different fluorescence emissions due to doubly hydrogen-bonded dimers of the monomer species. One of the emissions is assigned to the normal tautomer form of the dimer of C(2) symmetry, and the other is mostly assigned to the most stable dimer, which is nonsymmetric (C(1)) and exhibits single-proton transfer upon electronic excitation. As shown here, when the doubly hydrogen-bonded 7-azaindole dimer loses its centrosymmetry, by fusing a six-membered ring to the molecular structure, its first excited electronic state does not undergo a double-proton transfer but rather a single proton transfer, which, in a fluid medium, causes the dimer to dissociate; if the medium can trap the resulting excited structure, then the compound exhibits fluorescence in the 410 nm region, which is typical of its protonated or deprotonated forms.