H-bonded N-heterocyclic base-pair phototautomerizational potential barrier and mechanism: The 7-azaindole dimer.

A theoretical analysis of the double proton transfer (PT) in a hydrogen-bonded N-heterocyclic base pair is presented. The calculated (time-dependent density functional theory) double PT barrier calculated for the concerted process of the 7-azaindole C(2h) dimer in the first excited singlet electronic state S(1) conforms well to the kinetic data and the photophysical evidence reported in this article. The calculated PT energy barrier of 4.8 kcal/mol height, and the corresponding zero point energy value, yield for the S(1) state an activation energy barrier of 0.3 kcal/mol. This finding implies that the double PT concerted process is almost barrierless, confirming previous experiments. Upon N-H deuteration of the 7-azaindole dimer, the theoretical excited-state activation energy for the double deuterium transfer is determined to be 1.4 kcal/mol, in agreement with experiment, which in low-temperature spectroscopy is shown to negate excited-state double-deuteron transfer.

Resolution of concerted versus sequential mechanisms in photo-induced double-proton transfer reaction in 7-azaindole H-bonded dimer.

The experimental and theoretical bases for a synchronous or concerted double-proton transfer in centro-symmetric H-bonded electronically excited molecular dimers are presented. The prototype model is the 7-azaindole dimer. New research offers confirmation of a concerted mechanism for excited-state biprotonic transfer. Recent femtosecond photoionization and coulombic explosion techniques have given rise to time-of-flight MS observations suggesting sequential two-step biprotonic transfer for the same dimer. We interpret the overall species observed in the time-of-flight experiments as explicable without conflict with the concerted mechanism of proton transfer.

Singlet molecular oxygen evolution upon simple acidification of aqueous hypochlorite: application to studies on the deleterious health effects of chlorinated drinking water.

A study of the pH profile of the decomposition of aqueous hypochlorite has revealed the evolution (onset at pH 8) of single (1 delta g) molecular oxygen (singlet spin state dioxygen) detected spectroscopically (1268 nm), prior to the appearance of chlorine (onset at pH 5.5). The possible mechanism of the singlet state dioxygen evolution is presented, and the origin of its chloride ion dependence is discussed, especially in reference to chloride ion dependence of singlet molecular oxygen evolution in biological systems. Recent epidemiological analyses of the correlation of human cancer with chlorinated water supplies focus attention on the singlet oxygen mechanisms of DNA lesion formation.

Singlet molecular oxygen in the Haber-Weiss reaction.

Characteristic chemiluminescence emission of singlet (1 delta g) molecular oxygen at 1268 nm is reported from a Haber-Weiss reaction. The reaction consists of mixing aqueous hydrogen peroxide with a solution of potassium superoxide, solubilized by 18-crown-6 ether in carbon tetrachloride or in dry acetonitrile at room temperature. Since the discovery of the enzyme superoxide dismutase by J.M. McCord and I. Fridovich [(1968) J. Biol. Chem. 243, 5733-5760], the identity of the reactive oxidant in superoxide-generating systems in biology has remained a chemical mystery. The results presented here suggest strongly that the reactive species is singlet oxygen generated via the Haber-Weiss reaction and not, as usually assumed, the hydroxyl radical, .OH, generated by the same reaction.

Interplay between excited-state intramolecular proton transfer and charge transfer in flavonols and their use as protein-binding-site fluorescence probes.

A comparative study is presented of competitive fluorescences of three flavonols, 3-hydroxyflavone, 3,3',4',7-tetrahydroxyflavone (fisetin), and 4'-diethylamino-3-hydroxyflavone (DHF). The normal fluorescence S1-->S0 (400-nm region) is largely replaced by the proton-transfer tautomer fluorescence S'1-->S'0 in the 550-nm region for all three of the flavonols in aprotic solvents at room temperature. For DHF in polar solvents the normal fluorescence becomes a charge-transfer fluorescence (460-500 nm) which competes strongly with the still dominant proton-transfer fluorescence (at 570 nm). In protic solvents, and at 77 K, the interference with intramolecular hydrogen bonding gives rise to greatly enhanced normal fluorescence, lowering the quantum yield of proton-transfer fluorescence. The utility of DHF as a discriminating fluorescence probe for protein binding sites is suggested by the strong dependence of the charge-transfer fluorescence on polarity of the environment and by various static and dynamic parameters of the charge-transfer and proton-transfer fluorescence which can be determined.

Excited-state intramolecular proton transfer as a fluorescence probe for protein binding-site static polarity.

A fluorescence probe is introduced for protein conformation and binding-site monitoring as the proton-transfer (PT) tautomer fluorescence by using 4-hydroxy-5-azaphenanthrene (HAP) as a prototype. A typical grossly-wavelength-shifted PT fluorescence for HAP is observed in the 600-nm spectral region for this UV-absorbing molecule (absorption onset, 400 nm), for which case PT occurs even in protic solvents. It is shown that PT fluorescence of HAP can serve as a protein-binding-site static-polarity calibrator, shifting from a lambda max of 612 nm in cyclohexane to 585 nm in ethanol at 298 K, contrary to the usual dispersion red shift. A small mechanical solvent-cage effect is noted in ethanol at 77 K, but solvent dielectric relaxation is not apparent from the fluorescence spectrum. Thus, HAP serves to distinguish static solvent-cage polarity from dynamical solvent dielectric relaxation and other solvent-cage effects (mechanical restriction of molecular conformation). HAP as a PT-fluorescence probe is applied to human serum albumin (HSA) and beaver apomyoglobin.

Energy transfer, charge transfer, and proton transfer in molecular composite systems.

Models for the fundamental mechanisms of excitation energy transfer, including cases involving singlet oxygen states, twisting-intramolecular-charge-transfer (TICT) states, and intramolecular proton transfer, are described in terms of elementary concepts and energy diagrams. Three limiting cases of energy transfer are distinguished, Davydov free excitons (Simpson and Peterson strong coupling) and localized excitons (weak coupling), and the Förster mechanism of vibrational-relaxation energy transfer. The prominent rôle of the singlet molecular oxygen states is described, together with the rôle of simultaneous transitions for molecular oxygen pairs. The origin of sudden polarization via the TICT-state potential is discussed and the generality of this phenomenon emphasized. The intramolecular proton-transfer phenomenon is outlined, and its role in molecular excitation transient phenomena is described. The complex interaction of all of these excitation mechanisms in determining photochemical and radiation chemical pathways is suggested.

Proton-transfer spectroscopy of 3-hydroxyflavone in an isolated-site crystal matrix.

The isolated-site low-temperature crystal matrix (dilute solutions in heptane and in octane) ( Shpol ' skii matrix) is shown to be operative for the spectroscopic study of 3- hydroxyflavone luminescence. The observed luminescence is demonstrated to be unique proton-transfer fluorescence from the excited tautomer. A separate study at higher concentrations of aggregated molecule luminescence and excitation spectra distinguishes these for 3- hydroxyflavone from isolated-molecule spectra. The application of the Shpol ' skii matrix low-temperature spectroscopy technique is suggested for other large heteroaromatic molecules, such as biomolecules containing polar groups that impart low solubility in nonaqueous solvents.

Mechanism of four-level laser action in solution excimer and excited-state proton-transfer cases.

Four-level laser energy level schemes are compared from the mechanistic spectroscopic viewpoint: (i) noble-gas excimer, (ii) solution molecular excimer, (iii) conventional dye laser, and (iv) intramolecular proton transfer. The lasing action of the chlorophyll special pair is discussed as an example of a solution excimer laser, and the lasing action of 3-hydroxyflavone and other molecules is discussed as an example of an intramolecular proton-transfer laser.

Direct spectroscopic observation of singlet oxygen emission at 1268 nm excited by sensitizing dyes of biological interest in liquid solution.

The direct observation of dye-photosensitized 1268-nm emission of the (1)Delta(g) --> (3)Sigma(g) transition of molecular oxygen in liquid solution at room temperature is reported. Singlet oxygen was photosensitized by UV excitation of perfluorobenzophenone in fluorocarbon solvent, by 3,4-benzpyrene and hematoporphyrin in carbon tetrachloride, and by methylene blue in water. Also reported is the development of an extremely sensitive near-infrared spectrophotometer that uses a thermoelectrically cooled lead sulfide detector, optimized optics, and a boxcar integrator as a data processor.

Excited-state two-proton tautomerism in hydrogen-bonded n-heterocyclic base pairs.

The optical absorption and luminescence spectra of 7-azaindole and its doubly hydrogen-bonded dimer were investigated as a model for the study of electronic interactions in DNA base pairs. It is demonstrated that a biprotonic phototautomerism occurs in the dimer and in suitable ethanol solvates in fluid solvents but that the phenomenon is not observed in a rigid solvent matrix. The normal violet structured fluorescence of 7-azaindole monomer becomes a broad green fluorescence in the tautomer. It is shown that spectral band interchanges, excimer formation, excited-state single-proton transfer, and proton tunneling cannot account for the luminescence change, but that the molecular exciton effect facilitates the cooperative two-proton reversible transfer. It is proposed that biprotonic phototautomerism with molecular environmental sensitivity could provide a mechanism for the initial step in ultraviolet mutagenic effects in DNA.

A stationary state approach to radiationless transitions: radiation bandwidth effect on excitation processes in polyatomic molecules.

Excitation processes associated with photon absorption by a molecule are considered in terms of the stationary states of the molecule and the bandwidth of the incident radiation. The conditions of validity of the usual interpretation that transitions occur to excited Born-Oppenheimer states followed by radiationless transitions to other B-O states are examined in terms of the various correlation and relaxation times of the system. In contrast to the usual case, for sufficiently narrow radiation bandwidths, it is demonstrated that excitation should be regarded as occuring to molecular stationary states.