Picosecond dynamics of double proton transfer in 7-azaindole dimers

Hetherington, W. M.; Micheels, Ronald H.; Eisenthal, K.E.

doi:10.1016/0009-2614(79)85004-6

Cited by 117 publications

(88 citation statements)

References 8 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This reasoning leads to the surprisingly small value of 0.3 kcal͞mol for the ESDPT activation energy. However, this theoretical result is in good concordance with the experimental observations reported in the literature (17)(18)(19).…”

supporting

confidence: 92%

“…A lower limit of 1.4 kcal͞mol was estimated for the PT activation energy for 7AI dimer in 3-methylpentane. Hetherington et al (18) showed that the excited electronic state (S 1 ) generates the PT tautomer within the 5-ps width of the excitation pulse. They estimated the barrier for ESDPT as the energy difference between the first peak, in the PT tautomer and the normal tautomer, dimer excitation spectra (77 K) of their respective fluorescences, yielding an upper limit barrier height of 2.0 kcal͞mol.…”

mentioning

confidence: 99%

See 1 more Smart Citation

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

Catalán

Pérez

Valle

et al. 2003

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

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 C2h dimer in the first excited singlet electronic state S1 conforms well to the kinetic data and the photophysical evidence reported in this article. The calculated PT energy barrier of 4.8 kcal͞mol height, and the corresponding zero point energy value, yield for the S1 state an activation energy barrier of 0.3 kcal͞mol. 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 kcal͞mol, in agreement with experiment, which in low-temperature spectroscopy is shown to negate excited-state double-deuteron transfer. Multiple H-bonded base-pairing as a fundamental element of DNA structure was introduced by Watson and Crick (1), using the stable keto-tautomer base forms. They included consideration of the possibility of mutations by tautomeric proton transfer (PT) shifts. The example (2, 3) of an anomalous (imino tautomer) adenine-cytosine (AЈC) pairing could arise from a biprotonic phototautomerism in a normal AT pair, in which the excited-state electronic redistribution would catalyze a simultaneous two-proton flip. A model base pair for such a biprotonic phototautomeric shift is 7-azaindole (7AI) H-bonded dimer (4), now a much-studied example (Scheme 1). This phenomenon has been demonstrated to represent a concerted process over a double-minimum potential energy (PE) curve (refs. 5-7; see also ref. 8 and references therein). Earlier quantum chemical calculations (9, 10) on DNA base-pair mutations concentrated on ground electronic-state proton tunneling. However, for a normal tautomer to be converted to an anomalous (more stable) form with a lower potential minimum as a driving force for tautomerization, electronic excitation is an essential step. In this article, we present the refinements in the quantum mechanical (QM) theory needed to yield a reliable and more accurate PE curve for the tautomeric conversion. The PE curve for the centro-symmetric dimer of 7AI biprotonic phototautomerism is given, and the resultant excitation energy was analyzed for N-H-and N-D-substituted species, explaining the blocking of the photo-tautomerization in the latter.In recent years the subject of anomalous DNA base-pairings as a result of proton shifts within the H bonds has been the subject of accelerated study. The new interest has been stimulated by the application of ultra-fast laser excitation techniques and advances in QM molecular theories and calculations. Watson and Crick's initiating work (1-3) on DNA structure also focused attention on the possible production of anomalous base-pairing as a result of PT tautomeric shifts. Molecular spectroscopy has shown that tautomers unst...

show abstract

supporting

confidence: 92%

mentioning

confidence: 99%

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

Catalán

Pérez

Valle

et al. 2003

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

show abstract

“…The doubly hydrogen-bonded dimer of 7-azaindole is a prototypical system showing such a reaction in the photoexcited state. This dimer has served as one of the most central model systems, and hence its photochemistry and photophysics have been extensively studied for more than three decades (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Spectroscopic measurements revealed that the 7-azaindole dimer exhibits a double proton transfer reaction after photoexcitation ( Fig.…”

mentioning

confidence: 99%

“…The steady-state fluorescence spectra that were examined under various conditions provided information about photochemical properties of the relevant excited states (5,6,9,10). The proton transfer time was first reported as Ͻ5 ps in 1979 by picosecond fluorescence spectroscopy (14), and then it was determined as 1.4 ps by measuring the rise of the tautomer fluorescence with femtosecond time resolution (15).…”

mentioning

confidence: 99%

The answer to concerted versus step-wise controversy for the double proton transfer mechanism of 7-azaindole dimer in solution

Takeuchi

Tahara

2007

Proc. Natl. Acad. Sci. U.S.A.

146

115

View full text Add to dashboard Cite

The dynamics and mechanism of the double proton transfer reaction of the 7-azaindole dimer was investigated in solution by excitation wavelength dependence in steady-state and femtosecond time-resolved fluorescence spectroscopy. Femtosecond measurements in the UV region revealed that the dynamics of the dimer fluorescence exhibits remarkable change as the excitation wavelength was scanned from 280 to 313 nm. The fluorescence showed a biexponential decay (0.2 and 1.1 ps) with 280-nm excitation, whereas it exhibited a single exponential decay (1.1 ps) with 313-nm excitation (the red-edge of the dimer absorption). This observation clearly indicates that the 0.2-ps component is irrelevant to the proton transfer. In the visible region, we found that the tautomer fluorescence rises in accordance with the decay of the dimer fluorescence with a common time constant of 1.1 ps. This finding unambiguously denies the appearance of any intermediate species in between the dimer and tautomer excited states, indicating that the double proton transfer reaction is essentially a single-step process. We conclude that the double proton transfer of the 7-azaindole dimer in solution proceeds in the concerted manner from the lowest excited state with the 1.1-ps time constant. On the basis of the experimental data obtained, we discuss the long-lasting concerted versus step-wise controversy for the double proton transfer mechanism in solution.femtosecond spectroscopy ͉ fluorescence ͉ ultrafast ͉ photochemistry E xcited-state proton transfer has been receiving a great deal of attention, because it plays crucial roles in photochemistry and photobiology (1-3). The doubly hydrogen-bonded dimer of 7-azaindole is a prototypical system showing such a reaction in the photoexcited state. This dimer has served as one of the most central model systems, and hence its photochemistry and photophysics have been extensively studied for more than three decades (4-18). Spectroscopic measurements revealed that the 7-azaindole dimer exhibits a double proton transfer reaction after photoexcitation (Fig. 1) (4-7, 9, 10). The reactive nature of the 7-azaindole dimer in hydrocarbon solvent is manifested by its steady-state fluorescence spectrum, which shows significantly red-shifted fluorescence attributed to the tautomer fluorescence. The steady-state fluorescence spectra that were examined under various conditions provided information about photochemical properties of the relevant excited states (5, 6, 9, 10). The proton transfer time was first reported as Ͻ5 ps in 1979 by picosecond fluorescence spectroscopy (14), and then it was determined as 1.4 ps by measuring the rise of the tautomer fluorescence with femtosecond time resolution (15).A fundamental question relevant to this ultrafast double proton transfer is the cooperativity of the motion of the two protons. So far, it has been intensively argued whether the two protons move in a concerted or step-wise way. The head-on arguments on this mechanism were first made on the basis of time-resolved data for th...

show abstract

“…Since then, numerous studies have focused on the mechanisms of ESDPT in 7AI 2 in solution and matrix [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and in the gas phase [35][36][37][38][39][40][41][42][43][44][45][46][47] by using various spectroscopic techniques. Theoretical studies have been also carried out to investigate the hydrogen-bonded structures, potential energy curves (or surface), and molecular dynamics of 7AI 2 .…”

mentioning

confidence: 99%

Excited-State Double-Proton Transfer in the 7-Azaindole Dimer in the Gas Phase. Resolution of the Stepwise versus Concerted Mechanism Controversy and a New Paradigm

Sekiya¹,

Sakota²

2006

Bulletin of the Chemical Society of Japan

View full text Add to dashboard Cite

The 7-azaindole dimer (7AI 2 ) is a prototype of double hydrogen-bonded molecules. 7AI 2 has been considered as a model DNA base pair and has attracted much attention to the mechanism of the excited-state double-proton transfer (ESDPT). Two ESDPT mechanisms, stepwise and concerted mechanisms, have been proposed so far. Great efforts have been devoted to clarify the mechanism of ESDPT using experimental and theoretical methods. However, the reaction mechanism had been controversial for more than a decade. We provide the resolution of the two mechanisms on the basis of new data obtained from electronic spectroscopy and picosecond time-resolved spectroscopy in the gas phase. The initial state of the ESDPT reaction has been well characterized by investigating the exciton resonance interaction with UV-UV hole-burning spectroscopy for various 7AI 2 isotopomers. The lowest-excited state of 7AI 2 has been classified into the weak coupling case of the exciton theory. We have concluded that the ESDPT reaction in 7AI 2 occurs via the concerted mechanism on the basis of the results of picosecond time-resolved experiments and the H/D kinetic isotope effect on ESDPT studied by measuring the vibronic-state selective dispersed fluorescence spectra. ESDPT of 7AI 2 has a ''dynamic cooperative'' nature that may arise from the coupling of the two moving protons with the reorganization of electrons. We have provided a new paradigm of ESDPT, where two quantum effects, the exciton resonance interaction and the proton tunneling, are concerned with the ESDPT reaction.Proton-transfer reactions are one of the most basic reactions, and they are important in physics, chemistry, and biology. [1][2][3][4][5][6] Spectroscopic studies on various proton-transfer systems combining with theoretical studies have provided rich information about the hydrogen-bonded structure and proton-transfer dynamics. In particular, a very detailed picture has been obtained for intramolecular single-proton transfer in polyatomic molecules, with laser spectroscopic measurements in the gas phase combining with theoretical studies. For example, a proton transfers by quantum mechanical tunneling mechanism when a potential energy barrier exists on a potential energy surface. The proton tunneling was clearly observed in the electronic and vibrational spectra of several polyatomic molecules. [6][7][8][9][10] Spectroscopic data and theoretical studies showed that the proton tunneling has a multi-dimensional nature, i.e., the motion of the moving proton couples with the coordinates of the heavy atoms that constitute the molecule. Such a multi-dimensional nature and quantum mechanical tunneling are important properties that characterize proton-transfer reactions. Now much effort is devoted to obtain a more complete picture for singleproton transfer reactions and also a new picture for more complicated multiproton-transfer reactions. There are many examples of multiproton-transfer reactions such as proton relay systems in enzymes, in hydrogen-bonded water complexes, and prot...

show abstract

Picosecond dynamics of double proton transfer in 7-azaindole dimers

Cited by 117 publications

References 8 publications

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

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

The answer to concerted versus step-wise controversy for the double proton transfer mechanism of 7-azaindole dimer in solution

Excited-State Double-Proton Transfer in the 7-Azaindole Dimer in the Gas Phase. Resolution of the Stepwise versus Concerted Mechanism Controversy and a New Paradigm

Contact Info

Product

Resources

About