Asymmetric Double Proton Transfer of Excited 1:1 7‐Azaindole/Alcohol Complexes with Anomalously Large and Temperature‐Independent Kinetic Isotope Effects

Kwon, Oh‐Hoon; Lee, Young-Shin; Park, Hae‐Sim; Kim, Yong-Ho; Jang, Dai-Ja

doi:10.1002/anie.200461102

Cited by 84 publications

(88 citation statements)

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“…Electronic excitation reduces pK a , reshapes the energy surface, and lowers the tautomerization barrier, thus leading to much faster proton transfer rates than in the ground state (15). For example, photo-initiated ESPT in 7-azaindole/alcohol complexes occurs within ∼100 ps with negligible barrier (<1 kcal/mol) (17), and the double-proton transfer in 7-azaindole dimer proceeds with a 1.1-ps time constant (18).…”

Section: Resultsmentioning

confidence: 99%

“…However, they still suffer from certain limitations, including the difficulty of interpreting variations in electronic spectra, the inclusion of pH indicators, and questions about reactive species that may be created by the capacitive discharge. The more recent studies to capture proton transfer dynamics have used ultrafast photo-excitation to induce tautomerization on the excited electronic state (15)(16)(17)(18)(19)(20). This has proven a fruitful test bed for dynamics, but experiments in roomtemperature aqueous solution would more accurately report on physiological tautomerism.…”

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confidence: 99%

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Direct observation of ground-state lactam–lactim tautomerization using temperature-jump transient 2D IR spectroscopy

Peng

Baiz

Tokmakoff

2013

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

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We provide a systematic characterization of the nanosecond groundstate lactam-lactim tautomerization of pyridone derivatives in aqueous solution under ambient conditions using temperature-jump transient 2D IR spectroscopy. Although electronic excited-state tautomerization has been widely studied, experimental work on the ground electronic state, most relevant to chemistry and biology, is lacking. Using 2D IR spectroscopy, lactam and lactim tautomers of 6-chloro-2-pyridone and 2-chloro-4-pyridone are unambiguously identified by their unique cross-peak patterns. Monitoring the correlated exponential relaxation of these signals in response to a laser temperature jump provides a direct measurement of the nanosecond tautomerization kinetics. By studying the temperature, concentration, solvent, and pH dependence, we extract a thermodynamic and kinetic characterization and conclude that the tautomerization proceeds through a two-state concerted mechanism. We find that the intramolecular proton transfer is mediated by bridging water molecules and the reaction barrier is dictated by the release of a proton from pyridone, as would be expected for an efficient Grothusstype proton transfer mechanism.keto-enol tautomerism | multidimensional | time-resolved spectroscopy | ultrafast T automerism of aromatic heterocycles has been extensively studied owing to its importance in biochemical processes such as enzyme catalysis (1), ligand binding (2), and spontaneous mutagenesis (3). To characterize the tautomeric equilibria, researchers have used techniques such as UV absorption (4), circular dichroism (5), X-ray crystallography (3), NMR (6), Raman (7), and IR absorption spectroscopy (8). However, these methods face several challenges to characterizing thermodynamic and kinetic data for tautomeric equilibria and exchange processes. For example, electronic spectra are broad and featureless, which complicates spectral interpretation, particularly for systems with multiple tautomers. Although NMR provides excellent structural resolution, it can only directly monitor chemical exchange in real time on millisecond and longer time scales, rather than the picosecond to nanosecond time scales expected for proton transfer under physiological conditions. From the theoretical point of view, quantum mechanical calculations have been performed extensively to characterize the relative stability of tautomeric systems, their activation barriers, and the reaction mechanisms (9, 10); nevertheless, experimental validation of these predictions is scarce. To provide a characterization under ambient aqueous conditions, we need a technique with both the structural sensitivity to unambiguously identify various tautomers and the time resolution to probe the rapid proton transfer dynamics. To this end, we demonstrate the capability of 2D IR spectroscopy coupled with a nanosecond temperature-jump (Tjump) laser to reveal the nonequilibrium lactam-lactim tautomerization kinetics of pyridone derivatives.Time-resolved studies of tautomerism have a long and storie...

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Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

Direct observation of ground-state lactam–lactim tautomerization using temperature-jump transient 2D IR spectroscopy

Peng

Baiz

Tokmakoff

2013

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

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“…The decay of the dimers with one N 1 H moiety, i.e., the two species of HH and HD (see Fig. 1), can be expressed (27,28) …”

Section: Resultsmentioning

confidence: 99%

“…From the results in Fig. 6 (20,28,36). Given that the first step is on the femtosecond time scale, one would like to know the KIE for the second proton transfer.…”

Section: Discussionmentioning

confidence: 99%

Double proton transfer dynamics of model DNA base pairs in the condensed phase

Kwon

Zewail

2007

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

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159

149

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The dynamics of excited-state double proton transfer of model DNA base pairs, 7-azaindole dimers, is reported using femtosecond fluorescence spectroscopy. To elucidate the nature of the transfer in the condensed phase, here we examine variation of solvent polarity and viscosity, solute concentration, and isotopic fractionation. The rate of proton transfer is found to be significantly dependent on polarity and on the isotopic composition in the pair. Consistent with a stepwise mechanism, the results support the presence of an ionic intermediate species which forms on the femtosecond time scale and decays to the final tautomeric form on the picosecond time scale. We discuss the results in relation to the molecular motions involved and comment on recent claims of concerted transfer in the condensed phase. The nonconcerted mechanism is in agreement with previous isolated-molecule femtosecond dynamics and is also consistent with the most-recent high-level theoretical study on the same pair.7-azaindole ͉ femtochemistry ͉ reaction dynamics ͉ tautomerization S ince the work by Watson and Crick on the determination of DNA structure in 1953 (1), proton-translocating tautomerization of base pairs has been suggested as a cause of mutations (2). Model systems with similar pair structures can be studied by photoinducing proton transfer (3, 4), and such studies are helpful for understanding the dynamics of mutagenesis (5).In this regard, the dimers of 7-azaindole (7-AI) molecules are prototypical because they are structurally similar to the H-bonded adenine-thymine (A-T) pair with two bonds; the guanine-cytosine (G-C) base pair has three H-bonds. Following the first observation of excited-state double proton transfer in 7-AI by Taylor et al. in 1969 (6), a number of researchers have studied its characterization in the dimers and also of 7-AI monomers catalyzed by various H-bonded counterparts of protic guest molecules (ref. 7 and references therein). One of the fundamental issues, with important implications, is whether the transfer process (8-17) of the two hydrogens proceeds in a concerted manner with a single transition state, or through a stepwise pathway by forming an intermediate species, as shown in Fig. 1.The first report of the dynamics of double proton transfer in 7-AI in the isolated molecule (molecular beam) was published in 1995 (8). The pair was excited with a femtosecond pulse, and another pulse, at different time delays, was invoked to observe the mass spectra. The parent ion showed a biexponential behavior, even near the zero-point energy, and the time constants of both decays changed significantly, by a factor of more than eight, with isotopic substitution. These two time constants (650 fs and 3.3 ps) were found to decrease with the excess excitation energy, as more vibrational modes (18) The mechanism has been discussed in a number of theoretical papers. Ab initio configuration interaction singles calculations showed that the electronic excitation was localized on one moiety in the 7-AI dimer, making the t...

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“…l max $ 480 nm in cyclohexane) [16]. Over the past three decades, spectroscopic measurements and theoretical approaches have also revealed that the intrinsic structure of 7AI facilitates excited-state proton transfer via either solvent catalysis [17][18][19][20][21][22][23][24] or self-dimerization .…”

Section: Esdpt Dynamics In a 7ai Dimer In Condensed Phasementioning

confidence: 99%

Excited‐State Proton Transfer via Hydrogen‐Bonded Dimers and Complexes in Condensed Phase

Hsieh

Jiang

Chou

2010

Hydrogen Bonding and Transfer in the Excited State

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Proton transfer represents one of the most fundamental processes involved in chemical reactions as well as in living systems [1]. Vast numbers of scientists have been participating in relevant research, leading to thousands upon thousands of publications and numerous books on the topic. Accordingly, various types of proton transfer reaction, depending on, for example, the reaction in the ground or excited state, adiabatic versus non-adiabatic, strong and weak hydrogen bonding, acidity (proton donor) and basicity (proton acceptor), have been identified [2][3][4][5][6][7][8]. At this time, proton transfer reactions seem to have the potential for unlimited extensions and aspects in terms of fundamentals and applications. Thus, to cover the broad spectrum of proton transfer research within a limited number of pages seems to be an unachievable task. Rather than trying to achieve the impossible, this chapter focuses mainly on areas related to excited-state proton transfer (ESPT) [9] with pre-existing intramolecular hydrogen bonds. Under this constraint, the photoinduced process involving excited-state proton dissociation and/or protonation in bulk solvents or clusters, also a currently active topic that has received considerable attention [10][11][12][13][14], unfortunately cannot be addressed in this chapter.One particular focus of this chapter is bifunctional molecules possessing a proton donor group and a proton acceptor group, such that ESPT takes place via self-associated hydrogen-bonded (HB) dimers and/or solvent bridge relay. In spite of numerous explorations and elegant research on guest-molecule-assisted ESPT reactions, the results of which have provided great insight into the fundamentals as well as perspectives in applications,

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Asymmetric Double Proton Transfer of Excited 1:1 7‐Azaindole/Alcohol Complexes with Anomalously Large and Temperature‐Independent Kinetic Isotope Effects

Cited by 84 publications

References 45 publications

Direct observation of ground-state lactam–lactim tautomerization using temperature-jump transient 2D IR spectroscopy

Direct observation of ground-state lactam–lactim tautomerization using temperature-jump transient 2D IR spectroscopy

Double proton transfer dynamics of model DNA base pairs in the condensed phase

Excited‐State Proton Transfer via Hydrogen‐Bonded Dimers and Complexes in Condensed Phase

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