Femtosecond cluster studies of the solvated 7-azaindole excited state double-proton transfer

Abstract

Presented here are femtosecond pump-probe studies on the watersolvated 7-azaindole dimer, a model DNA base pair. In particular, studies are presented that further elucidate the nature of the reactive and nonreactive dimers and also provide new insights establishing that the excited state double-proton transfer in the dimer occurs in a stepwise rather than a concerted manner. A major question addressed is whether the incorporation of a water molecule with the dimer results in the formation of species that are unable to undergo excited state double-proton transfer, as suggested by a recent study reported in the literature [Nakajima, A., Hirano, M., Hasumi, R., Kaya, K., Watanabe, H., Carter, C. C., Williamson, J. M. & Miller, T. (1997) J. Phys. Chem. 101, 392-398]. In contrast to this earlier work, our present findings reveal that both reactive and nonreactive dimers can coexist in the molecular beam under the same experimental conditions and definitively show that the clustering of water does not induce the formation of the nonreactive dimer. Rather, when present with a species already determined to be a nonreactive dimer, the addition of water can actually facilitate the occurrence of the proton transfer reaction. Furthermore, on attaining a critical hydration number, the data for the nonreactive dimer suggest a solvation-induced conformational structure change leading to proton transfer on the photoexcited half of the 7-azaindole dimer. C luster science studies have long been utilized to yield unique perspectives of microscopic properties related to the bulk condensed phase (1). This approach involves the investigation of a broad spectrum of clusters ranging from isolated species to the study of fully solvated species, thus illustrating the progression from the gas phase to the condensed phase. In reference to our specific experiments as an example, we have been able to study the excited state double-proton transfer (ESDPT) of the 7-azaindole (7-Aza) dimer under conditions ranging from an isolated dimer to a state of solvation where the hydrogen-bonded dimer is clustered with as many as nine water molecules. As the number of water molecules on the nonreactive dimer increases, we begin to see evidence that the dimer molecule is behaving more as it would in a fully solvated condensed-phase environment. In support of these findings, it has been reported that clusters with as little as six waters begin to show liquid-like properties (2-4).The model DNA base pair 7-Aza has proven to be an interesting and enlightening species for study in both the gas and condensed phases. Of utmost interest is the double-proton transfer that the 7-Aza dimer undergoes on excitation to the S 1 state. This ESDPT was first observed in solution by Kasha and coworkers (5). Later, Kaya and coworkers performed extensive supersonic jet spectroscopic studies on the 7-Aza monomer, dimer, and solvated forms of these species (6-9). The first direct determination of the rates of the double-proton transfer was made in the gas phase by Zewail and c...

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: Structure, quantum-theoretical analysis, and simultaneity principles

“…These results are consistent with our spectroscopic experimental observation (2). It is interesting to note at this point that, in free-jet experiments such as those of Castleman et al (7,8), who heated the sample reservoir to 393 K, the dimerization constant is only 6 M Ϫ1 ; this result suggests that almost all of the 7AI emerging from the nozzle must be the monomeric species.…”

“…17). Both Zewail's group (4-6) and Castleman's group (7,8) use a femtosecond laser pulse at 305-310 nm, bracketing (32,258 cm Ϫ1 ) exactly the dipoleallowed molecular exciton band at 32,290 cm Ϫ1 , yet they do not recognize the simultaneity of the two-molecule excitation implied by that frequency in their mechanism of ''single-step'' hydrogen transfer. Second, the molecular orbital changes (HOMO͞LUMO; highest occupied molecular orbital͞lowest unoccupied molecular orbital) upon excitation at the pyridino-N site and at the pyrrolo-N site (3) offer dramatic indication of the simultaneity (concertedness) of the biprotonic transfer driving force in the dimer.…”

“…Under the conditions of the adiabatic nozzle experiment we could expect a considerable variety of transient card-pack and other dimer structures in these ultrarapid transit molecular beams at very low temperatures. If the variety of possible transient dimer structures is considered, we could hope to find a clue to the differential isotope-kinetic results that have been recorded (4)(5)(6)(7)(8).…”

“…Electronic spectroscopic observations and quantum theoretical calculations require a concerted (3) one-step simultaneous intermolecular transfer of the two protons involved in the hydrogen bonding of the 7AI base pair. In contrast, femtosecond laser-pulsed photo-excitation followed by femtosecond laserpulsed ionization and then time-of-flight (TOF) mass spectrometry (4-6), or with giant laser pulse-induced coulomb explosion (7,8) followed by TOF mass spectrometry of the resultant ions (and molecular fragments), has suggested to these researchers a two-step neutral H-atom transfer, based on kinetic analysis of the ion appearance and isotopic effects. We shall return to the question of neutral H-atom transfer vs. H ϩ (proton) transfer at the end of the companion paper (9).…”

Dynamics of clusters: From elementary to biological structures

“…The findings were supported by ab initio and MD calculations in other laboratories. Independently, the Castleman group, by using the Coulomb explosion technique, has arrested the intermediate (61) and shown that its rise is similar to our time constants, providing further evidence of the two-step mechanism. M. Kasha believes in the concerted mechanism, but as we have discussed elsewhere (62)(63)(64), for both elementary systems and those in the condensed phase, trajectories of an exact concerted motion are in the minority of dynamics on multidimensional surfaces.…”