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Conspectus: The role of quantum mechanical coherences or coherent superposition states in excited state processes has received considerable attention in the last two decades largely due to advancements in ultrafast laser spectroscopy. These coherence effects hold promise for enhancing the efficiency and robustness of functionally relevant processes, even when confronted with strong energy disorder and environmental fluctuations. Understanding coherence deeply drives us to unravel mechanisms and dynamics controlled by order and synchronization at a quantum mechanical level, envisioning optical control of coherence to enhance functions or create new ones in molecular and material systems. In this frontier, the interplay between electronic and vibrational dynamics, specifically the influence of vibrations in directing electronic dynamics, has emerged as the leading principle. Here, two energetically disparate quantum degrees of freedom work in-sync to dictate the trajectory of an excited state reaction. Moreover, with the vibrational degree being directly related to the structural composition of molecular or material systems, new molecular designs could be inspired by tailoring certain structural elements. In the realm of chemical kinetics, our understanding of the dynamics of chemical transformations is underpinned by fundamental theories such as transition state theory, activated rate theory, and Marcus theory. These theories elucidate reaction rates by considering the energy barriers that must be overcome for reactants to transform into products. Those barriers are surmounted by the stochastic nature of energy gap fluctuations within reacting systems, emphasizing that the reaction coordinate—the pathway from reactants to products—is not rigidly defined by a specific vibrational motion but encompasses a diverse array of molecular motions. While less is known about the involvement of specific intramolecular vibrational modes, their significance in certain cases cannot be overlooked. In this Account, we summarize key experimental findings that offer deeper insights into the complex electronic-vibrational trajectories encompassing excited states afforded from state-of-the-art ultrafast laser spectroscopy in three exemplary processes: photo-induced electron transfer, singlet-triplet intersystem crossing, and intramolecular vibrational energy flow in molecular systems. We delve into rapid decoherence—loss of phase and amplitude correlations—of vibrational coherences along promoter vibrations during a sub-picosecond intersystem crossing dynamics in a series of binuclear platinum complexes. This rapid decoherence illustrates the vibration-driven reactive pathways from Franck-Condon state to the curve crossing region. We also explore the generation of new vibrational coherences induced by impulsive reaction dynamics—rather than by the laser pulse—in these systems, which sheds light on specific energy dissipation pathways and thereby on the progression of the reaction trajectory in the vicinity of the curve crossing on the product side. Another property of vibrational coherences, amplitude, reveals how energy can flow from one vibration to another in the electronic excited state of a terpyridine-molybdenum complex hosting a nonreactive dinitrogen substrate. A slight change in vibrational energy triggers a quasi-resonant interaction, leading to constructive wavepacket interference and ultimately intramolecular vibrational redistribution from a Franck-Condon active terpyridine vibration to dinitrogen stretching vibration, energizing the dinitrogen bond.
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This content will become publicly available on October 2, 2026
The Shaping of Enzymatic Free Energy Barriers through the Creation of Rate-Promoting Vibrations via Inter-Residue Cross-Talk on Multiple Time Scales
Years ago, we identified a rapid vibrational motion termed a rate-promoting vibration (RPV) as central to the reaction coordinate of some enzymes. This study addresses two key questions for one example enzyme we have studied, lactate dehydrogenase (LDH): first, what is a lower bound on the RPV’s contribution to catalytic efficiency, and second, what is the mechanism of RPV formation via allosteric transmission. The goal is to understand how we can artificially create such a system. LDH catalyzes the interconversion of pyruvate and lactate via hydride and proton transfer. Altering the motion range between Val31 and Arg106, central residues in the promoting vibration, with a modest constraint reduces the reaction rate (through a raising of the free energy barrier) by over 3 orders of magnitude. Committor analysis shows that shorter distances in the constrained system shift the transition state toward proton transfer, while natural or longer distances favor a transition state formed in hydride transfer. PCA confirms the anticorrelated motion between Val31 and Arg106, aligning with vibrational modes to optimize the reaction path. Critically, we find that a breathing motion among alpha helices is used to create the necessary distance over which a rapid and short RPV can be effective. Network analysis reveals that Val31, Ala34, and Cys35 have higher eigenvector centrality in reactive trajectories, indicating enhanced inter-residue communication. These findings underscore RPVs as crucial modulators of enzymatic function through dynamic and allosteric mechanisms and suggest an approach to the generation of nonbiologic protein catalysts that include a promoting vibration.
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- Award ID(s):
- 2244981
- PAR ID:
- 10651537
- Publisher / Repository:
- ACS JPC B
- Date Published:
- Journal Name:
- The Journal of Physical Chemistry B
- Volume:
- 129
- Issue:
- 39
- ISSN:
- 1520-6106
- Page Range / eLocation ID:
- 9973 to 9982
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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