New interpretation of ground- and excited-state tunneling splitting in 2-pyridone·2-hydroxypyridine

Reactive force fields for proton transfer dynamics

A force field-inspired method based on fitted, high-quality multidimensional potential energy surfaces to follow proton transfer (PT) reactions in molecular dynamics simulations is presented. In molecular mechanics with proton transfer (MMPT) a system is partitioned into a region where proton transfer takes place and the remaining degrees of freedom which are treated with a conventional force field. The implementation of the method and applications to specific chemically and biologically relevant scenarios are presented. MMPT is developed in view of two primary areas in mind: to follow the molecular dynamics of proton transfer in the condensed phase on realistic time scales and to adapt the shape (morphing) of the potential energy surface for specific applications. MMPT is applied to PT in protonated ammonia dimer, double proton transfer in 2-pyridone-2-hydroxypyridine, and the first step of PT from a protein side-chain towards a buried [3Fe4S] cluster in ferredoxin I. Specific findings of the work include the fundamental role of the N-N vibration as the gating mode for PT in NH4+...NH3 and the qualitative understanding of PT from the protein to a metastable active-site water molecule in Ferredoxin I.

Theoretical studies of proton-transfer reactions of 2-hydroxypyridine--(H2O)n (n = 0-2) in the ground and excited states

The potential energy profiles for proton-transfer reactions of 2-hydroxypyridine and its complexes with water were determined by MP2, CASSCF and MR-CI calculations with the 6-31G** basis set. The tautomerization reaction between 2-hydroxypyridine (2HP) and 2-pyridone (2PY) does not take place at room temperature because of a barrier of approximately 35 kcal/mol for the ground-state pathway. The water-catalyzed enol-keto tautomerization reactions in the ground state proceed easily through the concerted proton transfer, especially for the two-water complex. The S1 tautomerization between the 2HP and 2PY monomers has a barrier of 18.4 kcal/mol, which is reduced to 5.6 kcal/mol for the one-water complex and 6.4 kcal/mol for the two-water complex. The results reported here predict that the photoinduced tautomerization reaction between the enol and keto forms involves a cyclic transition state having one or two water molecules as a bridge.

Detection and automatic repair of nucleotide base-pair mutations by coherent light

We show that phase-coherent optical techniques allow for the detection and automatic repair of mutations in nucleotide pairs. We demonstrate computationally that there is a laser pulse sequence that can detect the occurrence of a mutation caused by a double proton transfer between hydrogen-bonded nucleotide pairs and automatically repair it by converting the mutated nucleotide pair to the nonmutated one. The specific system chosen for this demonstration is the hydrogen-bonded 2-pyridone.2-hydroxypyridine dimer at typical internucleotide distances, a well-established model for tautomeric acid base pairs.

Proton transfer dynamics via high resolution spectroscopy in the gas phase and instanton calculations

Tunneling splittings have been observed in the eigenstate-resolved electronic spectrum of the 2-hydroxypyridine/2-pyridone dimer in the gas phase. Deuterium substitution experiments show that these splittings are caused by a concerted double proton transfer reaction along the O-H...O and N...H-N hydrogen bonds that hold the dimer together, substitution of the weaker and longer N...H-N bond having the larger effect. Tunneling splittings calculated by the instanton method for the zero-point level of the ground state are in good agreement with experiment for all observed isotopomers, showing that the dynamics occurs in this state, rather than in the electronically excited state.

Double Proton Transfer using Dissociable Force Fields

The construction, implementation, and use of dissociable classical force fields are discussed. Starting from zeroth-order interaction potentials for O2H5+ and N2H7+ calculated with MP2/6–311++G**, energy scaling of the potential energy surfaces allows adjustment of quantities such as the barrier heights to describe a range of physical situations observed in realistic systems. As an example, ‘potential morphing’ is used to investigate the dynamics of double proton transfer in 2-pyridone · 2-hydroxypyridine for which previous estimates of the barrier to tautomerization are available. Scaling factors to give barrier heights for double proton transfer between 3.6 and 17.6 kcal mol−1 are chosen to demonstrate the utility of the method to describe a range of different barrier heights and shapes. Considerable savings in computing time can be achieved compared to alternative methods such as mixed quantum/classical methods.