Title

Intramolecular charge-assisted hydrogen bond strength in pseudochair carboxyphosphate

Citation for published article

Kochanek, S. E., Clymer, T. M., Pakkala, V. S., Hebert, S. P., Reeping, K., Firestine, S. M., & Evanseck, J. D. (2014). Intramolecular Charge-Assisted Hydrogen Bond Strength in Pseudochair Carboxyphosphate. The Journal of Physical Chemistry B, 119(3), 1184–1191. https://doi.org/10.1021/jp506796r

DOI

10.1021/jp506796r

Document Type

Journal Article

School

Bayer School of Natural and Environmental Sciences

Abstract

© 2014 American Chemical Society. Carboxyphosphate, a suspected intermediate in ATP-dependent carboxylases, has not been isolated nor observed directly by experiment. Consequently, little is known concerning its structure, stability, and ionization state. Recently, carboxyphosphate as either a monoanion or dianion has been shown computationally to adopt a novel pseudochair conformation featuring an intramolecular charge-assisted hydrogen bond (CAHB). In this work, additive and subtractive correction schemes to the commonly employed open-closed method are used to estimate the strength of the CAHB. Truhlars Minnesota M06-2X functional with Dunnings aug-cc-pVTZ basis set has been used for geometry optimization, energy evaluation, and frequency analysis. The CHARMM force field has been used to approximate the Pauli repulsive terms in the closed and open forms of carboxyphosphate. From our additive correction scheme, differential Pauli repulsion contributions between the pseudochair (closed) and open conformations of carboxyphosphate are found to be significant in determining the CAHB strength. The additive correction modifies the CAHB prediction (ΔEclosed-open) of -14 kcal/mol for the monoanion and -12 kcal/mol for the dianion to -22.9 and -18.4 kcal/mol, respectively. Results from the subtractive technique reinforce those from our additive procedure, where the predicted CAHB strength ranges from -17.8 to -25.4 kcal/mol for the monoanion and from -15.7 to -20.9 kcal/mol for the dianion. Ultimately, we find that the CAHB in carboxyphosphate meets the criteria for short-strong hydrogen bonds. However, carboxyphosphate has a unique energy profile that does not result in the symmetric double-well behavior of low-barrier hydrogen bonds. These findings provide deeper insight into the pseudochair conformation of carboxyphosphate, and lead to an improved mechanistic understanding of this intermediate in ATP-dependent carboxylases.

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