Ionisation and (de-)protonation energies of gas-phase amino acids from an optimally tuned range-separated hybrid functional

ABSTRACT

We provide a quantitative examination of the ionisation potential, protonation, and de-protonation energies of a set of 22 different amino acids. Specifically, we compare results obtained using the conventional hybrid functionals B3LYP and BHLYP (which use 20% and 50% of Fock exchange, respectively) with those obtained from the recently developed optimally tuned range-separated hybrid (OT-RSH) functional approach, as well as to literature coupled-cluster calculations with single and double excitations (CCSD) data. We find the OT-RSH results to be quantitatively close to those of the BHLYP functional and a significant improvement over those of B3LYP, with respect to the CCSD data. We conclude that the results of the OT-RSH for these quantities are as reliable and inexpensive as those of BHLYP, but possess one distinct and important advantage: they overcome the empiricism and limited predictive power associated with the arbitrary choice of the amount of Fock exchange in the BHLYP. We further discuss quantitative and qualitative trends in the optimal-tuning procedure of the amino acids studied.

GRAPHICAL ABSTRACT

KEYWORDS:

• Amino acids
• density-functional theory
• deprotonation energy
• ionisation potential
• protonation energy
• range-separated hybrid

Acknowledgments

This article is dedicated to Prof. Andreas Savin, on the occasion of his 65th birthday. We thank Prof. Nurit Ashkenasy (Ben Gurion University) for motivating this study, Prof. Zijing Lin for providing valuable CCSD data, and Dr Sivan Refaely-Abramson and Dr David Egger for helpful discussions. This work was supported by the European Research Council and the Lise Meitner Minerva Center for Computational Chemistry.

Disclosure statement

No potential conflict of interest was reported by the authors.