Non-ionic and zwitterionic forms of neutral arginine – an ab initio study

Abstract

Six low-energy structures of arginine were studied at the zero-point corrected CCSD/6-31++G(d,p)+5(sp)//MP2/6-31++G(d,p)+5(sp) level. Two new non-ionic structures were identified, one of which is 1.75 kcal/mol lower than any previously reported structure. Two new zwitterion conformers are lower in energy than any previously reported zwitterion. The lowest non-ionic structure is lower in energy than the lowest zwitterion by 2.8 kcal/mol at our highest level of theory, and for no basis or theory level is a zwitterion structure suggested to be the global minimum. Finally, we also examined, at Koopmans' theorem level, the electron binding energies of the six structures.

Introduction

It is well known that, in aqueous solutions at pH=7, aminoacids exist primarily in their zwitterionic forms with the carboxyl group deprotonated and one of the nitrogen atoms protonated. In contrast, the zwitterionic forms are usually higher in energy in the gas phase than the corresponding non-zwitterion H₂N–CHR–COOH tautomers. Moreover, for some aminoacids (e.g., glycine), the zwitterionic structure does not even correspond to a local minimum on the gas-phase potential energy surface [1], [2]. There have been attempts to identify a zwitterionic tautomer of a neutral aminoacid which is globally stable in the gas phase [3] but these findings were inconclusive [4]. Other recent efforts concentrated on hydrated [5], [6], protonated [3], [7], [8], and alkali cationized [7], [8], [9] aminoacids.

We have recently suggested stabilizing the zwitterion form of an aminoacid in the gas phase with an excess electron [10] and it was via this route that we began our exploration of various arginine isomer energetics. Our reasoning is that the zwitterion form of an aminoacid would possess a larger dipole moment than the non-zwitterion form. It is well established that a molecule with a dipole moment larger than ca. 2.5 D binds an excess electron [11] with an electron binding energy roughly correlated with the magnitude of the dipole moment [12]. Therefore, we anticipated the instability of the zwitterion relative to the non-zwitterion structure might be reversed by the excess electron binding energy. It is our plan to report on our arginine anion findings in a future publication. However, because there is strong current interest in arginine itself, we decided to put forth our findings on the neutral species at this time.

In a recent study, we demonstrated that the instability of the zwitterion structure of glycine is significantly reduced by the attachment of an excess electron as a result of which a local minimum develops on the anionic potential energy surface [10]. However, its energy is still higher than that of the anion based on the non-zwitterion isomer of glycine. This outcome may be related to the fact that the proton affinity of glycine of 211.9 kcal/mol is the smallest among all 20 common naturally occurring amino acids [7]. The largest proton affinity of 251.2 kcal/mol is displayed by arginine [7], which possesses an extremely basic guanidine group. Therefore, we selected arginine for our study on stabilization through electron binding which leads to the results presented here.

The question of which tautomeric form of neutral arginine is dominant in the gas phase has recently been addressed in experimental studies [3], [4], [7], [9], [13], [14]. Williams and co-workers [3] concluded, on the basis of black body infrared radiative dissociation plus Fourier transform-mass spectrometry measurements, that protonated dimers of arginine are bound in a salt-bridge. Moreover, the results of their extensive computational study at the BLYP/6-31$\text{G}{}^{\mathrm{\ast }}$ and MP2/6-31$\text{G}{}^{\mathrm{\ast }}$ levels suggested that a zwitterion form of arginine is the global minimum on the potential energy surface, lower by 1 kcal/mol than the lowest non-ionic tautomer. Saykally and co-workers [4], however, did not confirm the dominance of the zwitterion of arginine in their infrared cavity ringdown laser absorption spectroscopy experiments. The observed band at ca. 1700 cm⁻¹, which is associated with the carbonyl stretch mode of a carboxylic acid, implied the presence of a non-ionic structure in their gas-phase sample. The absence of bands in the 1500–1660 cm⁻¹ region, which are associated with the carboxylate stretch modes, suggested a small or vanishing population of the zwitterion. It was pointed out, however, that there may be a significant barrier that separates the neutral and zwitterion forms of arginine and the thermodynamically unstable form may have a sufficiently long lifetime to be observed experimentally. Moreover, the sources of arginine used in the Williams and Saykally experiments are different. One employes solution electro spray at 37–149°C; the other employes a heated pulsed beam source of pure arginine at 170°C.

The geometrical shapes of the protonated, sodiated, and cesiated arginine were probed in the gas phase by using the ion mobility based ion chromatography method [7], [9]. Unfortunately, the qualitative structure of the protonated arginine could not be unambiguously determined from these experiments. It has been suggested that the alkali cationized arginine forms a salt bridge structure, related to the zwitterion form of this aminoacid. Results from the collisionally activated dissociation experiments of Williams et al. [13] and the kinetic experiments of Cerda and Wesdemiotis [14] indicated, however, that the structure of gas-phase arginine–alkali metal cation complexes depends on the size of the alkali metal cation. For Li⁺ and Na⁺, the non-zwitterion arginine solvates the metal ion. For the larger metal ions, a salt bridge is formed in which the arginine exists as a zwitterion.

Maksic and Kovacevic (MK), inspired by the variety of intriguing experimental findings, performed an extensive computational investigation in order to find the global minimum for neutral arginine [15]. They concluded from their thorough MP2 and B3LYP calculations using several basis sets, that the most stable structure is a non-ionic. These authors admitted, however, that the energy difference between the lowest non-ionic and a pair of low lying zwitterion structures is relatively small (within 1–3 kcal/mol depending on the theoretical model applied) [15].

As detailed above, our long-term goal is to determine whether an excess electron can stabilize a zwitterion structure of arginine. This goal requires extensive knowledge about the potential energy surface for the neutral species, and in this contribution we characterize what we have found to be the most promising neutral non-ionic and zwitterionic structures. In addition to the structures characterized so far [3], [15], we identified two non-zwitterion and two zwitterion structures that are promising candidates. Their relative energies have been determined at the MP2, B3LYP, and CCSD levels and their dipole moments have been calculated at the MP2 and B3LYP levels. These dipole moments will guide us as to the ability to bind an excess electron. With this goal in mind, the vertical electron attachment energies are determined at Koopmans' theorem (KT) level. An unexpected outcome of the current study on the six low energy tautomers of arginine is that the largest dipole moment of ca. 9 D is displayed by the lowest energy zwitterion's structure and by one of the non-ionic structures. However, their vertical electron detachment energies, determined at the KT level, differ considerably.