DFT analysis of nitrogen isotopic reduction partition function ratios in proton exchange equilibria of glutamic acid species in water and its application to the estimation of nitrogen isotope fractionation

Abstract

L-Glutamic acid (Glu) plays a pivotal role in amino acid metabolism in living organisms. Theoretical knowledge of how metabolism propagates the nitrogen isotope composition (δ¹⁵N) is essential for elucidating the mechanism of amino acid metabolism in vivo. In this paper, to estimate the nitrogen isotope fractionation involving Glu and its protonated/deprotonated species, their nitrogen isotopic reduction partition function ratios (RPFRs) were calculated at the apfd/6-311+G(2d,p) level of theory by the SCRF method. The results show that the RPFR values of the eight possible Glu species at 25 °C are: cation > zwitterion with α-COOH and γ-COO⁻ > zwitterion with α-COO⁻ and γ-COOH > anion with –NH₃⁺ ≫ uncharged molecule > anion with –NH₂ and γ-COO⁻ > anion with –NH₂ and α-COO⁻ > di-anion. Several correlations between RPFR and bond distances were found. Most importantly, it was found that the shorter the distance of the (C–)N⋯H(–O) hydrogen bond (HB), the greater the RPFR value for that species. This correlation indicates that the tri-coordinate nitrogen atom of the amino group may take on the character of the tetra-coordinate nitrogen atom to some extent by forming a HB of this type. The nitrogen isotope exchange equilibrium between Glu and glycine/serine was evaluated at a typical physiological pH of 7.4. Calculations suggest that the equilibrium constant for the former is an increasing function of temperature from 0 to 100 °C, while the latter is a decreasing function of temperature over the same temperature range.