DFT-SCRF Calculations\nof Nitrogen Isotopic Reduced\nPartition Function Ratios for Amino Acids in Aqueous Solution at pH\n7.4

Using the self-consistent reaction field (SCRF) method,\nthe nitrogen\nisotopic reduction partition function ratio (RPFR), a fundamental\nphysical quantity for theoretical consideration of equilibrium isotope\neffects, was evaluated by density functional theory (DFT) for 20 proteinogenic\nand two related amino acids under physiological conditions at pH 7.4.\nAt this pH value, all amino acids were of the zwitterion-type form\nwith α-NH<sub>3</sub><sup>+</sup> and α-COO<sup>–</sup> groups, regardless of the net charge of −1 (monoanion), 0\n(zwitterion), or +1 (monocation). The largest RPFR value by far was\nfor proline (Pro). This is due to the fact that the nitrogen atom\nof Pro has two C–N bonds and two N–H bonds, whereas\nthe α-nitrogen atoms of other amino acids form one C–N\nbond and three N–H bonds. There was a clear correlation between\nthe RPFR value of α-nitrogen and the distance of the N–H\ncovalent bond of the hydrogen atom involved in the formation of the\nintramolecular hydrogen bond (HB); the stronger the intramolecular\nHB, the lower the RPFR value of α-nitrogen. It was suggested\nthat equilibrium isotope effects were partially responsible for the\nnitrogen isotope fractionation in the enzymatic glutamic acid/aspartic\nacid transamination and in ammonia assimilation during the formation\nof glutamine from glutamic acid.