Wu Ronghu, McMahon Terry B
Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada.
Chemphyschem. 2008 Dec 22;9(18):2826-35. doi: 10.1002/cphc.200800543.
The protonation sites and structures of a series of protonated amino acids (Gly, Ala, Pro, Phe, Lys and Ser) are investigated by means of infrared multiple-photon dissociation (IRMPD) spectroscopy and electronic-structure calculations. The IRMPD spectra of the protonated species are recorded using the combination of a free-electron laser (FEL) and an electrospray-ion-trap mass spectrometer. The structures of different possible isomers of these protonated species are optimized at the B3LYP/6-311+G(d, p) level of theory and the IR spectra calculated using the same computational method. For every amino acid studied herein, the current results indicate that a proton is bound to the alpha-amino nitrogen, except for lysine, in which the protonation site is the amino nitrogen in the side chain. According to the calculated and experimental IRMPD results, the structures of the protonated amino acids may be assigned unambiguously. For Gly, Ala, and Pro, in each of the most stable isomers the protonated amino group forms an intramolecular hydrogen bond with the adjacent carbonyl oxygen. In the case of Gly, the isomer containing a proton bound to the carbonyl oxygen is theoretically possible. However, it does not exist under the experimental conditions because it has a significantly higher energy (i.e. 26.6 kcal mol(-1)) relative to the most stable isomer. For Ser and Phe, the protonated amino group forms two intramolecular hydrogen bonds with both the adjacent carbonyl oxygen and the side-chain group in each of the most stable isomers. In protonated lysine, the protonated amino group in the side chain forms two hydrogen bonds with the alpha-amino nitrogen and the carbonyl oxygen, which is a cyclic structure. Interestingly, for protonated lysine the zwitterionic structure is a local minimum energy isomer, but the experimental spectrum indicates that it does not exist under the experimental conditions. This is consistent with the fact that the zwitterionic isomer is 9.2 kcal mol(-1) higher in free energy at 298 K than the most stable isomer. The carbonyl stretching vibration in the range of 1760-1800 cm(-1) is especially sensitive to the structural change. In addition, IRMPD mechanisms for the protonated amino acids are also investigated.
通过红外多光子解离（IRMPD）光谱和电子结构计算，研究了一系列质子化氨基酸（甘氨酸、丙氨酸、脯氨酸、苯丙氨酸、赖氨酸和丝氨酸）的质子化位点和结构。使用自由电子激光（FEL）和电喷雾离子阱质谱仪相结合的方法记录质子化物种的IRMPD光谱。在B3LYP/6-311+G(d, p)理论水平上优化这些质子化物种不同可能异构体的结构，并使用相同的计算方法计算IR光谱。对于本文研究的每种氨基酸，当前结果表明，除赖氨酸外，质子与α-氨基氮结合，在赖氨酸中质子化位点是侧链中的氨基氮。根据计算和实验得到的IRMPD结果，可以明确确定质子化氨基酸的结构。对于甘氨酸、丙氨酸和脯氨酸，在每种最稳定的异构体中，质子化氨基与相邻的羰基氧形成分子内氢键。就甘氨酸而言，质子与羰基氧结合的异构体在理论上是可能的。然而，在实验条件下它不存在，因为相对于最稳定的异构体，它具有明显更高的能量（即26.6 kcal mol(-1)）。对于丝氨酸和苯丙氨酸，在每种最稳定的异构体中，质子化氨基与相邻的羰基氧和侧链基团形成两个分子内氢键。在质子化赖氨酸中，侧链中的质子化氨基与α-氨基氮和羰基氧形成两个氢键，这是一种环状结构。有趣的是，对于质子化赖氨酸，两性离子结构是局部能量最低的异构体，但实验光谱表明在实验条件下它不存在。这与两性离子异构体在298 K时的自由能比最稳定异构体高9.2 kcal mol(-1)这一事实一致。1760 - 1800 cm(-1)范围内的羰基伸缩振动对结构变化特别敏感。此外，还研究了质子化氨基酸的IRMPD机制。
