Nishikawa S, Ohno T, Huang H, Yoshizuka K, Jordan F
Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan.
J Acoust Soc Am. 2003 May;113(5):2884-8. doi: 10.1121/1.1564012.
Ultrasonic absorption coefficients in aqueous solutions of glycine, L-alanine, imidazole, L-phenylalanine, L-histidine and L-tryptophan at neutral pH were measured in the range from 0.8 to 220 MHz at 25 degrees C. A characteristic ultrasonic relaxation phenomenon was observed only in the solution of L-histidine with a relaxation frequency at around 2 MHz at neutral pH. It was proposed from the concentration independent relaxation frequency and the linear concentration dependence of the maximum absorption per wavelength that the relaxation mechanism was associated with a perturbation of the rotational isomeric equilibrium of the L-histidine molecule. The existence of two rotational isomeric forms of L-histidine in water was examined by semiempirical quantum chemical methods, in order to determine the free energy difference between the two states. The forward and backward rate constants were determined from the relaxation frequency and the energy change. Also, the standard volume change of the reaction was estimated from the concentration dependence of the maximum absorption per wavelength. It was speculated that L-histidine fulfills a specific function among amino acids because of the rotational motion in the molecule, in addition to its well-established acid-base properties.
在25摄氏度下，测量了甘氨酸、L-丙氨酸、咪唑、L-苯丙氨酸、L-组氨酸和L-色氨酸在中性pH值的水溶液中0.8至220兆赫兹范围内的超声吸收系数。仅在L-组氨酸溶液中观察到特征性的超声弛豫现象，在中性pH值下弛豫频率约为2兆赫兹。从与浓度无关的弛豫频率以及每波长最大吸收的线性浓度依赖性可以推断，弛豫机制与L-组氨酸分子旋转异构体平衡的扰动有关。通过半经验量子化学方法研究了L-组氨酸在水中两种旋转异构体形式的存在，以确定两种状态之间的自由能差。根据弛豫频率和能量变化确定了正向和反向速率常数。此外，根据每波长最大吸收的浓度依赖性估算了反应的标准体积变化。据推测，L-组氨酸除了具有公认的酸碱性质外，因其分子中的旋转运动，在氨基酸中发挥着特定功能。
