Gu Z, Ebisawa K, McDermott A
Department of Chemistry, Columbia University, New York, NY 10027, USA.
Solid State Nucl Magn Reson. 1996 Dec;7(3):161-72. doi: 10.1016/s0926-2040(96)01259-3.
Rates of rotation for amines in a variety of crystalline environments are reported, and the trends are explained in terms of the strengths of local hydrogen bonding interactions. Proton spin-lattice relaxation times (T1) and deuterium broad-line NMR spectra have been measured for D-, DL- and L- aspartic acid, two polymorphs of glycine, alanine, and leucine in the temperature range from -40 to 110 degrees C. The energy barriers for amine rotation are 27 +/- 2 kJ mol-1 for D- or L-aspartic acid and 22 +/- 2 kJ mol-1 for DL-aspartic acid; these energies are slightly lower than the previously reported value for the L from based on direct proton T1 measurements at 60 MHz. The values for the alpha and gamma forms of glycine were 24 +/- 2 and 30 +/- 2 kJ mol-1 respectively, that for L-alanine was 40 +/- 2 and that for L-leucine was 49 +/- 3 kJ mol-1. These are all in rough agreement with previously reported values (although the differences for the polymorphs of glycine and for L- vs. DL-aspartic acid were not reported). Crystal structures of these amino acids indicate differences in hydrogen bonding environments around the R-NH3+ groups that are probably responsible for the different activation barriers. A molecular mechanics calculation of the rotation energy barriers for L- and DL-aspartic acid based on the crystal structures gave satisfactory agreement with experimental results if a uniform (and arbitrarily chosen) dielectric constant of 2.5 was assumed. Differences between L- and DL-aspartic acids and between two polymorphs of glycine were well represented qualitatively. Including additional neighboring molecules not involved in the hydrogen bonding or including periodic boundary conditions to describe the crystal packing did not significantly affect these results. If vacuum dielectric constants are used, the barriers are uniformly overestimated, and if the experimental macroscopic dielectric constant values are used, the barriers are generally underestimated. Dielectric constants differ substantially from one amino acid to another and significantly affect the estimated barriers; in fact, the bulk dielectric constants appear to be the major difference between the highest and the lowest values. The difficulty of accurately including dielectric relaxation into molecular mechanics calculations resulted in the disagreement between experimental measurements and theoretical calculations.
报道了各种晶体环境中胺的旋转速率，并根据局部氢键相互作用的强度解释了这些趋势。在-40至110摄氏度的温度范围内，测量了D-、DL-和L-天冬氨酸、甘氨酸的两种多晶型物、丙氨酸和亮氨酸的质子自旋晶格弛豫时间（T1）和氘宽线核磁共振谱。D-或L-天冬氨酸胺旋转的能垒为27±2 kJ/mol，DL-天冬氨酸为22±2 kJ/mol；这些能量略低于先前基于60 MHz直接质子T1测量报道的L-型的值。甘氨酸α和γ型的值分别为24±2和30±2 kJ/mol，L-丙氨酸为40±2，L-亮氨酸为49±3 kJ/mol。这些都与先前报道的值大致相符（尽管未报道甘氨酸多晶型物以及L-与DL-天冬氨酸之间的差异）。这些氨基酸的晶体结构表明R-NH3+基团周围氢键环境存在差异，这可能是导致不同活化能垒的原因。基于晶体结构对L-和DL-天冬氨酸旋转能垒进行的分子力学计算，如果假设均匀（且任意选择）的介电常数为2.5，则与实验结果吻合良好。L-和DL-天冬氨酸之间以及甘氨酸的两种多晶型物之间的差异在定性上得到了很好的体现。包括不参与氢键的额外相邻分子或包括周期性边界条件来描述晶体堆积，对这些结果没有显著影响。如果使用真空介电常数，能垒会被统一高估，如果使用实验宏观介电常数的值，能垒通常会被低估。介电常数在不同氨基酸之间有很大差异，并显著影响估计的能垒；实际上，体介电常数似乎是最高值和最低值之间的主要差异。在分子力学计算中准确纳入介电弛豫的困难导致了实验测量和理论计算之间的不一致。
