Kaczor A, Reva I D, Proniewicz L M, Fausto R
Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal.
J Phys Chem A. 2006 Feb 23;110(7):2360-70. doi: 10.1021/jp0550715.
The conformational behavior and infrared spectrum of l-phenylalanine were studied by matrix-isolation infrared spectroscopy and DFT [B3LYP/6-311++G(d,p)] calculations. The fourteen most stable structures were predicted to differ in energy by less than 10 kJ mol(-1), eight of them with abundances higher than 5% at the temperature of evaporation of the compound (423 K). Experimental results suggest that six conformers contribute to the spectrum of the isolated compound, whereas two conformers (IIb(3) and IIIb(3)) relax in matrix to a more stable form (IIb(2)) due to low energy barriers for conformational isomerization (conformational cooling). The two lowest-energy conformers (Ib(1), Ia) differ only in the arrangement of the amino acid group relative to the phenyl ring; they exhibit a relatively strong stabilizing intramolecular hydrogen bond of the O-H...N type and the carboxylic group in the trans configuration (O=C-O-H dihedral angle ca. 180 degrees ). Type II conformers have a weaker H-bond of the N-H...O=C type, but they bear the more favorable cis arrangement of the carboxylic group. Being considerably more flexible, type II conformers are stabilized by entropy and the relative abundances of two conformers of this type (IIb(2) and IIc(1)) are shown to significantly increase with temperature due to entropic stabilization. At 423 K, these conformers are found to be the first and third most abundant species present in the conformational equilibrium, with relative populations of ca. 15% each, whereas their populations could be expected to be only ca. 5% if entropy effects were not taken into consideration. Indeed, phenylalanine can be considered a notable example of a molecule where entropy plays an essential role in determining the relative abundance of the possible low-energy conformational states and then, the thermodynamics of the compound, even at moderate temperatures. Upon UV irradiation (lambda > 235 nm) of the matrix-isolated compound, unimolecular photodecomposition of phenylalanine is observed with production of CO(2) and phenethylamine.
通过基质隔离红外光谱和密度泛函理论[B3LYP/6 - 311++G(d,p)]计算研究了L - 苯丙氨酸的构象行为和红外光谱。预测出的14种最稳定结构的能量差异小于10 kJ mol⁻¹，其中8种在化合物蒸发温度（423 K）下丰度高于5%。实验结果表明，六种构象异构体对分离出的化合物的光谱有贡献，而两种构象异构体（IIb(3)和IIIb(3)）由于构象异构化的低能垒（构象冷却）在基质中弛豫为更稳定的形式（IIb(2)）。两种能量最低的构象异构体（Ib(1)，Ia）仅在氨基酸基团相对于苯环的排列上有所不同；它们表现出相对较强的O - H...N型稳定分子内氢键以及反式构型的羧基（O = C - O - H二面角约为180°）。II型构象异构体具有较弱的N - H...O = C型氢键，但它们具有更有利的羧基顺式排列。由于II型构象异构体的灵活性明显更高，它们通过熵得以稳定，并且由于熵稳定作用，这种类型的两种构象异构体（IIb(2)和IIc(1)）的相对丰度随温度显著增加。在423 K时，发现这些构象异构体是构象平衡中含量第一和第三丰富的物种，相对丰度约为15%，而如果不考虑熵效应，它们的丰度预计仅约为5%。实际上，苯丙氨酸可被视为一个显著的例子，说明即使在中等温度下，熵在决定可能的低能构象状态的相对丰度以及化合物的热力学方面起着至关重要的作用。对基质隔离的化合物进行紫外照射（λ > 235 nm）时，观察到苯丙氨酸的单分子光分解，生成CO₂和苯乙胺。
