物理化学学报 >> 2005, Vol. 21 >> Issue (07): 774-781.doi: 10.3866/PKU.WHXB20050715

研究论文 上一篇    下一篇

分子手性的温度效应：D-丙氨酸的变温X衍射和中子衍射研究

王文清; 龚; 姚楠   

  1. 北京大学化学与分子工程学院，应用化学系，北京 100871
  • 收稿日期:2004-11-12 修回日期:2005-02-28 发布日期:2005-07-15
  • 通讯作者: 王文清 E-mail:wangwqchem@pku.edu.cn

Temperature Effect on Molecular Chirality：X-ray Diffraction and Neutron Diffraction Studies of D-alanine

WANG Wen-qing; GONG Yan; YAO Nan   

  1. Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871
  • Received:2004-11-12 Revised:2005-02-28 Published:2005-07-15
  • Contact: WANG Wen-qing E-mail:wangwqchem@pku.edu.cn

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摘要： 利用X衍射（300, 270, 250 K）和中子衍射（300, 260, 250, 240 K）研究D-氨酸单晶在静态的和动力学的变温过程中的结构特征以及考证Salam预言的由D到L构型转变的可能性. 实验发现丙氨酸晶体的空间群P212121对称性没有改变. 实验结果否定了构型相变的可能，但是发现在~250 K有一个微小的、连续的对称性破缺发生. 晶体分子振动产生的环电流模型可以用来解释D-和L-丙氨酸单晶直流磁化率和天然旋光角相反的现象， 与之相关的中子衍射数据进一步揭示了变温过程中αC－H(2), N－H(1), N－H(4), N－H(6) 键长的不同变化. 中子衍射还显示了质子移动所导致的动力学无序，来源于分子内氨基和羧基形成的氢键和分子间αC－H和氨基形成的氢键，从而产生的晶格扭曲和NH3+的扭转. 实验结果表明Salam预言相变不是传统意义的结构相变，而是由于温度效应导致了在相变点附近分子的宇称破缺能差（PVED）增大，然后通过氨基酸分子的隧道效应扩大了宇称破缺能差的影响，这一研究为生命现象中快速的均一手性形成提供了非线性机理的合理解释.

关键词: 温度效应, 相变, X衍射, 中子衍射, D-丙氨酸单晶, 宇称破缺能差

Abstract: The results of X-ray diffraction of single crystal D- and L-alanine at 300, 270, 250 K and neutron diffraction of D-alanine at 300, 260, 250, 240 K were presented to look for the static and dynamical structure feature of Salam phase transition. No change in the space group P212121 symmetry was observed. The data rule out the hypothesis of a conventional structural phase transition and reveal a subtle and continuous symmetry breaking that occurs below ~250 K. A vibrationally generated electronic ring current model was explained the temperature effect on magnetic susceptibilities and opposite optical rotation phenomena related to the bond lengths of αC－H(2), N(1)－H(1), N(1)－H(4), and N(1)－H(6) lengthening (or contraction). A picture of dynamic disorder due to the protonic motion was presented in the intermolecular hydrogen bonds between the NH3+ group and the CO2－ group. Salam predicted the enhancement of parity-violating energy difference (PVED) due to temperature-induced phase transition. Tunneling dynamics of amino acids with PVED may provide a bifurcation mechanism for much faster development of homochirality.

Key words: Temperature effect, Phase transition, X-ray diffraction, Neutron diffraction, D-alanine single crystal, Parity-violating energy difference