物理化学学报 >> 2002, Vol. 18 >> Issue (10): 871-877.doi: 10.3866/PKU.WHXB20021002

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“D和L-丙氨酸宇称破缺能差正负”的争论

王文清;孙琳;闵玮;王哲明   

  1. 北京大学化学与分子工程学院，应用化学系; 1稀土材料化学与应用国家重点实验室，北京 100871
  • 收稿日期:2002-01-29 修回日期:2002-03-19 发布日期:2002-10-15
  • 通讯作者: 王文清 E-mail:wangwq@sun.ihep.ac.cn

A Debate on the Parity-Violating Energy Difference of D-and L-Alanine

Wang Wen-Qing;Sun Lin;Min Wei;Wang Zhe-Ming   

  1. Department of Applied Chemistry, College of Chemistry and Molecular Engineering; 1State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing　100871
  • Received:2002-01-29 Revised:2002-03-19 Published:2002-10-15
  • Contact: Wang Wen-Qing E-mail:wangwq@sun.ihep.ac.cn

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摘要： 在宇宙开始大爆炸的时候，电荷变号与镜象反射共轭（CP）是对称的.但现在我们的宇宙绝大部分是正物质核子和电子等组成的，所以我们的宇宙是不对称的. D和L-丙氨酸通常称为对映体（enantiomer），实际上它们并不是由正、反粒子组成的真正的对映体，而是空间反演的，即x→－x, y→－y, z→－z 的非对映异构体（diastereoisomer），所以D-和L-丙氨酸是不对称的，两者间有能量的差别.自然界的力只有弱力是宇称不守恒的.在分子物理中，电弱力宇称不守恒是导致D-和L-丙氨酸能差的根源.所有以前的研究都认为L型丙氨酸比D型稳定.但是，最近以 Quack和 Schwerdtfeger为首的理论物理学家计算了L-丙氨酸在气相和溶液两种状态下,宇称破缺能差与分子构象的关系，提出“D-和L-丙氨酸究竟哪一个稳定”的质疑.由于气相和液相中两面角较难测定，我们用X射线四圆单晶衍射法，测定了270 K和250 K 时D-和L-丙氨酸的O(1)O(2)C(1)C(2)H(4)的原子坐标，算出了二面角，按照 Quack的MC-LR方法，D-和L-丙氨酸宇称破缺能差为1.2×10－19 Hartree, 相当于3.3× 10－18 eV/分子或3.2×10－16 kJ•mol－1，从而得出D-丙氨酸能态高于L-丙氨酸的结论.

关键词: 宇称破缺能差, D-和L-丙氨酸, 二面角, 二级相变, X射线四圆单晶衍射

Abstract: The chiral purity of biochemical monomers——only L-amino acids are present in proteins and D-sugars in the DNA and RNA of living organisms——is today accepted as an absolute necessity for the existence of life. All previous studies on amino acids reported the naturally occurring L-form to be stabilized by weak neutral currents. Two recent papers dealing with electro weak quantum calculations of the parity-violating energy shifts of alanine in gaseous phase and in solution have defied this long held belief. The parity-violating energy difference (PVED) of D- and L-alanine was undertaken as a function of the dihedral angle θ of the carboxylate plane with respect to the Cα-Hα plane. Dihedral angles was calculated from the atomic coordinates of O(1)O(2)C(1)C(2)H(4) of D- and L-alanine under the temperature dependence of X-ray diffraction data. According to Quack’s theoretical results by means of highest level ab initio studies (MC-LR), the ΔEPV value is 1.2×10－19 Hartree (3.3×10－18 eV/ molecule), namely L-alanine is more stable than D-alanine. The present paper proved definitely that in the single crystal state L-alanine is lower in energy than D-alanine both by X-ray diffraction data and the differential scanning calorimetry.

Key words: Parity-violating energy difference, D- and L-alanine, Dihedral angle, 　Second order phase transition, X-ray diffraction