“D和L-丙氨酸宇称破缺能差正负”的争论

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.

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

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 electroweak 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 Dand 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(1) of Dand 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 △Epv 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.