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