Acta Phys. -Chim. Sin. ›› 2004, Vol. 20 ›› Issue (05): 540-545.doi: 10.3866/PKU.WHXB20040519

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Temperature Dependence of Natural Optical Rotation Study of D-, L- and DL- Valine

Chen Yu;Wang Wen-Qing;Du Wei-Min   

  1. Institude of Modern Optics, College of Physics;Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871
  • Received:2003-11-28 Revised:2004-02-06 Published:2004-05-15
  • Contact: Wang Wen-Qing

Abstract: Most biomolecules are chiral, but only one enantiomeric form occurs in nature. Life is based on L-amino acids and D-sugars rather than the D-amino acids and L-sugars. The homochirality is a hallmark of life, which remains a puzzle for theories of the chemical origin of life. This broken symmetry is believed to be a feature of fundamental physics, a result of symmetry breaking by the weak force, which makes one enantiomer slightly more stable than the other. The natural L-amino acids, L-alanine and L-valine were found to be more stable than their D-amino acids. For glyceraldehydes (n=3), the parent of the higher sugars, the D form is PVED-stabilized by about 10-17 kT. D-oxyribose is also PVED-stabilized in the C2-endo form found in DNA. It has recently become recognized that homochirality is not just a consequence of life. This is because life and self-organization seem to require polymers and polymerization do not go in racemic solution. It is found that polymerization to give stereoregular biopolymers, in particular poly-D-ribonucleotides and poly-L-peptides, proceeds efficiently only in almost homochiral monomer solutions. In racemic solution, addition of the “wrong” hand to the growing chain tends to terminate the polymerization. The weak force appears to predict hand whatever came first. For nucleic acids, D-glyceraldehyde and α-D-ribose are more stable and the right-hand helical backbone of DNA is also PVED-stabilized. For proteins, the L-amino acids are more stable.    In 1991 Abdus Salam proposed a hypothesis:the subtle energy difference(PVED) of chiral molecules induced by Z0 interactions combined with Bose condensation may cause homochirality among nineteen amino acids (excluding the achiral glycine). This is a consequence of secondorder phase transition at a critical temperature Tc, which is analogous to that of BCS(Bardeen-Cooper-Schrieffer) superconductivity. The value of temperature Tc is estimated to be around 250 K deduced from Ginzberg -Landau equation. To verify the Salam hypothesis, one may lower the temperature while measuring the optical activity when polarized light is shone upon a particular amino acid. If the polarization vectors get rotated, it can be said that the “phase transition” has taken place.   Chiral molecules are characterized by a natural optical activity. The difficulty of the experiment is how to measure the rotation angle of monoclinic crystals of valine enantiomers. In this paper natural optical rotation in biaxial crystal of monoclinic valine is studied by projecting He-Ne laser light along one of the optic axes, b axis. The other two principal axes of refractive index ellipsoid, a and c are to be determined. Here we design a method to eliminate the effect of ellipticity induced by the crystal anisotropy. The temperature dependence of the natural optical rotation angle of D-valine, L-valine and DL-valine can thus be determined. The obtained results provide a direct evidence of the Salam hypothesis.

Key words: Natural optical rotation angle, Biaxial crystal, D-, L- and DL-valine,  Salam hypothesis, Second-order phase transition