A Comprehensive Optical Second Harmonic Generation Study of the Non-Centrosymmetric Character of Biological Structures

Abstract

The application of optical second harmonic generation (SHG) to the study of non-centro-symmetry in 71 α-amino acids, 22 dipeptides. 6 tripeptides. 16 proteins, and 5 viruses is reported. All α-amino acids except α-glycine, all peptidcs containing at least one enantiomeric α-amino acid residue, and all proteins and viruses are found to be non-centrosymmetric. Of the 22 racemic α-amino acids investigated, 19 form centrosymmetric crystalline racemates. DL-alanine and DL-norleucine form non-centrosymmetric racemates whereas m-glutamic acid does not form racemic crystals except when hydrated. These results lead us to suggest that non-centrosymmetry and its consequences (i.e., allowable piezoelectricity and possible pyroelectricity) is an essential and universal property of all biological systems. Furthermore, the concept of polarization homeostasis (i.e., biomolecules in vivo have a macroscopic equilibrium polarization in which the polarization ≠ 0) is proposed as a basic biological phenomenon and shown to be consistent with the dominance of one enantiomeric configuration of α-amino acids in nature, natural selection and mutationally induced molecular diseases.