Specific anion effects on the optical rotation of glucose and serine

Optical activity is directly related to molecular conformation through the anisotropic polarizabilities of molecules and the refractive index of materials. L-amino acids and D-sugars are characteristic essential bioactive molecules. Since molecular recognition and enzyme activity are related to the conformation of substrates, the relevance of optical activity to biological processes is evident. Specific ion, or Hofmeister, effects that occur with electrolytes at moderately high concentrations modify the behavior of interfaces, molecular forces between membranes, of bulk solutions, of enzymes, and even of DNA. Such effects are universal. Here we report a study on the change in optical rotation induced by some sodium salts for the enantiomers of serine and glucose in water solution. The optical rotation is shown to depend on the kind of anion and on the salt concentration. To obtain further insights into the mechanism behind the phenomenon, Fourier transform infrared (FTIR) spectral studies of serine and glucose solutions in electrolytes were also carried out. The results suggest that it is the differences in interactions of anions at specific chemical sites of the solutes that are responsible for the effects. These forces depend strongly on anion polarizability in water. Such specific ion preferential interactions can affect conformation and internal field, and result in significant changes in optical rotation.