Page 261 - MODERN ELECTROCHEMISTRY
P. 261

ION–SOLVENT INTERACTIONS 197



















                              Fig. 2.80.  A secondary structure of
                              avian pancreatic polypeptide. (Re-
                              printed from M. Irisa, T. Takahashi,
                              F. Hirata, and T. Yanagida, J. Mol.
                              Liquids 65: 381, 1995.)

          2.24.5. Solvation Effects and the          Transition
             In many  biological  materials in  solution,  there is an  equilibrium between a
                              51
          coil-like film and a helix  (Fig. 2.80). The position of the equilibrium between coil
          and helix-like forms in biomolecules in solution is determined by hydration. For
          example, in the solvent           light scattering studies show that poly-
          benzyl glutamate is found to consist of random coils, but in other solvents it forms a
          helix.
             The form and structure of proteins demands hydration. Doty, Imahor, and Klem-
          perer found that a typical structure was 15%   in 0.14 M NaCl, but at pH 4 it
          changed to 50%      The change is a function of hydrogen bonding. The main
          difference between having a “dry” cavity and a wet one is the hydrogen bond energy.


          2.25.  WATER IN BIOLOGICAL SYSTEMS


          2.25.1.  Does Water in Biological Systems Have a Different Structure
                 from Water in Vitro?

             It has  been  suggested  that water in  biological  systems has a different structure
          from that of free water. Thus,  Szent-Györgyi  (the discoverer of vitamin C)  was the
          first to suggest that an icelike structure surrounded proteins and other biomolecules.
          Cope examined cell hydration and asked whether water is affected by conformational

          51
           A helix is an elongated form of a coil.
          52
           Conformational refers to spatial arrangements of molecules that can be obtained by rotating groups around
           bonds.
   256   257   258   259   260   261   262   263   264   265   266