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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
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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
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A helix is an elongated form of a coil.
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Conformational refers to spatial arrangements of molecules that can be obtained by rotating groups around
bonds.