458 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 2.51 [•/•- lectog ---Serum olbumin FEgg (]lbumin 2,0 1.5 •""/• -Zein x / x / x / 0.5 x x/x x / x / Figure 9. 0.2 0.4 0.6 0.8 I0.0 rh Water adsorption isotherms of proteins with varying 0•-helical contents. [2, 10% O, 45% A, 30% X, 70%. Humidity at 25øC. ß -.-. X X X x•x, x. 0 25 50 75 I00 % a- helicol content Figure 10. Plot of water adsorption isotherms of proteins as a function of their 0•-helical content. •, 90 % humidity ¸, 80 % humidity X, 60 % humidity.

THE BINDING OF SMALL MOLECULES TO HAIR--I 459 Therefore, it can be stated that experimental evidence suggests that peptide linkages both in a-helical and in amorphous conformations will bind water, but apparently in different proportions. The secondary structure of a protein, and in particular the degree of order, has a role in determining its water uptake capacity. This view is also confirmed by the fact that polyalanine absorbs water (15). Co-operative hydration regions So far the interactions between water and discrete functional groups of the protein have been discussed and very little has been said about hydration processes which are due to wide ranging co-operative interactions resulting from the ordered macromolecular structure of proteins. A distinction must be drawn here between short-range co-operative interactions (e.g. next neighbour interactions) and interactions which facilitate the stabilization of extended water structures around the polypeptide chain. From the fore- going it would seem that the latter interactions will only have secondary importance in determining the hydration structure of proteins, as practically all the water uptake of proteins can be accounted for by direct interactions with functional groups. However, there are a number of phenomena sug- gesting that co-operative hydration also exists in proteins and that this is governed mainly by the tertiary structure of the protein. One of the successful techniques for the study of co-operative hydration structures proved to be the measurement of volume changes which accompany various reactions of the protein molecules. Ikegami (22) carried out a systematic study of the hydration of various polyacids using a refractometric technique. On the basis of his results he concluded that the hydration region around a polyelectrolyte ion can be separated into two regions. Firstly, there is an intrinsic spherical region around each charged group, and secondly, there is a cylindrical region surrounding the entire macro-ion which is produced by the co-operative action of two or more charge-bearing groups along the polymer skeleton. From his measurements of the volume changes accompanying the ionization and de-ionization of poly-acids, he concluded that when counterions enter the secondary regions, the co-operative water structures will be destroyed owing to the strong electrostatic interactions with the polyion. Correlating the spatial distribution of ionic groups along the polymer skeleton with the volume changes observed, he came to the conclusion that if the individual charged groups are spaced more than 0'31 nm from each other, no