45O JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS found that f, the force required to extend a fibre to a given extension, drops roughly by a factor of four as the humidity of the atmosphere increases from 0 to 1003/o (Fig. 2). More recently in our laboratory, Huck and Baddiel 60- I00% 45 •• •'4% ---- / / / _41% ß 3o ß 15 I I I I I I I I I I I I I I I 0 6 12 18 24 30 36x105 Lood x 104 g- :::' Figure 2. Load extension curves of wool for varying humidities at 25øC [reproduced with permission from ref. (2)]. (3) determined the values of E', the dynamic elastic modulus, and E", the dynamic storage modulus of single hair fibres as functions of humidity at low frequencies using Tokita's oscillating beam techniques [for details of the technique and definitions of E' and E" see (3)]. In Figs. 3 and 4, the values off at 10% extension [from (1)] and the values of E' and E" deter- mined at various humidities are respectively plotted against the corre- sponding values of IcR. The linear relationships which were obtained lend strong support to the view that changes in the single fibre properties are the most important factors determining the dependence of Icx on humidity. Following Astbury and Street's early measurements, Treloar (4) carried out a careful study measuring changes in the elongations which occur when keratin fibres are transferred from one humidity to another. Using his data, Breuer (5) gave a thermodynamic description of the processes involved. However, none of these authors discussed the effects of humidity on keratin fibres in terms of molecular mechanisms. This was mostly due to the lack of knowledge which existed then with respect to the molecular basis of hydration of proteins. Since then, however, considerable progress

THE BINDING OF SMALL MOLECULES TO HAIR--I 451 60- 5 10 15 20 ,'"( 10 %) gcm 2x 10-9 --.- Figure 3. Plot of/ca, the curl retention index of hair tresses measured at various humidi- ties, against f, the retractive force of single fibres at 10 % extension. The force was measured at the same humidity as the corresponding value of IcR. 600 o. I I 4o E"X 10 -I N m -2 0.2 0.3 0.4 I I I ß 0 o i i I I 0 '3.5 4.0 4.5 ,5.0 E'x 10 -IN m-2 Figure 4. Plot of IcR, the curl retention index of hair tresses measured at various humidities against the values of E', the dynamic elastic and E*, the dynamic storage modulus of single hair fibres, measured at the same humidities as the corresponding Ic•. has been made in this field and molecular interpretations of the mechanisms involved in the action of humidity on single fibre properties now seem feasible. HYDRATION STRUCTURE OF PROTEINS AND POLYAMIDES Owing to its great biological importance (6-9), considerable work has been carried out on the hydration of proteins. The early work was limited,