PROTEIN STRUCTURE 



bonds between water molecules. The stabilization of serum 

 albumin against denaturation by heat (15), or by urea and 

 guanidine hydrochloride (14), by the addition of fatty acid 

 anions with long nonpolar side chains is probably associated 

 with a stabilizing interaction of this sort — the fatty acid anions 

 certainly bind to the albumin, and they probably tie down some 

 potentially loose and unstable points in the albumin structure. 



Further reflections on these general problems are contained 

 in the thoughtful essay of Waugh (108). We now turn to a 

 detailed consideration of spatial configurations. 



Spatial Configuration of Peptide Chains 



Along a peptide chain NHg CH(Ri) CO . NH CH(R2) CO . NH 

 CH(R)CO — NH CH(RJ COOH there is a simple unit repeat, 

 the peptide chain residue (NH.CH.CO.). We know that the 

 ordered regions in fibrous proteins which give x-ray diffraction 

 patterns are regions of skeletal backbone chain order. They 

 arise from regular intra- or inter-chain packing of identical 

 neighboring chain residue units. Short-range or long-range 

 order in the side-chain sequence Ri R2 — R„ may sometimes oc- 

 cur also. In silk fibroin, for example, an ordered sequence 

 appears very probable. 



The Astbury classification of fibrous proteins into three 

 groups is based on the three different types of wide-angle x-ray 

 diffraction patterns observed. These correspond to three 

 distinct types of molecular peptide chain configurations and 

 interchain packing. The outstanding features of the a-keratin 

 pattern given by proteins of the keratin-myosin-epidermin- 

 fibrinogen (k-m-e-f) group are a meridional reflection of 5.1 A 

 and an equatorial reflection at 9.8 A. X-ray diffraction patterns 

 of fibrous proteins are commonly taken with the x-ray beam 

 normal to the fiber axis. The meridional reflections correspond 

 therefore to spacings along the fiber axis, and the equatorial 

 spacings relate to spacings of planes parallel to the fiber axis. 

 a-Keratin is extensible along the fiber axis direction. In the 



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