THE X-ray diffraction photographs usually obtained from apparently nonfibrous proteins have so much in common with one another and with the photographs given by certain natural protein fibres when disoriented that the inference seems clear that all proteins at some stage of their existence are fibrous in the molecular sense [Astbury, 1933; 1934,1,2]. Recently [Astbury and Lomax, 1934, 1, 2;1935] this concept has been expressed as a generalised interpretation of denaturation in the conclusion that the two more stable and insoluble states of protein structure, the fibrous and the denatured, are based on fundamentally similar modes of molecular arrangement; that, in fact, the denatured state2 is essentially a fibrous state inasmuch as it always consists of peptide chains, often fully extended, and aggregated after coagulation in parallel bundles, as in fibroin [Meyer and Mark, 1928], f3-keratin [Astbury and Street, 1931; Astbury and Woods, 1933], /3-myosin [Astbury and Dickinson, 1935], fibrin [Katz and De Rooy, 1933] etc. Itwas found that heat-denaturation of the albumins, for instance, merely makes the X-ray photograph more like that of a random arrangement of fibres of/-keratin (stretched hair, horn, etc.; cf. Plate V, Figs. 1 and 3). ,B-Keratin is built from almost fully-extended polypeptide chains linked side-to-side, firstly by combinations between their side-chains ("side-chain linkage "), and secondly in a direction at right angles [Astbury and Sisson, 1935], through attractions between the =CO and =NH groups of neighbouring main-chains ("backbone linkage"). The three-dimensional structure is that of a pile of polypeptide "grids ", the average distance apart of the main-chains in the plane of each grid being about 9*8 A. ("side-chain spacing") and the distance between the grids being 4*65 A. (" backbone spacing "). These two spacings correspond respectively to the two reflections, 001 and 200, seen on the equator of Fig. 4: they are the two prin'cipal side-spacings of the ,B-keratin crystallites, whilst the reflection 020 (spacing about 3*4 A.) gives the average length of an amino-acid residue in the direction of the main-chains. Fig. 4 is a "fibre photograph" of crystallites all pointing roughly in the same direction (the fibre-axis) and taken with the X-ray beam perpendicular to this direction; but when the crystallites are 1 Beit Scientific Research Fellow. 2 The seed globulins, at least, appear to pass through an intermediate fibrous state before reaching complete denaturation. It is possibly better therefore to reserve the word "denatured"
