Three-strand ropes

Figure 25-17 Representation of the quaternary structure of a-keratin showing (a) three a-helical polypeptide strands coiled into a rope and (b) eleven units of the three-stranded rope arranged to form one microfibril...

Crick (1952) pointed out that this difficulty could be overcome by supposing that the a-helices in a-keratin were distorted in a helical manner to form coiled coils as illustrated in Fig. 14. This distortion, which was claimed to require only about 0.1 kcal per residue, enabled the side chains to pack more neatly. In subsequent papers Crick obtained an expression for the Fourier transform of a coiled coil (Crick, 1953a) and was able to show that this type of distortion could account in a general way for some previously unexplained features of the X-ray pattern of a-keratin (Crick, 1953b) including the simultaneous appearance of 1.5 and 5.15 A meridional reflections. Detailed descriptions of two-strand and three-strand ropes of these coiled coils were given in which the pitch of the major helix was 186 A, and it was suggested that the three-strand model was appropriate... 

A somewhat more detailed version of Crick s three-strand rope model was described by Lang (1956a,b) in which the major helix had a pitch of 197 A and radius 5.5 A giving a tilt of about 10°. The /3-carbon atom was supposed to be in position 2 which was favored at that time. Detailed calculations appeared to confirm Crick s view that a model of this type could account for a strong meridional reflection at 5.15 A, but these are unlikely to be valid as it is now considered that position 1 is appropriate. 

FIGURE 10-6 Schematic views of compound helices and their packing. Left, a coiled coil, se n-strand cable and three-strand rope right, packing diagram for coils and caWcs in a-keratin. [From Corey and Pauling, Proc, International Wool Textile Res, Conf, Australia 1955, Vol. B, 249-66.]... 

Other distinct types of protein secondary structure include the type present in collagen, a fibrous connective tissue protein and the most abundant of all human proteins. Collagen peptide chains are twisted together into a three-stranded helix. The resultant three-stranded rope is then twisted into a superhelix (Chapter 10). 

A back splice in a three-strand rope is made by interlacing the strands back along the rope for three turns. However, it is often more useful to have a loop, which may be placed around a metal or plastic fitting, at the end of a rope. Fig. 13.6 (a) shows a Z-twist, three-strand rope ready to be spliced. The free ends are then tucked under successive strands in an S-path. Fig. 13.6 (b) shows a similar procedure for an eight-strand braided rope. For some of the newer rope types, splicing procedures have to be modified. Braid-on-braid ropes are spliced by pushing a section of the outer braid into the centre of the rope from which a section of the inner braid has been removed (Fig. 13.6 (c)). 

Fig. 13.6 (a) Start for eye splice in a three-strand rope, (b) Making an eye splice in an eight-strand braided rope, (c) Eye splice in a double-braid rope. Parts (a) and (b) courtesy of Gleistein part (c) courtesy of Samson Rope. 

Fig. 2. At the left, a compound a-helix with pitch about 67 A. The diameter, shown as about 10 A., includes the volume occupied by side chains as well as the main chains of the protein. Centre, a seven-strand a-cable, with lead of about 400 A. In the proposed structures of proteins of the a-keratin type these cables are packed together, with compound helixes as shown at the left in the interstices. At the rigid, a three-strand rope, with lead of about 200 A., and with sense opposite to that of the seven-strand cable. In the suggested structure for feather rachis keratin, these ropes arc packed together with seven-strand a-cabics, in tlie ratio of two ropes to one a-cable...

A radius of 6 A. for the large helix would permit three compound helixes to twist about one another, to form a three-strand rope (Fig. 2). Such a rope with the sense of twist of the strands opposite to that of the rope would be formed by the seven-residue compound helix, or with the sense of the strands the same as that of the rope by, for example, the fifteen-residue compound helix (with a repeating unit of fifteen residues, comprising nearly four turns of the a-helix). 

Keratin can be seen as a biological answer to the need for a tough plastic equivalent to nylon. As hair and fur, it is a fiber. As epidermis, it is a film, and as hoof or hom, it is tough solid. The stmcture contains fibrils, which are built from three-stranded ropes of alpha-helical chains in a coiled-coil arrangement. The fibrils are embedded in a cross-linked matrix of amorphous protein that is heavily... 

A simple three strand rope is shown in Fig. 2.8. Here, three preliminary ropes of diameter d. are twisted together in close-packed helical fashion to form the final rope of diameter (<7j). These three preliminary ropes A, B, and C are laid togetherto form right hand helixes, in this case. Each ofthe preliminary ropes consists of secondary three strand, closely-packed helical ropes of diameter (J3) that have a left hand helical lay. The preliminary rope (A) is made up of secondary ropes D, E, and F. Other layers of closely-packed helixes may be added to form ropes of greater diameter. 

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