Extended structures single-stranded

The P structure is one of the most important secondary structures in proteins. It occurs in about 80% of the soluble globular proteins whose structures have been determined. In many cases almost the entire protein is made up of P structure. Single strands of extended polypeptide chain are sometimes present within globular proteins but more often a chain folds back on itself to form a hairpin loop. A second fold may be added to form an antiparallel "P meander"102 and additional folds to form P sheets. Beta structures are found in silk fibers (Box 2-B) as well as in soluble proteins. 

Figure 14. Chiral biomesogenic backbone structures as survivors of a Darwinian selection for optimized homochiral transient order-disorder distributions (top left) stacked nucleic acid single strand, (top right and bottom left) extended protein single strand, (bottom right) membrane component with a small but nevertheless chiral backbone [5, 6, 7, 17, 18, 33].

Note 1 For a single-strand polymer molecule with skeletal bonds all joined by the same value of valence angle, the contour length is equal to the end-to-end distance of the chain extended to the all-trans conformation. For chains of complex structures, only approximate values of the contour length may be accessible. 

In this report, both the structure-based and source-based nomenclature rules are extended to regular double-strand (ladder and spiro) organic polymers. Biopolymers, however, such as DNA are not considered here. Rules for quasi-single-strand coordination polymers, resembling spiro polymers yet not covalently bonded, are not included. 

Figure 4.3 The building block structure of potato amylopectin clusters. Branched building blocks (encircled) are mainly found inside amorphous lamellae (A) of semi-crystalline rings in starch granules. Double helices (symbolized as cylinders) extend from the building blocks into the crystalline lamellae (C). Enlargements of a double helix segment, in which the single strands are parallel and left-handed, and a building block are shown to the right.

D loop. An extended loop of single-stranded DNA displaced from a duplex structure by an oligonucleotide. 

The second major regular, repeating structure, the /3 structure, differs from the a helix in that the polypeptide chains are almost completely extended, as in Fig. 4-7(a), and hydrogen bonding occurs between polypeptide strands, rather than within a single strand, as shown in Fig. 4-7(c). 

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