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S-44 The Three-Dimensional Structure of Proteins chapter 4 1. Properties of the Peptide Bond In x-ray studies of crystalline peptides, Linus Pauling and Robert Corey found that the C O N bond in the peptide link is intermediate in length (1.32 Å) between a typical C O N single bond (1.49 Å) and a C P N double bond (1.27 Å). They also found that the peptide bond is planar (all four atoms attached to the C—N group are located in the same plane) and that the two a -carbon atoms attached to the C O N are always trans to each other (on opposite sides of the peptide bond). (a) What does the length of the C O N bond in the peptide linkage indicate about its strength and its bond order (i.e., whether it is single, double, or triple)? (b) What do the observations of Pauling and Corey tell us about the ease of rotation about the C O N peptide bond? Answer (a) The higher the bond order (double or triple vs. single), the shorter and stronger are the bonds. Thus, bond length is an indication of bond order. For example, the C P N bond is shorter (1.27 Å) and has a higher order ( n ± 2.0) than a typical C O N bond (length ± 1.49 Å, n ± 1.0). The length of the C O N bond of the peptide link (1.32 Å) indicates that it is intermediate in strength and bond order between a single and double bond. (b) Rotation about a double bond is generally impossible at physiological temperatures, and the steric relationship of the groups attached to the two atoms involved in the double bond is spatially “fixed.” Since the peptide bond has considerable double-bond character, there is essentially no rotation, and the O C P O and O N O H groups are fixed in the trans configuration. 2. Structural and Functional Relationships in Fibrous Proteins William Astbury discovered that the x-ray diffraction pattern of wool shows a repeating structural unit spaced about 5.2 Å along the length of the wool fiber. When he steamed and stretched the wool, the x-ray pattern showed a new repeating structural unit at a spacing of 7.0 Å. Steaming and stretching the wool and then letting it shrink gave an x-ray pattern consistent with the original spacing of about 5.2 Å. Although these observations provided important clues to the molecular structure of wool, Astbury was unable to interpret them at the time. (a) Given our current understanding of the structure of wool, interpret Astbury’s observations. (b) When wool sweaters or socks are washed in hot water or heated in a dryer, they shrink. Silk, on the other hand, does not shrink under the same conditions. Explain. 2608T_ch04sm_S44-S53 1/31/08 9:45PM Page S-44 ntt 102:WHQY028:Solutions Manual:Ch-04:
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Chapter 4 The Three-Dimensional Structure of Proteins S-45 Answer (a) The principal structural units in the wool fiber polypeptide, a -keratin, are successive turns of the a helix, which are spaced at 5.4 Å intervals; two a -keratin strands twisted into a coiled coil produce the 5.2 Å spacing. The intrinsic stability of the helix (and thus the fiber) results from intra chain hydrogen bonds (see Fig. 4–4a).
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This note was uploaded on 05/20/2011 for the course BCH 3218 taught by Professor Johnsteward during the Spring '08 term at University of Florida.

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