Right-handed alpha helix

Figure 25-11 Peptide chain of a protein coiled to form a right-handed alpha helix. Configuration of the helix is maintained by hydrogen bonds, shown as vertical dotted (or solid) lines. The helix on the left shows the detailed atom structure of the peptide chain. The helix on the right is a schematic representation without structural detail. 

C. a right-handed alpha-helix D. a left-handed alpha-helix... 

Fiqure 1.4 The right-handed a helix found in proteins. This example is from myoglobin (residues 126-146) showing just the backbone alpha carbon, C=0, and peptide nitrogen atoms, (a) The circles are drawn for the full van der Waals radius of each atom (see Chapter 11). (b) The circles are drawn at 20% of the van der Waals radius of each atom from its center. 

An alpha helix of a polypeptide might resemble either a right-handed screw or a left-handed screw. The segment shown in the drawing is a right-handed helix of L-amino-acid residues. This is the kind of alpha helix that has been found in proteins no protein has been shown to contain left-handed alpha helixes. 

Helices can be described by the number of amino acid residues in a complete turn. In order to fit into a good helix, the amino acids must have the same configuration. For proteins, that configuration is described as an L-configuration, with the helix being a right-handed helix, referred to as an alpha-helix. 

Via. 12-18.—A drawing showing two possible forms of the alpha helix the one on the left is a left-handed helix, and the one on the right is a right-handed helix. The amino acid residues have the configuration in each case. 

The Pauling-Corey alpha helix. In addition to the right-handed helix shown, a left-handed one is also possible (with the same t-amino acids). In proteins, the helix always seems to be right-handed. [Reprinted by permission from L. Pauling, The Nature of the Chemical Bond, p. 500, Cornell Univ. Press, Ithaca, NY I960).]... 

Figure 36.4. Alpha helix structure proposed by Pauling for -kcraiin. Makes room for large side chains. Right-handed helix with 3.6 residues per turn hydrogen bonding within a chain.

Alpha helix. A helical conformation of a polypeptide chain, predominantly right-handed, with maximal intrachain hydrogen bonding of the peptide bonds one of the most common secondary structures in proteins. 

An example of a protein which has a well studied secondary structure is keratin, found in hair. Each protein molecule in keratin is arranged in the shape of a spiral, called a helix. Hydrogen bonding holds the helix together, by linking to different sections of the same chain. The arrangement of the spiral in keratin is called an alpha helix. This means that it is a helix with a right-handed turn, as in Fig. 18.4. 

Alpha (a) helix (Section 24.8A) A secondary structure in proteins where the polypeptide chain is coiled in a right-handed helix. 

The alpha helix has been found to be present in many fibrous proteins and globular proteins. It might occur as either a right-handed or a left-handed helix, but in fact only the right-handed helix has been found in proteins and in synthetic polypeptides of L-amino acids. The probable... 

The double helix structure of a DNA has been imaged by AFM. That is shown in Fig. 21.9. A DNA molecule looks like a right-handedly wound ropes. The ropes are the (alpha) helically coiled double strand. These images (Figs. 21.6-21.9) give us... 

Alpha (ar) helix (Section 24.8A) A secondary structure in proteins where the polypeptide chain is coiled in a right-handed helix. Alpha (ar) hydrogens (Sections 18.1, 18.5C, 18.5D) A hydrogen atom bonded to an a carbon. These hydrogens are significantly more acidic than the typical alkane hydrogen. 

The latter is energetically more favorable and has helical parameters which are very similar to that of the polypeptide alpha helix. As can be judged from Fig. 15 the two models have a distinctly different carbonyl orientation either perpendicular or parallel to the helix axis. Unlike the poly(S-lactide) where only one helix handedness is stable, both right and left-handed helices are equally stable in this calculation. This is evident from the symmetry in Fig. 14. 

The alpha-helix corresponds to the structure shown in Figure 14.1, with two possible directions of notation corresponding to right-handed and left-handed helixes. 

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