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(biochemistry) Any of a group of cyclic peptides proposed as a basis of rigid protein structures

Figure 1: In the classic cyclol reaction, two peptide groups are linked by a N-C' bond, converting the carbonyl oxygen into a hydroxyl group. Although this reaction occurs in a few cyclic peptides, it is disfavored by free energy, mainly because it eliminates the resonance stabilization of the peptide bond. This reaction was the basis of Dorothy Wrinch's cyclol model of proteins.

(biochemistry) Any of a group of cyclic peptides proposed as a basis of rigid protein structures

Figure 2: The alanine cyclol-6 molecule proposed by Dorothy Wrinch is a cyclic hexapeptide in which three peptide groups are fused by cyclol reactions into a central ring. The three outer (unfused) peptide groups are not planar, but have dihedral angle ω=60°. The three red atoms in the central ring represent the hydroxyl groups formed by the cyclol reactions, whereas the three outer red atoms represent the oxygens of carbonyl groups. The inner oxygen atoms are separated by only 2.45 Å, which is extremely close even for hydrogen-bonded atoms. This hypothetical molecule has not been observed in nature.

(biochemistry) Any of a group of cyclic peptides proposed as a basis of rigid protein structures

Figure 3: Stick model of the alanine cyclol fabric proposed by Dorothy Wrinch. The cyclol fabric is conceptually similar to a beta sheet, but more uniform and laterally denser. The fabric has large "lacunae" arranged in a hexagonal pattern, in which three C^β atoms (shown in green) and three H^α atoms (shown in white) converge on a (relatively) empty spot in the fabric. The two sides of the fabric are not equivalent; all the C^β atoms emerge from the same side, which is the "upper" side here. The red atoms represent hydroxyl groups (not carbonyl groups) and emerge (in sets of three) from both sides of the fabric; the blue atoms represent nitrogen. This hypothetical structure has not been observed in nature.