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The cyclol hypothesis is the now discredited first structural model of a protein folding, globular protein protein, formulated in the 1930s. It was based on the cyclol reaction of peptide bonds proposed by physicist Charles Frank in 1936, in which two peptide bond are chemically crosslinked. These crosslinks are covalent analogs of the non-covalent between peptide groups and have been observed in rare cases, such as the ergopeptides.
Based on this reaction, mathematician Dorothy Wrinch hypothesized in a series of five papers in the late 1930s a structural model of globular proteins. She postulated that, under some conditions, amino acids will spontaneously make the maximum possible number of cyclol crosslinks, resulting in cyclol molecules and cyclol fabrics. She further proposed that globular proteins have a tertiary structure corresponding to and semiregular polyhedra formed of cyclol fabrics with no free edges. In contrast to the cyclol reaction itself, these hypothetical molecules, fabrics and polyhedra have not been observed experimentally. The model has several consequences that render it energetically implausible, such as steric clashes between the protein sidechains. In response to such criticisms J. D. Bernal proposed that hydrophobic interactions are chiefly responsible for protein folding, which was indeed borne out.
The chemical structure of was still under debate at that time. The most accepted (and ultimately correct) hypothesis was that proteins are linear , i.e., unbranched of linked by . However, a typical protein is remarkably long—hundreds of amino acid—and several distinguished scientists were unsure whether such long, linear could be stable in solution. Further doubts about the polypeptide nature of proteins arose because some enzymes were observed to cleave proteins but not peptides, whereas other cleave peptides but not folded proteins. (1999). 9780585359809, Yale University Press. ISBN 9780585359809 Attempts to synthesize proteins in the test tube were unsuccessful, mainly due to the chirality of amino acids; naturally occurring proteins are composed of only left-handed amino acids. Hence, alternative chemical models of proteins were considered, such as the diketopiperazine hypothesis of Emil Abderhalden. However, no alternative model had yet explained why proteins yield only amino acids and peptides upon hydrolysis and proteolysis. As clarified by Linderstrøm-Lang, these proteolysis data showed that denatured proteins were polypeptides, but no data had yet been obtained about the structure of folded proteins; thus, denaturation could involve a chemical change that converted folded proteins into polypeptides.
The process of protein denaturation (as distinguished from coagulation) had been discovered in 1910 by Harriette Chick and Charles Martin,
but its nature was still mysterious. Tim Anson and Alfred Mirsky had shown that denaturation was a reversible, two-state process that results in many chemical groups becoming available for chemical reactions, including cleavage by enzymes. (2025). 9780120342020 ISBN 9780120342020 In 1929, Hsien Wu hypothesized correctly that denaturation corresponded to protein unfolding, a purely conformational change that resulted in the exposure of amino-acid side chains to the solvent. Preliminary reports were presented before the XIIIth International Congress of Physiology at Boston (19–24 August 1929) and in the October 1929 issue of the American Journal of Physiology. Wu's hypothesis was also advanced independently in 1936 by Mirsky and Linus Pauling. Nevertheless, protein scientists could not exclude the possibility that denaturation corresponded to a chemical change in the protein structure, a hypothesis that was considered a (distant) possibility until the 1950s.
X-ray crystallography had just begun as a discipline in 1911, and had advanced relatively rapidly from simple salt crystals to crystals of complex molecules such as cholesterol. (2025). 9781597452663 ISBN 9781597452663 However, even the smallest proteins have over 1000 atoms, which makes determining their structure far more complex. In 1934, Dorothy Crowfoot Hodgkin had taken crystallographic data on the structure of the small protein, insulin, although the structure of that and other proteins were not solved until the late 1960s.; ; ; However, pioneering X-ray fiber diffraction data had been collected in the early 1930s for many natural such as wool and hair by William Astbury, who suggested that "globular proteins in general might be folded from elements essentially like the elements of fibrous proteins."Charles Tanford & Jacqueline Reynolds (2001) Nature's Robots: a history of proteins, pages 80 to 83, Oxford University Press
Since protein structure was so poorly understood in the 1930s, the physical interactions responsible for stabilizing that structure were likewise unknown. William Astbury hypothesized that the structure of was stabilized by in β-sheets. The idea that are also stabilized by hydrogen bonds was proposed by Dorothy Jordan Lloyd in 1932, and championed later by Alfred Mirsky and Linus Pauling. At a 1933 lecture by Astbury to the Oxford Junior Scientific Society, physicist Charles Frank suggested that the fibrous protein α-keratin might be stabilized by an alternative mechanism, namely, covalent crosslinking of the by the cyclol reaction above. The cyclol crosslink draws the two peptide groups close together; the N and C atoms are separated by ~1.5 Angstrom, whereas they are separated by ~3 Angstrom in a typical hydrogen bond. The idea intrigued J. D. Bernal, who suggested it to the mathematician Dorothy Wrinch as possibly useful in understanding protein structure.
These rings can be extended indefinitely to form a cyclol fabric (Figure 3). Such fabrics exhibit a long-range, quasi-crystalline order that Wrinch felt was likely in proteins, since they must pack hundreds of residues densely. Another interesting feature of such molecules and fabrics is that their amino acid side chains point axially upwards from only one face; the opposite face has no side chains. Thus, one face is completely independent of the primary sequence of the peptide, which Wrinch conjectured might account for sequence-independent properties of proteins.
In her initial article, Wrinch stated clearly that the cyclol model was merely a working hypothesis, a potentially valid model of proteins that would have to be checked. Her goals in this article and its successors were to propose a well-defined testable model, to work out the consequences of its assumptions and to make predictions that could be tested experimentally. In these goals, she succeeded; however, within a few years, experiments and further modeling showed that the cyclol hypothesis was untenable as a model for globular proteins.
The lability of the cyclol bond was seen as an advantage of the model, since it provided a natural explanation for the properties of denaturation; reversion of cyclol bonds to their more stable amide form would open up the structure and allows those bonds to be attacked by , consistent with experiment. Early studies showed that proteins denatured by pressure are often in a different state than the same proteins denatured by high temperature, which was interpreted as possibly supporting the cyclol model of denaturation.
The Langmuir-Wrinch hypothesis of hydrophobic stabilization shared in the downfall of the cyclol model, owing mainly to the influence of Linus Pauling, who favored the hypothesis that protein structure was stabilized by . Another twenty years had to pass before hydrophobic interactions were recognized as the chief driving force in protein folding. (2025). 9780120342143 ISBN 9780120342143
Wrinch speculated that proteins are responsible for the synthesis of all biological molecules. Noting that cells digest their proteins only under extreme starvation conditions, Wrinch further speculated that life could not exist without proteins.
Wrinch also wrote a paper with William Astbury, noting the possibility of a keto-enol isomerization of the >CαHα and an amide carbonyl group >C=O, producing a crosslink >Cα-C(OHα)< and again converting the oxygen to a hydroxyl group. Such reactions could yield five-membered rings, whereas the classic cyclol hypothesis produces six-membered rings. This keto-enol crosslink hypothesis was not developed much further.
A large variety of closed polyhedra meeting this criterion can be constructed, of which the simplest are the truncated tetrahedron, the truncated octahedron, and the octahedron, which are or semiregular polyhedra. Considering the first series of "closed cyclols" (those modeled on the truncated tetrahedron), Wrinch showed that their number of amino acids quadratic growth as 72 n2, where n is the index of the closed cyclol Cn. Thus, the C1 cyclol has 72 residues, the C2 cyclol has 288 residues, etc. Preliminary experimental support for this prediction came from Max Bergmann and Carl Niemann, whose amino-acid analyses suggested that proteins were composed of integer multiples of 288 amino acid residues ( n=2). More generally, the cyclol model of globular proteins accounted for the early analytical ultracentrifugation results of Theodor Svedberg, which suggested that the of proteins fell into a few classes related by integers.
The cyclol model was consistent with the general properties then attributed to folded proteins. (1) Centrifugation studies had shown that folded proteins were significantly denser than water (~1.4 gram/liter) and, thus, tightly packed; Wrinch assumed that dense packing should imply regular packing. (2) Despite their large size, some proteins crystallize readily into symmetric crystals, consistent with the idea of symmetric faces that match up upon association. (3) Proteins bind metal ions; since metal-binding sites must have specific bond geometries (e.g., octahedral), it was plausible to assume that the entire protein also had similarly crystalline geometry. (4) As described above, the cyclol model provided a simple chemical explanation of denaturation and the difficulty of cleaving folded proteins with proteases. (5) Proteins were assumed to be responsible for the synthesis of all biological molecules, including other proteins. Wrinch noted that a fixed, uniform structure would be useful for proteins in templating their own synthesis, analogous to the Watson-Francis Crick concept of DNA templating its own replication. Given that many biological molecules such as carbohydrate and have a hexagonal structure, it was plausible to assume that their synthesizing proteins likewise had a hexagonal structure. Wrinch summarized her model and the supporting molecular-weight experimental data in three review articles.
Wrinch also predicted that insulin was a C2 closed cyclol consisting of 288 residues. Limited X-ray crystallographic data were available for insulin which Wrinch interpreted as "confirming" her model.
However, this interpretation drew rather severe criticism for being premature.
Careful studies of the Patterson diagrams of insulin taken by Dorothy Crowfoot Hodgkin showed that they were roughly consistent with the cyclol model; however, the agreement was not good enough to claim that the cyclol model was confirmed.
Wrinch replied to the steric-clash, free-energy, chemical and residue-number criticisms of the cyclol model. On steric clashes, she noted that small deformations of the bond angles and bond lengths would allow these steric clashes to be relieved, or at least reduced to a reasonable level. She noted that distances between non-bonded groups within a single molecule can be shorter than expected from their van der Waals radii, e.g., the 2.93 Angstrom distance between methyl groups in hexamethylbenzene. Regarding the free-energy penalty for the cyclol reaction, Wrinch disagreed with Pauling's calculations and stated that too little was known of intramolecular energies to rule out the cyclol model on that basis alone. In reply to the chemical criticisms, Wrinch suggested that the model compounds and simple bimolecular reactions studied need not pertain to the cyclol model, and that steric hindrance may have prevented the surface hydroxyl groups from reacting. On the residue-number criticism, Wrinch extended her model to allow for other numbers of residues. In particular, she produced a "minimal" closed cyclol of only 48 residues, and, on that (incorrect) basis, may have been the first to suggest that the insulin monomer had a molecular weight of roughly 6000 Da.
Therefore, she maintained that the cyclol model of globular proteins was still potentially viable and even proposed the cyclol fabric as a component of the cytoskeleton. However, most protein scientists ceased to believe in it and Wrinch turned her scientific attention to mathematical problems in X-ray crystallography, to which she contributed significantly. One exception was physicist Gladys Anslow, Wrinch's colleague at Smith College, who studied the ultraviolet absorption spectra of proteins and peptides in the 1940s and allowed for the possibility of cyclols in interpreting her results. As the sequence of insulin began to be determined by Frederick Sanger, Anslow published a three-dimensional cyclol model with sidechains, based on the backbone of Wrinch's 1948 "minimal cyclol" model.
Clarification of the modern terminology is appropriate. The classic cyclol reaction is the addition of the NH amine of a peptide bond to the C=O carbonyl group of another; the resulting compound is now called an azacyclol. By analogy, an oxacyclol is formed when an OH hydroxyl group is added to a peptidyl carbonyl group. Likewise, a thiacyclol is formed by adding an SH thiol moiety to a peptidyl carbonyl group.Wieland T and Bodanszky M, The World of Peptides, Springer Verlag, pp.193–198.
The oxacyclol alkaloid ergotamine from the fungus Claviceps purpurea was the first identified cyclol. The cyclic depsipeptide serratamolide is also formed by an oxacyclol reaction. Chemically analogous cyclic thiacyclols have also been obtained. Classic azacyclols have been observed in small molecules and tripeptides.
Peptides are naturally produced from the reversion of azacylols,
a key prediction of the cyclol model. Hundreds of cyclol molecules have now been identified, despite Linus Pauling's calculation that such molecules should not exist because of their unfavorably high energy.
After a long hiatus during which she worked mainly on the mathematics of X-ray crystallography, Wrinch responded to these discoveries with renewed enthusiasm for the cyclol model and its relevance in biochemistry.
She also published two books describing the cyclol theory and small peptides in general.
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