Triple helix model

In a refined staircase model the exact orientation of the planar groups is described using three angles. The authors 145) conclude that the triple helix model can be used to account for the optical rotation of helicenes, but that more sophisticated quantum mechanical calculations can better be based on the refined staircase model. 

Kinetic Aspects of Triple-Helix Formation of Peptide Models Compared with... 

This gives an important hint at what kind of model peptides are synthesized to obtain detailed information about the thermodynamics and kinetics of the collagen triple-helix formation. A first success was already achieved by synthesizing peptides of the following general structure37 ... 

In the case of cooperative processes, the formation of a nucleus, already discussed from the kinetical point of view, plays a crucial role. The steady state described by Eq. (1) depicts the formation of a triple helix as the simplest model by the formation of a nucleus Hx through fast pre-equilibria and subsequent propagation steps, Hx in this case is a triple-helical intermediate with x tripeptide units (that means x hydrogen bonds) in the helical state. The final product H3n 2 possesses two hydrogen bonds less than tripeptide units because the three single chains are staggered at one amino add residue each. 

This model does not say anything about the mechanism of triple-helix formation, because even in the case of an AON mechanism, nucleation may take place at many positions of the chains and may lead to products the chains of which are staggered. The AON model is based on the assumption that these products are too instable to exist in measurable concentration. As already mentioned, Weidner and Engel142 succeeded in proving by relaxation measurements of al CB2 that the kinetics of in vitro triple-helix formation is governed by more than one relaxation time. This rules out an AON mechanism, but the fitting to the experimentally found equilibrium transition curves nevertheless showed good accommodation and AH° computed from these curves could be confirmed by calorimetric measurement. 

Fig. 39. Dependence of AHC of triple-helix formation on the proline content of the CNBr peptides and synthetic model peptides, respectively...

The amino acids proline and hydroxyproline exert a stabilizing influence on the triple helix as described in detail in Sect. 4.5. By examining the CB peptides of collagen, a structural stability which is directly proportional to the itnino acid content may thus be found. It has, however, not been possible to synthesize model peptides displaying structural stability comparable to that of the native peptides having corresponding amino acid contents. 

Figure 1 The collagen triple helix, (a) Colored by element C (green), O (red), N (blue), S (yellow) (b) colored by chains and (c) colored by Gly (red), Xaa (yellow), and Yaa (white) positions. The modeis were buiit using PyMOL with the type iii coiiagen model peptide (PDB accession number 3DMW). Glycine residues in every third position are located on the inside of the triple helix. Each chain is staggered by one residue to accommodate the Gly in the center of the triple helix.

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