Rod like Polypeptides

Tohyama K, Miller WG (1981) Network structure in gels of rod-like polypeptides. Nature... 

The main-chain of PG considered in this work takes the a-helical form like a long rod. At temperatures above the melting point of the side-chain crystallites the side chains take the liquid like phase such as liquid -paraffms and are working as a solvent for the main chain. The rod like polypeptide is diffusing as reported previously. The diffusion process of the polypeptide is assumed to foUow the Kirkwood theory [100] for the diffusion process of rod like polymers. 

Figure 14.1 Each polypeptide chain in the collagen molecule folds into an extended polyproline type II helix with a rise per turn along the helix of 9.6 A comprising 3.3 residues. In the collagen molecule three such chains are supercoiled about a common axis to form a 3000-A-long rod-like molecule. The amino acid sequence contains repeats of -Gly-X-Y- where X is often proline and Y is often hydroxyproline. (a) Ball and stick model of two turns of one polypeptide chain.

The secondary level of structure in a protein is the regular folding of regions of the polypeptide chain. The two most common types of protein fold are the a-helix and the P-pleated sheet. In the rod-like a-helix, the amino acids arrange... 

Under appropriate conditions, all reduced polypeptides bind the same amount of sodium dodecylsulfate (SDS), that is, 1.4 g SDS/g polypeptide. Furthermore, the reduced polypeptide-SDS complexes form rod-like particles with lengths proportional to the molecular weight of the polypeptides. This forms the basis for the empirical estimation of the molecular weight (Mr) of proteins using SDS-PAGE (Weber et al, 1972) according to... 

It may be ccmsidered that the molecular cluster in which rod-like molecules align nearly parallel to the cluster axis, itself forms a crystallite. The intensity of the X-ray diffraction for the incident beam that is perpendicular to the magnetically oriented fflm of polypeptide, /, is well expresred in the form (38) ... 

Liquid crystalline phases other than the cholesteric phase can exist in concentrated solutions of polypeptides and rod-like molecular clusters are formed (or separated from domains) easily under the action of an electric field, a magnetic field or shearing stresses. 

When a-helices form in synthetic polypeptides at the air-water interface, their rigid rod-like nature promotes side-by-side association of the molecules into highly ordered arrays or micelles, just as liquid crystalline structures form in solution at sufficiently high concentration (II). When such a monolayer is compressed on a Langmuir trough, the pressure rises when the surface area has reached a value expected for close-packed a-helices. At a pressure which appears a characteristic for the polymer, a transition is observed which is either an almost flat plateau in the pressure-area curve or simply an inflexion, flrst noted by Crisp (13), if the side chain is short (12). An inflexion also occurs if the side chain is inflexible. Normally the pressure rises again as the area is decreased, and in some instances further transitions are observed (14). 

The /S-pleated structure is markedly different from the a helix in that it is like a sheet rather than a rod. The polypeptide chain is almost fully extended, and each chain forms many intermolecular hydrogen bonds with adjacent chains. Figure 25.13 shows the two different types of /S-pleated structures, called parallel and antiparallel. Silk molecules possess the /S structure. Because its polypeptide chains are already in extended form, silk lacks elasticity and extensibility, but it is quite strong due to the many intermolecular hydrogen bonds. 

A rod-like particle model in which the polypeptide chain forms the core of a rod of 3.6 nm diameter with surfactant bound along its length. The rod is not totally rigid and has flexible regions between short rigid segments. 

A pearl necklace model in which the polypeptide chain forms the string of the necklace and the surfactant molecules form micelle-like clusters along the polypeptide chain, which passes through the micellar clusters in a a-helical conformation. In contrast to the rod-like particle model, this model assumes that the polypeptide chain is flexible. 

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