P Spiral

Poly(VPGVG) (Fig. 6) has been smdied most thoroughly and it was shown that it exhibits an inverse phase transition. The biopolymer undergoes phase separation from solution upon increasing temperature, resulting in a p-spiral structure and simultaneous release of water molecules associated with the polymer chain (Fig. 7). 

Fig. 7 (a) Recurring P-tum found in poly(VPGVG). (b) P-spiral structure adopted by the poly(VPGVG) upon raising the temperature above the inverse transition temperature. Reprinted from [22] with permission from Elsevier, copyright 1992... 

A spider s orb-web is formed by extrusion of a concentrated protein solution and stretching of the resulting fiber. The cross-strands, which are stronger than steel, resemble silkworm silk. The molecules contain microcrystalline p sheet domains that are rich in Gly-Ala repeats as well as polyalanine segments. The capture spiral is formed from much more elastic molecules that contain many -tum-forming sequences. These assume a springlike p spiral. See Box 2-B. 

A chain of repeated P bend motifs can form a flexible spring, a P spiral as proposed for elastin. Genes for spider silk have been cloned and are being used to engineer new proteins with commercial uses, e.g., to help anchor cells in regenerating body tissues. d/h/ ... 

Despite different sequences and repetitive motives, all gliadins have the same secondary structure of loose spirals which are a balanced compromise between the p-spiral and poly-L-proline structure (polyproline helix II) (Parrot et al., 2002), the balance is dependent on temperature, type of solvent, and hydration level (Miles et al., 1991). Similar sequences can be found in other proteins, mainly animal proteins such as elastin and collagen, and they are responsible for particular biomechanical properties connected to reverse P-spirals or p-sheet structures (Tatham and Shewry, 2000). 

This represents a set of p spirals, one on each sublattice v, having respective phases 0 . It is the only configuration derivable from equation 145 that has, to within rotations of the plane of the spins, Sly independent of n. With this equation, the strong constraints of equation 134 become p equations in the p unknowns 0. . . 0P ... 

Kasarda, D.D., King, G., and Kumonsinski, T.F. (1994). Comparison of p-spiral structures in wheat high molecular weight glutenin subunits and elastin by molecular modelling. In ... 

Bennema, P. Spiral growth and surface roughening development since Burton, Cabrera and Frank. J. Cryst. Growth 1984, 69, 182-197. 

Seminal work by Urry on cyclic analogs of elastin revealed a Pro-Gly type II P-tum by NMR and crystallography, which served as the basis for the P-spiral model (22,23,24). The model described a structure in which consecutive p-tums formed a helical arrangement. In this structure, there was one type II P-tum per pentameric unit of elastin, which served as spacers between the spiral turns. Further evidence for this p-spiral stmcture was compiled by molecular dynamic... 

D-amino acid residue on the right (an optical isomer that does occur in biology, but in those peptides not encoded for by the genetic code). C The effect of insertion of a D-amino acid residue in an otherwise L-amino acid residue protein in the P-spiral structure of the elastic-contractile model protein of our focus would be to disrupt the regular structure. This is difficult to avoid completely in chemical synthesis, and it increases the temperature at which occurs the inverse temperature transition and decreases the heat of the transition due to less optimal association of oil-like groupings. 

The caleulated T, value for Pro comes from poly(GVGVP) when the experimental values of Val and Gly are used. This hydrophobicity value of -8° C is unique to the P-spiral structure, where there is hydrophobic contact between the Val) yCHs and the adjacent Pro 5CH2 and the intertum Pro +3pCH2 moieties. 

For simplicity, we often display a single chain in the P-spiral structure in reality, however, we do not expect such to form readily in isolation, but rather as part of a twisted filament as in Figure 5.6F. Accordingly, the schematic representations of the P-spiral structure of Figures 5.6C-E are regularly used to simplify but adequately make the intended point. 

Figure 5.6. Description of the molecular structure of the parent model protein (GVGVP) , which should be recognized as equivalent to (VPGVG) . (A) A series of P-turns involving residues VPGV. (B) The P-turn obtained from the crystal structure of the cyclic conformational correlate cyclo(GVGVP)3 shown in Figure 5.4B. (C,D) Schematic helical representations without P-turns (in C) and showing p-turns as spacers between turns of a helix, called the P-spiral. (E) Cross-eye stereo view of the poly(GVGVP) p-spiral in side view below and in axis view above. (F) Cross-eye stereo view of the twisted filament composed of three P-spirals. Before passing...

Figure 5.8. Hydrophobic association to produce increased elastic force at fixed length, as experimentally shown in Figure 5.7. (Top) A series of clamshaped globular proteins, strung together by an elastic band attached near the mouth, are at equilibrium between open and closed states due to hydrophobic association. As the distribution shifts toward more closed states, the elastic force increases. (Bottom) A P-spiral with an equilibrium between...

Figure 5.32. As indicated below the schematic P spiral representations of the structures, stepwise increases in hydrophobicity as Phe replaces Val result in supralinear increases in carboxyl pKa values...

D.W. Urry, C.M. Venkatachalam, M.M. Long, and K.U. Prasad, Dynamic p-Spirals and A Librational Entropy Mechanism of Elasticity. ... 

Figure 9.17. Stereo views (cross-eye) of molecular structure and proposed acoustic function of the poly(VPGVG) family of jj-spiral structures. The leftmost structure is the schematic p-spiral showing P turns to function as spacers between turns of the spiral. The central pair of structures provide details of bonding in side view and axis view (above). TTie rightmost structure shows P-turns in the role of...

Description of the mechanics of elastin requires the understanding of two interlinked but distinct physical processes the development of entropic elastic force and the occurrence of hydrophobic association. Elementary statistical-mechanical analysis of AFM single-chain force-extension data of elastin model molecules identifies damping of internal chain dynamics on extension as a fundamental source of entropic elastic force and eliminates the requirement of random chain networks. For elastin and its models, this simple analysis is substantiated experimentally by the observation of mechanical resonances in the dielectric relaxation and acoustic absorption spectra, and theoretically by the dependence of entropy on frequency of torsion-angle oscillations, and by classical molecular-mechanics and dynamics calculations of relaxed and extended states of the P-spiral description of the elastin repeat, (GVGVP) . The role of hydrophobic hydration in the mechanics of elastin becomes apparent under conditions of isometric contraction. 

The structure involves a series of p-tums, ten-atom hydrogen-bonded rings from the Val C— O to the VaP N—H (Figure 3A and B) which, upon raising the temperature, wrap up into a helical structure called the p-spiral (shown schematically in Figure 3C and D and in detail in Figure 3E). It is shown that the P-tums function as hydrophobic spacers between the turns... 

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