Polypeptides conformational behavior

The conformational behavior can be quite different in aqueous solution. Below pH 5, a sample of poly(L-glutamic acid) containing about 30 mol% azobenzene units adopts a P-structure that is not affected by light. Above pH 7, the polypeptide is random coil and the conformation is, once more, not affected by the photoisomerization of the azo side chains. However, at pH values in the range 5-7 (close to the pK of the conformational transition), irradiation causes a remarkable diminishing of the ordered structure, which is completely reversed in the dark.120,221... 

The different conformational behavior of the azobenzoyl- and the azobenzenesul-fonyl-L-lysine polymers was explained on the basis that the monomeric units VI may interact with HFP differently than units V do (Scheme 4). The strongly proto-nating solvent HFP (pKa = 9.30) 36 is known to form electrostatic complexes with various organic compounds, including amines and dimethylsulfoxide 1371 on the other hand, sulfonamides are significantly protonated in acid media 38 so it may be presumed that protonation and formation of electrostatic complexes can occur for azobenzenesulfonyl-L-lysine residues, as well. In HFP therefore, polypeptides of structure V can adopt the ordered a-helix structure, while polypeptides of structure VI should be forced by the electrostatic interactions arising from complexation with HFP to adopt a disordered conformation. 

The study of the conformational behavior of polypeptides has intrinsic interest in a complex and challenging theoretical and experimental problem. There are also strong biological implications inasmuch as there are many known naturally occurring polypeptides, both linear and cyclic, with potent effects as hormones, toxins, antibiotics, and ionophores. It is very probable that these functions are closely related to the polymer chain conformations. Investigations of model polypepetides have been extremely useful for the interpretation of conformational behavior and for the elucidation of the interaction between proteins and other macromolecules. The conformational structures of polyproline and related compounds have been of particular interest due to the important role L-proline plays in effecting protein structure. 

The photochromic reactions schematized in Figure 14.9 and the above discussed absorption spectra allow us to explain the conformational behavior. In HFP, in the presence of TFA, the photochromic side chains are protonated by the acid either when the sample is kept in the dark (photochromic units present as open species III) or when the sample is exposed to light (photochromic units present as closed species IV). In both cases the polypeptide is essentially a polycation, so the repulsive forces among the side chains make the macromolecules adopt an extended coil conformation, and no photoresponse is observed. 

N., and Lenci, F. (1999) Photoresponsive polypeptides, photochromic and conformational behavior of spiropyran-containing poly(l-glutamate)s under acid conditions. Macromolecules, 32, 7116—7121. 

MacRitchie " and Dickinson and Stainsby " have reviewed the behavior of proteins at a variety of interfaces. There have also been a number of recent research papers which have taken a theoretical or computational approach examples include the computation of the electrostatic interaction energy between a protein and a charged surface the computation of electrostatic and van der Waals contributions to protein adsorption " Monte Carlo simulation of the conformational behavior of a polypeptide chain near a charged surface " protein structure prediction based on statistical potential development of a model system for the interaction... 

The changes in structure of denatured nuclease as a function of urea concentration (Fig. 3) suggest that, as hydrophobic interactions are weakened and the backbone becomes more highly solvated, the chain expands gradually. The data presented by Millet et al. in this volume suggest that this expansion does not continue asymptotically as predicted by simple polymer physical chemistry. This is the behavior expected for a polypeptide chain trapped in a small region of conformation space. Most, perhaps all, of the conformations accessible in the expanded denatured state may have a native-like topology. 

Polymeric phospholipids based on dioctadecyldimethylammonium methacrylate were formed by photopolymerization to give polymer-encased vesicles which retained phase behavior. The polymerized vesicles were more stable than non-polymerized vesicles, and permeability experiments showed that vesicles polymerized above the phase transition temperature have lower permeability than the nonpolymerized ones [447-449]. Kono et al. [450,451] employed a polypeptide based on lysine, 2 aminoisobutyric acid and leucine as the sensitive polymer. In the latter reference the polypeptide adhered to the vesicular lipid bilayer membrane at high pH by assuming an amphiphilic helical conformation, while at low pH the structure was disturbed resulting in release of the encapsulated substances. 

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