Coiled coil structures compound helices

Other polysaccharides studied by ROA are lami-narin [16] and pullulan [18]. Laminarin, which is a P(l—>3) linked polymer of D-glucose, adopts a triple helix conformation. This is concluded by comparison with the ROA of the compound that corresponds to laminarin s dimer subunit, D-laminaribose. Pullulan is a linear polymer of D-glucose, consisting of D-mal-totriose units connected through a(1 6) glycosidic links. It adopts a random coil structure in aqueous solution, as can be deduced by the similarity of its ROA spectrum to that of D-maltotriose. 

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

Later work by Carlstrom, Miller and Bryant has thrown some doubt on to whether the reaction product obtained by Bryant actually was compound (I) above [424], Attempts to repeat the synthesis of (I) met with uniform failure and frequently yielded instead a silver-containing polyelectrolyte of unknown structure (silver ion was used as a catalyst). It seems quite possible that the line width changes observed by Bryant [422] reflect an entirely different process than a helix-random coil transition. 

Two opposing hypotheses attempted to describe the conformation of the a-l,4-linked glucopyranoside polymers in neutral aqueous solutions the random coil and the segmented helix structure hypotheses. The former one was based mainly on hydrodynamic studies of amylose solutions, the latter on many very different observations, but mainly on the formation and properties of amylose-helix complexes. The formation of cyclodextrins, catalysed by cyclizing enzymes, delivers further proof for the helical structure, and simultaneously is the source of a new technology the molecular encapsulation of different compounds by cyclodextrin complexation. The significance of the cyclodextrins and their derivatives in commercial applications will be discussed. 

Synthetic polypeptides consist of repeating sequences of certain amino acids and their structures are not as complicated as those of proteins. Eor this reason, synthetic polypeptides are sometimes used as model compounds for proteins. Their preferred conformations are classified as a helix, P sheet, 0) helix and so on. High-resolution solid-state C NMR spectroscopy has proved to be a very powerful tool for determining the structure of polypeptides in the crystalline state. Eor example, C CP MAS NMR spectra of solid poly(L-alanine) ([Ala] J show the Ca, Cp and C=0 carbon signals to be well resolved between the a helix and P sheet forms. The chemical shifts of the Ca and C=0 carbons of the a helix are displaced sig nilicantly to high frequency by 4.2 and 4.6 ppm, re spectively, relative to those of the P sheet form, while the shift of the Cp carbon of the a heUx is displaced to low frequency by about 5 ppm with respect to that of the P sheet. Eor this reason, the value of the C shift can be used to describe the local conformation. In addition, the C shifts of randomly coiled [Ala] in trifluoroacetic acid solution have values between those of the a helix and P sheet forms. The absolute C shifts of the Ca and Cp carbons are affected by the chemical structure of the individual amino acid residues and can be used effectively for conformational studies of particular amino acid residues in polypeptides and proteins. On the other hand, the C=0 shifts do not seem to be affected by residue structure and can be used for diagnosing the main chain conformation. 

Apart from intrinsically asymmetric chromophores, it is the existence of a dissymmetry in the space around the chromophores that makes their electronic transitions optically active. This dissymmetry is the result of the presence of one or more asymmetric centres (usually carbon atoms) and thus depends on structural factors (configuration, conformation). If the experimental or external conditions are modified (solvent, temperature, solution-composition), then there is a change in the conformational equilibrium which has some effect on the optical activity, thus justifying the interest of ORD and CD in the structural analysis of optically active compounds and particularly of helical conformation or helix-coil transitions of biopolymers. 

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