Poly helices stability

Poly(a-amino acids) serve as model substances for proteins. In the solid state, they occur in two forms. The a-form is a helix stabilized by intramolecular hydrogen bonding (see also Section 4.2.1). The )8-form has the pleated sheet structure (see also Figure 5.10). Because of intermolecular hydrogen bonding, this form is infusible and insoluble. The a form yields wool-like, the j3-form silklike fibers. 

Azo-modified polypeptides were also investigated in connection with their photochromic behaviour caused by the trans to cis photoisomerization of the azo groups present in the side chains. In methanol/water solvent mixture, the 20% azo-poly(L-lysine) adopts the a-helix conformation. The helix stability was found to be higher when the azo chains are in cis than when they are in trans configuration. Irradiation at 340 nm (trans to cis isomerization) and alternately at 450 nm (cis to trans isomerization) produced reversible variations of the a-helix content. In hexafluoro-2-propanol/water/sodium dodecylsulfate mixture, the 43% azo-poly(L-lysine) adopted a P-structure and promoted the a-helix conformation. The effect was reversed upon irradiation at 450 nm. The photoinduced P to helix change was explained on the basis of the different geometry and hydrophobic character of the trans and the cis... 

Fig. 18.—Antiparallel packing arrangement of 2-fold poly(ManA) (15) helices, (a) Stereo view of two unit cells roughly normal to the hoplane. The helix at the center (filled bonds) is antiparallel to the two in the back (open bonds). Intrachain hydrogen bonds stabilize each helix. Association of helices through direct hydrogen bonds involve the carboxylate groups for parallel chains, but involve the axial hydroxyl groups for antiparallel chains, (b) A view of the unit-cell contents down the t-axis highlights the interactions between the helices.

N 036 Helix-Coil Stability Constants for the Naturally Occurring Amino Acids in Water. III. Glycine Parameters from Random Poly(hydroxybutylglutamine-co-glycine)"... 

PI6.4 The tetramethylammonium ion (Me4N+) has been reported to stabilize oligonucleotides. Marky et a/.20 have studied its effect on the helix-to-coil transition in poly(dA-dT). They compared their results with similar studies conducted in solutions containing the sodium ion of the same concentration. The results are summarized below ... 

Kakiuchi, N., Marck, C., Rousseau, N., Leng, M., De Clerq, E. and Guschlbauer, W. (1982) Polynucleotide helix geometry and stability. Spectroscopic, antigenic and interferon-inducing properties of deoxyribose-, ribose-, or 2 -deoxy-2 -fluororibose-containing duplexes of poly(inosinic acid), poly(cytidylic acid). J. Biol. Chem., 257, 1924-1928. 

The above described model sequences have been studied both as oligomers [7,8,11-13,19] and as polymers [9,11,20]. An increase in the size of the helix is known to reinforce its stability, as revealed by their melting curves [18] and attested by X-ray diffraction measurements in solution [21]. Therefore, in this chapter we focus on the polymeric duplexes poly(dGdC).poly(dGdC) [= 1000 base-pairs], poly(dAdT).poly(dAdT) [= 200-400 base-pairs] and poly(dA).poly(dT) [= 2000 base-pairs] studied by us. First we discuss the absorption spectra, which reflect the properties of Franck-Condon states, in connection with theoretical studies. Then we turn to fluorescence properties fluorescence intensity decays (hereafter called simply fluorescence decays ), fluorescence anisotropy decays and time-resolved fluorescence spectra. We... 

Okamoto, Y., Mohri, H., Ishikura, M., Hatada, K., and Yuki, H., Optically active poly(diphenyl-2-pyridylmethyl methacrylate) asymmetric synthesis, stability of helix, and chiral recognition ability, J. Polymer Sci. Polymer Symp., 74, 125, 1986. 

Another important point to note is that the so-called standard 310 helix is outside the range of stability this result is also obtained for poly-L-alanine (Scott and Scheraga, 1966c) and, presumably, accounts for the fact that no 310 helices have been observed. 

In conclusion, it was found that complexes of poly(ethylene oxide)-h-poly(L-lysine) with retinoic acid with short poly(L-lysine) segments of 18 and 30 monomers form core shell micelles. The cores of the micelles contain a lamellar smectic A-like structure, formed by a poly(L-lysine) retinoate complex, which is surrounded by a corona of poly(ethylene oxide). Although the poly(L-lysine) chains are relatively short, they adopt an a-helical conformation to a pH as low as 9. This effective stabilization of the a-helix structure seems to be due to the formation of a protective surrounding coat of retinoate and a shell of poly(ethylene oxide). 

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