In order to examine the potential correlation between infrared absorption spectra and 3(10)- and alpha-helices and beta-bend ribbon structures, the secondary structures of synthetic peptides known to contain pure 3(10)-helices, mixed 3(10)/alpha-helices, and pure beta-bend ribbon structures, based upon X-ray diffraction and NMR studies, have been investigated by using FTIR spectroscopy incorporating resolution-enhancement techniques. Studies of the peptides known to contain a stable 3(10)-helix in CDCl3 show the main amide I band of fully stable 3(10)-helices occurs at 1666-1662 cm-1. Resolution-enhancement methods revealed small contributions at 1681-1678 and 1646-1644 cm-1, while the amide II band occurs at 1533-1531 cm-1. Peptides known to contain both alpha- and 3(10)-helices in their structure exhibit bands characteristic of both types of conformation. Peptides known to fold into the beta-bend ribbon structure show an amide I band maximum at 1648-1645 cm-1 with the amide II band at 1538-1536 cm-1. Incorporation of these peptides into model membrane structures, e.g., DMPC vesicles, in aqueous buffer sometimes produces changes in the peptide secondary structure. Those peptides which possess a 3(10)-helical structure in CDCl3 solution change the secondary structure in DMPC vesicles to predominantly alpha-helical, plus a contribution from short, unstable 3(10)-helix and/or beta-turns. Those peptides which contain a combination of alpha- and 3(10)-helical structures in CDCl3 solution tend to retain some 3(10)-helical structure within the lipid environment, although the overall H-bonding pattern is altered. Those peptides which form a beta-bend ribbon structure appear to be largely unaffected in the membrane environment.(ABSTRACT TRUNCATED AT 250 WORDS)
residues, as determined by conformational energy computations, X-ray diffraction analyses, and spectroscopic studies, are reviewed. The results obtained indicate that the 3io-helix and the fully extended (Cg) conformation are preferentially adopted by long sequences of these C" "-disubstituted -amino acid residues, depending upon bulkiness and nature (whether linear or cyclic) of their side chains.
The IR absorption and NMR analysis of the preferred conformation of monodisperse Z-(Aib)"-OtBu (Z = (benzyloxy)carbonyl, OtBu = tert-butoxy, Aib = -aminoisobutyric acid; = 1-12) strongly indicates the formation of fully developed stable 310-helices for the higher (n = 8-12) homooligomers (pleionomers). This experimental study was performed in the solid state as well as in deuteriochloroform solution, in the latter case as a function of concentration, temperature, and addition of dimethyl sulfoxide and the free radical Tempo. A picture of the mode of self-association of the helical structures has also been obtained. A detailed theoretical study of Aib IV-acetyl-lV'-methyl amide by conformational energy computations indicates that the fully extended structure is less stable than the helical structures, irrespective of the actual value of the N-C"-C' valence angle.
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