Secondary structure of peptides: 8. 13C n.m.r. CP/MAS investigation of solid poly(l-leucines) and poly(d-norvalines) prepared from N-carboxyanhydrides |
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Authors: | Hans R Kricheldorf Detlef Müller |
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Institution: | Institut für Angewandte Chemie der Universität, Martin-Luther-King-Platz 6, D-2000 Hamburg 13, FRG;Bruker Analytische Messtechnik GmbH, Silberstreifen, D-7512 Rheinstetten/Fo., FRG |
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Abstract: | 13C n.m.r. CP/MAS spectra (50.3 and 75.4 MHz) of solid poly(l-lleucines) and poly(d-norvalines) measured with suitable acquisition parameters allow quantification of the composition of the secondary structure. The optimum acquisition parameters were found by systematic variation of the contact time by means of samples containing 5?0% α-helix structure. The polypeptides were prepared by primary or tertiary amine-initiated polymerizations of the corresponding amino acid NCAs and the average degrees of polymerization () were determined by 1H n.m.r. endgroup analysis. The mole fraction of α-helices increases with increasing ; it depends on the nature of the solvent and to a lesser degree on the polymerization temperature. When prepared under identical conditions, poly(d-norvaline) samples contain more β-sheet structure than poly(l-leucine. Reprecipitation increases the α-helix content, demonstrating that a part of the original β-sheet structure is thermodynamically unstable. The presence of oligomers of ?10 is mainly responsible for the thermodynamically stable part of the β-sheet structure. The chain growth mechanism is discussed. |
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Keywords: | Polypeptides α-helix/β-sheet ratio NCA polymerization molecular weight distribution secondary structure |
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