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Alanine-rich peptides serve as models for exploring the factors that control helix structure in peptides and proteins. Scalar C alpha H-NH couplings (3JHN alpha) are an extremely useful measure of local helix content; however, the large alanine content in these peptides leads to significant signal overlap in the C alpha H region of 1H 2D NMR spectra. Quantitative determination of all possible 3JHN alpha values is, therefore, very challenging. Szyperski and co-workers [(1992) J. Magn. Reson. 99, 552-560] have recently developed a method for determining 3JHN alpha from NOESY spectra. Because 3JHN alpha may be determined from 2D peaks outside of the C alpha H region, there is a much greater likelihood of identifying resolved resonances and measuring the associated coupling constants. It is demonstrated here that 3JHN alpha can be obtained for every residue in the helical peptide Ac-(AAAAK)3A-NH2. The resulting 3JHN alpha profile clearly identifies a helical structure in the middle of the peptide and further suggests that the respective helix termini unfold via distinct pathways.

Kinetic analysis of the time-dependent data suggests a model whereby the amorphous aggregate has a previously unsuspected dual role: it releases monomer into solution and also provides initiation sites for fibril growth. These findings suggest that the beta-sheet-rich PrPSc may be stabilized by aggregation.

Physical chemistry laboratory to investigate the thermally controlled critical demixing of binary fluid mixtures; includes experimental setup, sample data, and analysis.

No abstract