Integrated 'H peak intensities were measured using a 500 MHz NMR spectrometer in 99.99% D20solution for six compounds (n = I to 6) in the structural series Ac-Hel (Ala4-LyS)3-Alan-NH2. This series comprises alanine-rich, lysine-containing peptides that are covalently linked at their N-termini to Ac-Hel, which is a conformationally restricted modification of the dipeptide acetyl-prolyl-proline. Previous work in our laboratory has shown that Ac-Hel acts as a reporting conformational template that initiates helicity in peptides to which it is linked. The NMR spectra of these conjugates show two sets of resonances corresponding to slowly equilibrating s-cis and s-trans acetamido conformations. We have previously shown that the relative abundance of these conformations provides an incisive measure of the helical stability of a peptide. Concurrent circular dichroism measurements made on these peptides, taken together with these t/c measurements, showed that the helical propensities of the alanine residues in the C-terminal region must be close to 1.0, consistent with other measurements in our laboratories on shorter templated peptides, but inconsistent with the widely accepted published values from Professor Robert Baldwin's laboratory at Stanford. Although circular dichroism is the primary tool used by the biological community to monitor helicity of polypeptides in solution, the key parameters used to relate the measured ellipticity values to fractional helicity have not been rigorously calibrated. The t/c values obtained by NMR measurement on Ac-Hel conjugates of peptides provide an independent measure of helicity. We are in the process of accumulating a data-base of medium-sized peptides that, when complete, is expected to permit refinement and recalibration of the CD-derived helicity scale. The six members of this structural series studied by Mr. Kennedy at CMR are expected to be key members of this data-base.
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