15N-NMR spectroscopy. IV. Comparison of poly(L-lysine) and isopoly(L-lysine)

The natural abundance ¹⁵N-nmr spectroscopy has been used to characterize the isomeric polymers (L -Lys)_n and iso (L -Lys)_n in aqueous solution. Although the peptide nitrogens of the two polymers have nearly equivalent shifts at pH < 10, the amino nitrogens differ by 5–6 ppm at pH < 7 and provide an easy means of identification. Furthermore, the polymers are distinguishable by the pK_a of the amino group and the basicity of the peptide nitrogen. At pH 10.3 and 25°C, (Lys)_n exhibits line broadening and an upfield chemical shift of the peptide nitrogen, indicative of the coil → helix transition. The formation of 100% helix may produce a shift as large as 5 ppm, which probably makes ¹⁵N-nmr spectroscopy more suitable for studies of this transition.