Secondary structure of peptidese: 13. 15N and 13C n.m.r. spectroscopic investigation of the helix stability of poly(l-alanine) in acidic solutions

¹⁵N-enriched poly(l-alanines) of various molecular weights were prepared from $\text{l}\text{-alanine}\text{-N-}\text{carboxyanhydride}$ (l-Ala-NCA) and their helix/coil equilibrium in trifluoroacetic acid (TFA) investigated by means of 40.5 MHz ¹⁵N nuclear magneic resonance (n.m.r.), 22.3 MHz ¹³C n.m.r. and circular dichroism (c.d.) spectra. The ¹⁵N n.m.r. spectra exhibit at least three peaks, and the dependence of their intensities on molecular weight, molecular weight distribution and temperature, as well as dynamic nuclear Overhauser effect (NOE) measurements, indicate that the high-field peak represents the helix fraction. All three spectroscopic methods agree that a helix→coil transition takes place with decreasing concentration. Furthermore, poly(l-alanines) containing d-alanine or glycine in various mole ratios were synthesizsed by copolymerizations of N-carboxyanhydrides (NCAs). The ¹⁵N n.m.r. spetra demonstrate that one d-Ala unit per 100 l-Ala units suffices to affect significantly the helix/coil equilibrium in TFA. In other words, the helix content under equilibrium conditions is highly sensitive to racemization. Furthermore, ¹³ C n.m.r. cross-polarization/magic angle spinning (CP/MAS) spectra demonstrate that the presence of d-Ala units also affects the α-helix content in the solid state.