Structure and Dynamics of Poly-Peptides
In the solid-state polypeptides adopt characteristic secondary structures like α-helices or β-sheets. Remarkably, most polypeptides can reside in both secondary structures and the predominant structure is determined by factors like chain length and solidification conditions. In order to gain a better understanding for the formation of secondary structures, the behavior of two series of poly(γ-benzyl-L-glutamate) with variing chain length from different synthetic routes have been studied by x-ray diffraction, DSC, solid state NMR and dielectric spectroscopy.
Typically, α-helices are the predominant secondary structure for long PBLG polypeptides, while the β-sheet content increases for shorter polymer chains. Quantitative 13C CP-MAS NMR has been used in order to determin the quantitative ratio between the secondary structures by simple signal integration, as the Cα signals in the different secondary structures are separated by more than 5 ppm, shifting from 58 ppm in α-helices to 52 ppm in β-sheets.
The secondary structure determined by solid-state NMR probing the local electronic environment of the structure forming Cα site shows a smooth transition from an equilibrated 50:50 ratio extrapolated to monomeric PBLG toward a pure α-helical state for PBLG chains longer than 40 monomeric units. An exponential chain length dependence shown in color in the figure above describes very well the determined behavior. The results obtained from modulates DSC methods to determine the chain length dependent secondary structure of the PBLG chains match those obtained from solid state NMR.
Using x-ray diffraction probing the stacking of β-sheets and the hexagonal ordering of α-helices to determine the ratio of secondary structures in the sample, however, a much more stepwise behavior is observed with a step towards complete α-helical structure for PBLG chains with more than 20 monomeric units. The discrepancy illustrated in the left part figure below, indicates that a faster crystallization of β-sheets leads to larger better ordered β-sheet crystals which hamper the complete crystallization α-helices in the sample.
In the right part of the figure above the amplitude of fluctuations of the Cα site in PBLG with 14 monomeric units is monitored as a function of temperature. It shows that the intramolecular α-helical structure is much softer and more mobile at any temperature.
Literature and References