Determination of -Helix and -Sheet Stability in the Solid State: A Solid-State NMR Investigation of Poly(L-Alanine)

The relative stability of -helix and -sheet secondary structure in the solid state was investigated using poly(L-alanine) (PLA) as a model system. Protein folding and stability has been well studied in solution, but little is known about solid-state environments, such as the core of a folded protein, where peptide packing interactions are the dominant factor in determining structural stability. C cross-polarization with magic angle spinning (CPMAS) NMR spectroscopy was used to determine the backbone conformation of solid powder samples of 15-kDa and 21.4-kDa PLA before and after various sample treatments. Reprecipitation from helix-inducing solvents traps the -helical conformation of PLA, although the method of reprecipitation also affects the conformational distribution. Grinding converts the secondary structure of PLA to a final steady-state mixture of 55% -sheet and 45% -helix at room temperature regardless of the initial secondary structure. Grinding PLA at liquid nitrogen temperatures leads to a similar steady-state mixture with 60% -sheet and 40% -helix, indicating that mechanical shear force is sufficient to induce secondary structure interconversion. Cooling the sample in liquid nitrogen or subjecting it to high pressure has no effect on secondary structure. Heating the sample without grinding results in equilibration of secondary structure to 50% -helix/50% -sheet at 100°C when starting from a mostly -helical state. No change was observed upon heating a -sheet sample, perhaps due to kinetic effects and the different heating rate used in the experiments. These results are consistent with -sheet approximately 260 J/mol more stable than -helix in solid-state PLA. © 2002 Wiley Periodicals, Inc. Biopolymers 64: 246–254, 2002