The relative stability of alpha-helix and beta-sheet secondary structure in the solid state was investigated using @entity12483 ( @entity1989 ) 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. @entity7352 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 @entity1989 before and after various sample treatments. Reprecipitation from helix-inducing solvents traps the alpha-helical conformation of @entity1989 , although the method of reprecipitation also affects the conformational distribution. Grinding converts the secondary structure of @entity1989 to a final steady-state mixture of 55% beta-sheet and 45% alpha-helix at room temperature regardless of the initial secondary structure. Grinding @entity1989 at liquid @entity1568 temperatures leads to a similar steady-state mixture with 60% beta-sheet and 40% alpha-helix, indicating that mechanical shear force is sufficient to induce secondary structure interconversion. Cooling the sample in liquid @entity1568 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% alpha-helix/50% beta-sheet at 100 degrees C when starting from a mostly alpha-helical state. No change was observed upon heating a beta-sheet sample, perhaps due to kinetic effects and the different heating rate used in the experiments. These results are consistent with beta-sheet approximately 260 J/mol more stable than alpha-helix in solid-state @entity1989 .

Determination of alpha-helix and beta-sheet stability in the solid state: a solid-state NMR investigation of XXXX .

@entity1989 :: ('CHEBI:60159', 'Chemical') :: ['PLA'] @entity12483 :: ('MESH:C019529', 'Chemical') :: ['poly(L-alanine)'] @entity7352 :: ('CHEBI:36928', 'Chemical') :: ['(13)C'] @entity1568 :: ('MESH:D009584', 'Chemical') :: ['nitrogen']
@entity12483 :: (MESH:C019529,Chemical) :: ['poly(L-alanine)']
