Using a combination of explicit solvent atomistic simulation and continuum theory, here we study the lateral deformation mechanics of three distinct protein structures: an amyloid fibril, a beta helix, and an alpha helix. We find that the two b-sheet rich structures -amyloid fibril and beta helix, with persistence lengths on the order of mm -are well described by continuum mechanical theory, but differ in the degree to which shear deformation affects the overall bending behavior. The alpha

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... al protein structure, however, with a persistence length on the order of one nanometer, does not conform to the continuum theory and its deformation is dominated by entropic elasticity due to significant fluctuations. This study provides fundamental insight into the nanomechanics of widely found protein motifs and insight into molecular-scale deformation mechanisms, as well as quantitative estimates of Young's modulus and shear modulus in agreement with experimental results.