Nanocrystals composed mainly of beta-sheet polyalanine are responsible for the high toughness of major ampullate (dragline) spider silk. Fourier-transform infrared (FTIR) spectroscopy is employed to study their response to (i) uniaxial stress and (ii) hydrostatic pressure. In the former a red shift and in the latter a blue shift of the vibration of polyalanine beta-sheets at 965 cm(-1) occurred. In both cases, a linear dependence is observed, which bends off for hydrostatic pressure greater than 1.4 GPa and is fully reversible up to 7 GPa. The seamless connection of negative and positive pressure regimes corroborate quantitatively our structural model of spider silk as composed of prestressed alanine-rich nanocrystals embedded in a glycine-rich amorphous matrix. It is also confirmed that nanocrystals withstand high pressures without undergoing structural transition or deteriorating their mechanical properties.