Understanding the unusual stiffness of hydrophobic dipeptide crystals

The hydrophobic diphenylalanine peptide crystal is known to be unusually stiff, with an experimental Young's modulus in the range of 19-27 GPa. Here it is shown by means of density functional theory calculations that phenylalanine-leucine, leucine-phenylalanine, alanine-valine, valine-alanine and valine-valine hydrophobic dipeptide crystals are also unusually stiff, with Young's moduli in the range of 19.7-33.3 GPa. To further our understanding of the origin of that unusual stiffness, a linear correlation is established between Young's modulus and the strength and orientation of the hydrogen bond network developed along the crystals, showing that stiffness in these materials is primarily dictated by hydrogen bonding.