How the H-Bond Layout Determines Mechanical Properties of Crystalline Amino Acid Hydrogen Maleates

The stiffness tensor and elastic anisotropy characteristics for the crystalline hydrogen maleates of Lisoleucinium, L-leucinium, and L-norvalinium with L-norvaline have been calculated using the periodic DFT calculations and atom-centered basis sets. The H-bond orientations have been compared with spatial directions of the minimum and maximum values of Young's modulus, shear modulus, and linear compressibility. In spite of the similar layered structures, L-isoleucinium and L-leucinium hydrogen maleates show significant difference in elastic moduli anisotropy. The flexibility of L-leucinium hydrogen maleate is explained by the relatively high universal elastic anisotropy index and the large anisotropy ratios of elastic moduli. In its turn, this index is determined by the almost coincidental Young's modulus maximum direction and the orientation of the strongest H-bonds.