X-ray crystallography at ultra-high resolution enables to observe the deformation of the atomic electron densities due to chemical bonding and intermolecular interactions. These can be quantitatively analysed via charge density studies, a mature and dynamic branch of modern crystallography, which provides important chemical information (charges, bond order, nucleophilicity…) on the atoms. For the analysis of the increasing number of protein structures measured at subatomic resolution, LCM3B has developed the program MoPro and a multipolar library describing the electron density of atoms. The laboratory has pioneered in extending these methodologies from small molecules to proteins with the multipolar refinement of crambin and human aldose reductase. The excellent quality diffraction data collected for aldose reductase, an enzyme involved in diabetes complications, will allow further unprecedented analysis of the electron density in alpha-helices and beta-strands. Preliminary results demonstrate the feasibility of the crystallographic refinement as the polypeptide main-chain groups C=O…H-N show significant differences in atoms polarization. These will be quantified experimentally with software MoPro and confirmed by quantum chemical computations. The strong background of the applicant in quantum chemistry will be advantageous to analyse the relationships between atoms polarization, helix macro-dipole and hydrogen bonding geometry and strength in proteins. In the second phase of the project, it is aimed to improve the accuracy of electrostatic interaction energies computed on protein-substrate systems by using a multipolar atom model compared to point charges. The structures of complexes of human aldose reductase with several medicinal inhibitors are determined at high resolution. The electron density will be transferred from our library of multipolar atoms. This will allow analysing the ligands binding strength and the specificity with respect to the proteins.