Photoswitchable molecular compounds containing transition metals (TM) are appealing candidates for the future of electronic and high-density data storage. Among them, Prussian Blue Analogs (PBAs) of general formula CxA4[B(CN)6](8+x)/3.nH2O (C= alkali cation; x=0-4; A,B = TM of the first row) present photoswitching properties depending on the TM and the stoichiometry; however these properties are usually observed at low temperature (below 150K). Our research aims at overcoming this working temperature issue that limits their applications. According to literature and our previous results, a promising solution seems a gentle modification of the distribution of the electronic density along the A-NC-B linkage via a slight structural distortion around the photoactive species. The goal of this MagDiDi project is therefore to completely elucidate the relationship between slight structural distortions and photoswitching properties, which will allow adjusting the working temperature. However, a tool able to quantify the slight structural distortions involved in the photoswitching properties of PBAs is still missing. So we propose here to develop a new methodology based on the X-ray Magnetic Circular Dichroism (XMCD) at the TM K-edge, which is a derived technique from X-ray Absorption Spectroscopy (XAS). Our first investigation under pressure (0-7 GPa) of the non-photoswitchable NiFe PBA family indeed demonstrated that XMCD is very sensitive to small structural distortions considered in the adjustment the photoswitching properties. Our first task is (i) to synthesize and characterize model PBAs, and (ii) to establish from a pivotal PBA a scale that related the variations of the XMCD signal to small structural distortions of the bimetallic cyanide network and to the lifetime of photoswitchable compounds; this scale will then be used to chemically adjust the working temperature of the latter. The second task consists in (i) identifying and disentangling the physical effects originating TM K-edge XMCD, which are still poorly understood, and (ii) establish the quantitative relationship between XMCD signals and structural distortions of the A-NC-B linkage, and consequently between structural distortions and photoswitching properties. These two tasks, which will especially enable to develop a new spectroscopic tool, will be achieved by a systematic study of PBAs from task 1 by XMCD at the K-edge of both TM present in the PBA; these PBAs will be chosen to independently vary the electronic and structural parameters that can have an effect on the XMCD signal. To go further, this experimental part will be coupled to a theoretical study of the XMCD signals for the most relevant PBAs using a DFT-based approach, already used to finely interpret x-ray absorption spectra. Once these two tasks are finished, the last task will be to apply our new tool to other families of compounds and/or compounds of smaller size (oxides, alloys, PBA nanoparticles…) in order to extend the use of the new spectroscopic tool to other scientific concerns. The results of this project, mainly fundamental, are at the interface between physics and chemistry. They will be a major step in our understanding of (i) the relationship between structure and photoswitching properties in PBAs as well as their derivatives (nanomaterials, molecules), and (ii) the physical processes behind the TM K-edge XMCD. We will thus improve our general knowledge on magnetism and on the interaction of X-ray with matter. Finally, the new methodology will be a priceless tool for all the communities interested in magnetic properties in relationship with the fine structure of materials, whatever the type of compound and its size.

Heparan sulfate (HS) are complex polysaccharides abundantly found in extracellular matrices and cell surfaces. These polysaccharides participate to major cellular processes through their ability to bind and modulate a wide array of signalling proteins. HS/ligands interactions occur through saccharide domains (termed S-domains) of specific sulfation pattern, present within the polysaccharide. Assembly of such functional domains is orchestrated by a complex biosynthesis machinery and their structure is further regulated at the cell surface by post-synthetic modifying enzymes, including extracellular sulfatases of the Sulf family. Sulfs specifically target HS S-domains and catalyze the selective removal of 6-O-sulfate groups, which are required for the recognition of many proteins. Although structurally subtle, these modifications have great functional consequences, and Sulfs have emerged as critical regulators of HS activity, in physiological processes such as embryogenesis and tissue regeneration, and in diseases such as cancer. There are two identified isoforms of Sulfs, Sulf-1 and Sulf-2, which share a very similar molecular organization. They are composed of two regions that are essential for enzyme activity: the catalytic domain (CAT-D), which includes the enzyme active site and is well conserved amongst sulfatases, and a highly basic, hydrophilic domain (Hyd-D), which is responsible for recognition and binding to HS substrates and is a unique feature of the Sulfs. However, despite increasing interest, Sulfs still remain poorly understood. During our recent studies of these enzymes, we have shed light on an original processive desulfation mechanism and on remarkable structural features. Based on these data, the SULF@AS project proposes to deliver an integrated study of the human isoforms HSulf-1 and HSulf-2, combining biochemical and biophysical approaches to characterize their structure and post-translational modifications ; in vitro, in cellulo and in vivo functional analysis to determine their substrate specificities and respective role during tumour progression ; and the development of specific inhibitors based on HS mimetics. This project should provide major insights into the regulatory role played by these enzymes in many biological processes and deliver the structural basis for the development of therapeutic strategies targeting HSulfs.