The H-cluster active center of [FeFe]-Hydrogenase can be considered a strongly correlated electronic system in which the geometrical structure, spin densities, and electronic interactions play a significant role in its molecular function. Single crystal EPR has been successfully applied to [NiFe]-Hydrogenases due to the availability of relative large crystals (0.5 x 0.5 x 1.0 mm3). However, protein crystals from [FeFe]-Hydrogenase are significantly smaller (approximately 50 times). With the development of nano-EPR we will be able to access, for the first time, protein micro-crystals of sizes used for X-ray crystallography structure determination (nanoliter to sub-nanoliter volumes). The current project will be specifically focused on the [FeFe]-Hydrogenase enzymes, with its active site shown to the right, but the methods and technology are applicable to any protein crystal.