NMR of Paramagnetic Proteins: ¹³C Derived Paramagnetic Relaxation Enhancements Are an Additional Source of Structural Information in Solution

In paramagnetic metalloproteins, longitudinal relaxation rates of ¹³C′ and ¹³C^α nuclei can be measured using ¹³C detected experiments and converted into electron spin-nuclear spin distance restraints, also known as Paramagnetic Relaxation Enhancement (PRE) restraints. ¹³C are less sensitive to paramagnetism than ¹H nuclei, therefore, ¹³C based PREs constitute an additional, non-redundant, structural information. We will discuss the complementarity of ¹³C PRE restraints with ¹H PRE restraints in the case of the High Potential Iron Sulfur Protein (HiPIP) PioC, for which the NMR structure of PioC has been already solved by a combination of classical and paramagnetism-based restraints. We will show here that ¹³C R₁ values can be measured also at very short distances from the paramagnetic center and that the obtained set of ¹³C based restraints can be added to ¹H PREs and to other classical and paramagnetism based NMR restraints to improve quality and quantity of the NMR information.