Field-Cycling 1H Relaxometry of biomolecules and paramagnetic proteins

Field-Cycling (FC) NMR relaxometry measures longitudinal relaxation rates (R1) as a function of magnetic field, yielding Nuclear Magnetic Relaxation Dispersion (NMRD) profiles whose fitting reveals molecular dynamics in both diamagnetic and paramagnetic systems. In paramagnetic systems, it can separate first-, second- and outer-sphere contributions to water-proton relaxation and determine key electronic parameters. We present fiRelax (https://firelax.cerm.unifi.it), a free web-based fitting platform with a graphical interface that requires no programming or theoretical expertise. It offers real-time analysis of experimental profiles and simulation of theoretical curves. Its computations run on the open-source One-Fit-Engine (OFE), written in Raku and C, into which the SBM and Florence models have been ported from Fortran for faster fitting. Case studies include paramagnetic systems with static Zero-Field-Splitting, analyzed with the (modified) Florence model, and macromolecules in human blood serum, analyzed with a model-free approach.