Quantification of inhomogeneous local proton density distributions in soft matter, unstructured macromolecules and non-Anfinsen biomacromolecules

A new method to quantitatively analyse inhomogeneous distributions of local proton densities around paramagnetic centers in unstructured bio-polymers, disordered biomolecules and soft matter is introduced, and its feasibility is demonstrated on aqueous solutions of stochastically spin-labeled polysaccharides. The technique is based on a dipolar EPR pulse sequence ih-RIDME (intermolecular hyperfine relaxation-induced dipolar modulation enhancement). The global analysis of a series of RIDME traces offers a mathematically stable transformation of the time-domain data to the proton density distribution. Novel pulse EPR sequences are proposed and primarily tested, which combine the ih-RIDME block and the double-electron-electron resonance (DEER) experiment. These new pulse sequences can be potentially used to correlate the local proton distributions with the overall macromolecule chain conformational distribution.