Chen, H.Y., M. Ragavan, and C. Hilty, Protein folding studied by dissolution dynamic nuclear polarization. Angew Chem Int Ed Engl, 2013. 52(35): p. 9192-5.
http://www.ncbi.nlm.nih.gov/pubmed/23857756
Protein folding occurs on a timescale that is not directly observable with most traditional nuclear magnetic resonance (NMR) spectroscopy methods.[1, 2] The insights into this complex process that can potentially be gained from the high site resolution of NMR has led to developments such as stopped-flow NMR,[3] incorporation of specific isotope labels,[4] and pulse sequences tailored for rapid data acquisition.[5] Hyperpolarization, the generation of a non-equilibrium spin state, shows significant promise to enhance the sensitivity and, by removing the need for signal averaging, dramatically decrease signal acquisition time.[6, 7] For the study of protein folding, chemically induced dynamic nuclear polarization (CIDNP) has been used to hyperpolarize tryptophan residues that undergo a cyclic reaction with a photosensitizer.[8] A different technique, dynamic nuclear polarization (DNP),[9] hyperpolarizes nuclei throughout a molecule via an admixed stable free radical. Combined with solid state NMR, DNP provides unique information on protein structure.[10] Using dissolution DNP,[11] NMR in the liquid state would be sensitive to structural changes across the entire macromolecule during the folding process. Liquid-state NMR signals of a DNP hyperpolarized, denatured protein, the ribosomal protein L23,[12, 13] have recently been observed.[14]