Insights into the conformational passage of a polypeptide chain across its free energy landscape have come from the judicious combination of experimental studies and computer simulations 1,2 . Even though some unfolded and partially folded proteins are now known to possess biological function 3 or to be involved in aggregation phenomena associated with disease states 1,4 , experimentally derived atomic-level information on these structures remains sparse as a result of conformational heterogeneity and dynamics. Here we present a technique that can provide such information. Using a 'Trp-cage' miniprotein known as TC5b (ref. 5), we report photochemically induced dynamic nuclear polarization NMR 6 pulse-labelling experiments that involve rapid in situ protein refolding 7,8 . These experiments allow dipolar cross-relaxation with hyperpolarized aromatic side chain nuclei in the unfolded state to be identified and quantified in the resulting folded-state spectrum. We find that there is residual structure due to hydrophobic collapse in the unfolded state of this small protein, with strong inter-residue contacts between side chains that are relatively distant from one another in the native state. Prior structuring, even with the formation of non-native rather than native contacts, may be a feature associated with fast folding events in proteins.Experimental advances in nuclear magnetic resonance (NMR) spectroscopy have led to the characterization of a diverse range of unfolded states of proteins 9 . In many cases the presence of residual structure has been shown 10-13 , but with some significant exceptions 14 the poorly resolved spectra of the unfolded state, arising from conformational exchange and dynamic averaging, have generally hampered structural analysis by NMR. We report here the use of an NMR technique that circumvents some of these problems by transferring Three methodologies are combined in this 'pulse-labelling' experiment ( Fig. 1). (1) Photo-CIDNP (chemically induced dynamic nuclear polarization) 6,22 , a technique for enhancing the NMR signals ('hyperpolarization') of solvent-accessible tryptophan, tyrosine and histidine side chains by means of a laser-induced reaction of the protein with a flavin photosensitizer. (2) Rapid homogeneous mixing of solutions in the NMR sample tube to trigger the folding of a denatured protein on a timescale faster than nuclear spin-lattice relaxation ( Supplementary Fig. 1) 7,8 . To these two techniques we add here, for the first time, (3) transfer of nuclear magnetization via nuclear Overhauser effects (NOEs) from the hyperpolarized side chain protons to neighbouring atoms before the refolding step. As a result, inter-residue contacts in unfolded conformations can be detected in the well-resolved NMR spectrum of the refolded native state.1 H photo-CIDNP measurements were initially performed on the native and denatured states of TC5b. The photo-CIDNP spectrum of native TC5b (Fig. 2b) is considerably simpler than the conventional NMR spectrum (Fig. 2a), because only t...

The Non-Hydrogenative Parahydrogen-Induced Polarization (NH-PHIP) technique, which is referred to as Signal Amplification by Reversible Exchange (SABRE), has been reported to be applicable to various substrates and catalysts. For more detailed studies, pyridine was mainly examined in the past. Here, we examined several pyrazole derivatives towards their amenability to this method using Crabtree's Catalyst, which is the polarization transfer catalyst that is best documented. Additionally, the dependence of the signal enhancement on the field strength, at which the polarization step takes place, was examined for pyridine and four different pyrazoles. To achieve this, the polarization step was performed at numerous previously determined magnetic fields in the stray field of the NMR spectrometer. The substrate dependence of the field dependence proved to be relatively small for the different pyrazoles and a strong correlation to the field dependence for pyridine was observed. Reducing the number of spins in the catalyst by deuteration leads to increased enhancement. This indicates that more work has to be invested in order to be able to reproduce the SABRE field dependence by simulations.

Homogeneous hydrogenations of unsaturated substrates with parahydrogen yield strong NMR signal enhancements of the transferred 1H nuclei if the symmetry of H2 is broken in the resulting hydrogenated products. This chemically induced hyperpolarization known as Parahydrogen-induced polarization (PHIP) is also transferred to other protons and heteronuclei (2H, 13C, 29Si, 31P) when the hydrogenation is initiated at low magnetic fields. Hydrogenating various fluorinated styrenes and phenylacetylenes, we show that PHIP-derived hyperpolarization is transferred to 19F not only in the Earth's magnetic field (ALTADENA condition) but also in a strong magnetic field, e.g., when carrying out the reaction in the NMR spectrometer (PASADENA condition). Upon conducting a systematic analysis of the observed PHIP transfer to 1H, 13C, and 19F in the hydrogenation products to elucidate the mechanisms that govern this parahydrogen-aided resonance transfer (PART), we conclude that high- and low-field PHIP transfer mechanisms differ in detail depending on either through-bond or through-space interactions. Substrates with high hydrogenation rates and long spin-lattice relaxation times (T1) yield the highest degree of heteronuclear hyperpolarization. Possible medical applications for hyperpolarized 19F-containing molecules as "active" contrast agents for magnetic resonance imaging (MRI) are outlined.

Photo-chemically induced dynamic nuclear polarization (CIDNP) is a nuclear magnetic resonance (NMR) phenomenon which, among other things, is exploited to extract information on biomolecular structure via probing solvent-accessibilities of tryptophan (Trp), tyrosine (Tyr), and histidine (His) amino acid side chains both in polypeptides and proteins in solution. The effect, normally triggered by a (laser) light-induced photochemical reaction in situ, yields both positive and/or negative signal enhancements in the resulting NMR spectra which reflect the solvent exposure of these residues both in equilibrium and during structural transformations in "real time". As such, the method can offer - qualitatively and, to a certain extent, quantitatively - residue-specific structural and kinetic information on both the native and, in particular, the non-native states of proteins which, often, is not readily available from more routine NMR techniques. In this review, basic experimental procedures of the photo-CIDNP technique as applied to amino acids and proteins are discussed, recent improvements to the method highlighted, and future perspectives presented. First, the basic principles of the phenomenon based on the theory of the radical pair mechanism (RPM) are outlined. Second, a description of standard photo-CIDNP applications is given and it is shown how the effect can be exploited to extract residue-specific structural information on the conformational space sampled by unfolded or partially folded proteins on their "path" to the natively folded form. Last, recent methodological advances in the field are highlighted, modern applications of photo-CIDNP in the context of biological NMR evaluated, and an outlook into future perspectives of the method is given.