Information AboutCidnp |
| CATEGORIES ABOUT CIDNP | |
| physical chemistry | |
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RADICAL PAIR MECHANISM The generation of CIDNP in a typical photochemical system (target + Photosensitizer , Flavin in this example) is a cyclic photochemical process shown schematically in Figure 1. The chain of reactions is initiated by a blue light Photon , which excites the Flavin Mononucleotide (FMN) photosensitizer to the Singlet excited state. The Fluorescence Quantum Yield of this state is rather low, and approximately half of the molecules undergo Intersystem Crossing into the long-lived Triplet state. Triplet FMN has a remarkable Electron Affinity . If a molecule with a low Ionization Potential (e.g. Phenol s, Polyaromatics ) is present in the system, the Diffusion-limited Electron Transfer Reaction forms a spin-correlated triplet electron transfer state – a radical pair. The actual Kinetics are rather complicated and may involve multiple (de)protonations and hence exhibit PH dependence. The radical pair may either cross over to a singlet electron state and then recombine, or separate and perish in side reactions. The relative probability of these two pathways for a given radical pair depends on the nuclear spin state and leads to the nuclear spin state sorting and observable nuclear polarization. APPLICATIONS Detected as enhanced absorptive or emissive signals in the NMR spectra of the reaction products, CIDNP has been exploited for the last 30 years to characterise transient free radicals and their Reaction Mechanism s. In certain cases, CIDNP also offers the possibility of large improvements in NMR sensitivity. The principal application of this photo-CIDNP technique, as devised by Kaptein in 1978, has been to Protein s in which the aromatic amino acid residues Histidine , Tryptophan and Tyrosine can be polarized using flavins or other Aza- aromatics as photosensitisers. The key feature of the method is that only Solvent -accessible Histidine , Tryptophan and Tyrosine residues can undergo the radical pair reactions that result in nuclear polarization. Photo-CIDNP has thus been used to probe the surface structure of Protein s, both in native and partially Folded states, and their interactions with molecules that modify the accessibility of the reactive side chains. REFERENCES # L.T. Muus, P.W. Atkins, K.A. McLauchlan, J.B. Pedersen (ed.), Chemically induced magnetic polarisation, D. Reidel, Dordrecht, 1977. # M. Goez, Photochemically induced dynamic nuclear polarization, Adv. Photochem. 23 (1997) 63-163. # R. Kaptein, Photo-CIDNP studies of proteins, Biol. Magn. Res. 4 (1982) 145-191. # R. Kaptein, K. Dijkstra, K. Nicolay, Laser photo-CIDNP as a surface probe for proteins in solution , Nature 274 (1978) 293-294. # P.J. Hore, R.W. Broadhurst, Progr. NMR Spec. 25 (1993) 345-402. Abstract # I. Kuprov, P.J. Hore, J. Magn. Res. 168 (2004) 1-7 Article |
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