The Solid-State Photo-CIDNP Effect and Its Analytical Application

Abstract

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is an effect that produces non-Boltzmann nuclear spin polarization which can be observed as modification of signal intensity in NMR spectroscopy. The effect is well known in liquid-state NMR where it is explained most generally by the classical radical pair mechanism (RPM). In the solid-state, other mechanisms are operative in the spin-dynamics of radical pairs such as three-spin mixing (TSM) and differential decay (DD). Initially the solid-state photo-CIDNP effect has been solely observed on natural photosynthetic reaction centers (RCs). Therefore the analytical capacity of the method has been explored in experiments on reaction centers (RCs) of the purple bacterium of Rhodobacter (R.) sphaeroides. Here we will provide an account on phenomenology, theory, and analytical capacity of the solid-state photo-CIDNP effect.

Photochemically Induced Dynamic Nuclear Polarization Observed by Solid-State NMR in a Uniformly 13C-Isotope-Labeled Photosynthetic Reaction Center

“…The solid-state photo-CIDNP (photochemically induced dynamic nuclear polarization) effect, discovered by Zysmilich and McDermott in 1994 1 , allows for dramatic signal enhancement in solid-state magic-angle spinning (MAS) NMR experiments upon sample illumination (for review, see refs 2,3 ). While photo-CIDNP has been observed in the liquid state in various electron-transfer systems, the effect in solid matrix seems to be confined to a few systems:…”

Analysis of electron donors in photosystems in oxygenic photosynthesis by photo-CIDNP MAS NMR

“…Photo-CIDNP effects in liquid and solid state show strong magnetic field dependence which is indicative for the mechanisms and the parameters causing the effect [60,64,[77][78][79]. Interestingly, the solid-state photo-CIDNP effects observed in PSI and PSII exhibit very different magnetic field dependencies (Fig.…”

“…In addition, the enhanced signals allow for the selective observation of chemical shifts related to the electronic ground-state structure. Hence, photo-CIDNP MAS NMR is a versatile analytical tool for the analysis of transient spin-correlated radical pairs permitting an atomic-level ''view" on the electronic structure of the RC apparatus [64].…”

Time‐Resolved Chemically Induced Dynamic Nuclear Polarization of Biologically Important Molecules

“…In this situation, using a dye molecule as a photosensitizer is not necessary as the RP can be formed after light absorption by the cofactor and intramolecular electron transfer. Likewise, CIDNP effects have been reported for photosynthetic reaction centers,, after recombination of the RPs generated after the primary charge separation event. Here we deliberately do not dwell on CIDNP in the systems of both kinds because polarization formation is often based on other mechanisms rather than spin‐sorting discussed in this contribution.…”

“…One should note that solid‐state CIDNP of biomolecules is also feasible: there are examples of CIDNP studies in photosynthetic reaction centers and flavo‐proteins. One should note however that solid‐state CIDNP is a separate sub‐field owing to the difference in experimental objects, hardware and polarization mechanisms, therefore we recommend an interested reader to go for specialized reviews on this subject.…”