Conspectus During the three decades 1980–2010, magic angle spinning (MAS) NMR developed into the method of choice to examine many chemical, physical and biological problems. In particular, a variety of dipolar recoupling methods to measure distances and torsion angles can now constrain molecular structures to high resolution. However, applications are often limited by the low sensitivity of the experiments, due in large part to the necessity of observing spectra of low-γ nuclei such as the I = ½ species 13C or 15N. The difficulty is still greater when quadrupolar nuclei, like 17O or 27Al, are involved. This problem has stimulated efforts to increase the sensitivity of MAS experiments. A particularly powerful approach is dynamic nuclear polarization (DNP) which takes advantage of the higher equilibrium polarization of electrons (which conventionally manifests in the great sensitivity advantage of EPR over NMR). In DNP, the sample is doped with a stable paramagnetic polarizing agent and irradiated with microwaves to transfer the high polarization in the electron spin reservoir to the nuclei of interest. The idea was first explored by Overhauser and Slichter in 1953. However, these experiments were carried out on static samples, at magnetic fields that are low by current standards. To be implemented in contemporary MAS NMR experiments, DNP requires microwave sources operating in the subterahertz regime — roughly 150–660 GHz — and cryogenic MAS probes. In addition, improvements were required in the polarizing agents, because the high concentrations of conventional radicals that are required to produce significant enhancements compromise spectral resolution. In the last two decades scientific and technical advances have addressed these problems and brought DNP to the point where it is achieving wide applicability. These advances include the development of high frequency gyrotron microwave sources operating in the subterahertz frequency range. In addition, low temperature MAS probes were developed that permit in-situ microwave irradiation of the samples. And, finally, biradical polarizing agents were developed that increased the efficiency of DNP experiments by factors of ~4 at considerably lower paramagnet concentrations. Collectively these developments have made it possible to apply DNP on a routine basis to a number of different scientific endeavors, most prominently in the biological and material sciences. This Account reviews these developments, including the primary mechanisms used to transfer polarization in high frequency DNP, and the current choice of microwave sources and biradical polarizing agents. In addition, we illustrate the utility of the technique with a description of applications to membrane and amyloid proteins that emphasizes the unique structural information that is available in these two cases.

Composed of the two bacteriochlorophyll cofactors, PL and PM, the special pair functions as the primary electron donor in bacterial reaction centers of purple bacteria of Rhodobacter sphaeroides. Under light absorption, an electron is transferred to a bacteriopheophytin and a radical pair is produced. The occurrence of the radical pair is linked to the production of enhanced nuclear polarization called photochemically induced dynamic nuclear polarization (photo-CIDNP). This effect can be used to study the electronic structure of the special pair at atomic resolution by detection of the strongly enhanced nuclear polarization with laser-flash photo-CIDNP magic-angle spinning NMR on the carotenoid-less mutant R26. In the electronic ground state, P L is strongly disturbed, carrying a slightly negative charge. In the radical cation state, the ratio of total electron spin densities between PL and PM is 2:1, although it is 2.5:1 for the pyrrole carbons, 2.2:1 for all porphyrinic carbons, and 4:1 for the pyrrole nitrogen. It is shown that the symmetry break between the electronic structures in the electronic ground state and in the radical cation state is an intrinsic property of the special pair supermolecule, which is particularly attributable to a modification of the structure of PL. The significant difference in electron density distribution between the ground and radical cation states is explained by an electric polarization effect of the nearby histidine.electron transfer ͉ nuclear polarization ͉ photosynthesis ͉ solid-state NMR ͉ electronic structure T he essential steps in photosynthesis, photon absorption, and electron transfer occur in the reaction center (RC) membrane protein. Simple purple photosynthetic bacteria possess only a single type of RC and perform anoxygenic photosynthesis. In RCs from the purple bacterium Rhodobacter sphaeroides R26, the primary electron donor (P), called the special pair, consists of two symmetrically arranged BChl a (Fig. 1A) cofactors, labeled P L and P M , coordinated by His-L168 and His-M202, respectively (Fig. 1B) (1, 2). The other cofactors are two accessory BChls, two BPhes a, two ubiquinones, and a nonheme iron that are arranged in a nearly C 2 symmetry. Despite the symmetrical arrangement, the electron pathway is entirely unidirectional, occurring along the L branch (for review, see ref.3).In the dark electronic ground state, the symmetry between both cofactors P L and P M is already broken, as was shown with photochemically induced dynamic nuclear polarization (photo-CIDNP) 13 C magic-angle spinning (MAS) NMR (4). The ratio of electron spin densities between the two cofactors in the radical cation state has been determined at the molecular level to be Ϸ2:1 using 1 H electron nuclear double resonance (ENDOR) (5, 6) and steady-state photo-CIDNP 13 C MAS NMR (7). On the other hand, values of Ϸ5:1 have been observed by 15 N-ESEEM (ESEEM, electron spin echo envelope modulation) (8) and values of Ϸ4:1 have been observed by time-resolved photo-CIDNP 15 N MAS NMR (9). It has be...

The solid-state photo-CIDNP effect is known to occur in natural photosynthetic reaction centers (RCs) where it can be observed by magic-angle spinning (MAS) NMR as strong modification of signal intensities under illumination compared to experiments performed in the dark. The origin of the effect has been debated. In this paper, we report time-resolved photo-CIDNP MAS NMR data of reaction centers of quinone depleted Rhodobacter sphaeroides. It is demonstrated that the build-up of nuclear polarization on the primary donor and the bacteriopheophytin acceptor depends on the presence and lifetimes of the molecular triplet states of the donor and carotenoid. Analysis of the data proves that up to three electron-nuclear spin-coupling mechanisms and two transient effects are working concomitantly in the spin-chemical machinery of the reaction center.

Oxygen-17 detected DNP NMR of a water/glycerol glass enabled an 80-fold enhancement of signal intensities at 82 K, using the biradical TOTAPOL. The >6,000-fold savings in acquisition time enables 17O-1H distance measurements and heteronuclear correlation experiments. These experiments are the initial demonstration of the feasibility of DNP NMR on quadrupolar 17O.