Solid-state NMR spectra of Q-blocked photocycling photosynthetic reaction centers exhibit strongly enhanced lines. These peaks, for both carbon and nitrogen spectra, can be assigned to the primary donor bacteriochlorophyll special pair and the primary acceptor pheophytin. This paper concerns the mechanism by which the polarization develops. Previously reported simulations involving a radical pair mechanism (RPM) could achieve good agreement for the bacteriochlorophyll-containing electron donor, but not for the pheophytin acceptor. In this paper we focus on the N II and N IV nitrogens of the pheophytin acceptor, which we know are strongly directly polarized and for which we cannot explain the polarization by the RPM. To explain the strongly polarized acceptor signals, we propose a three-spin coherent mixing mechanism: nanosecond time scale coherent mixing of nuclear states and electron zero-quantum states are combined with chemical decay kinetics (back electron transfer) using the stochastic Liouville equation, to yield the density matrix for the radical pair states, 1P+·H-·(t) and 3P+·H-·(t). We solve the equation numerically for the yields of the ground state P (after back-electron transfer) and the molecular triplet 3P, as well as the nuclear polarization associated with each of these yields. Nuclear polarization results from such a dynamical system only if the dipolar Hamiltonian is not fully truncated; terms involving nuclear raising and lowering operators must be retained. Nuclear polarization is computed for crystallites at a variety of (2500) randomly selected orientations with respect to the magnetic field, and polarized powder patterns and MAS spectra are simulated, showing good agreement with the experimental data. Several characteristics of this coherent mixing mechanism are described below that will help discern other experimental systems that are likely to develop large nuclear polarization. Unlike previous mechanisms, nuclear relaxation rates and electron−nuclear cross relaxation rates do not drive the development of polarization and therefore are not critical to the success of the scheme. The mechanism is driven by coherent evolution, and the critical kinetic parameters are the electron zero-quantum frequency, the hyperfine coupling, and the nuclear Zeeman terms which must be comparable (i.e., within 2 orders of magnitude) for optimal polarization to be observed. The chemical reaction constants are also critical to this scheme: the singlet decay constant must be slow enough to allow some evolution under the influence of the hyperfine interaction, and the triplet decay kinetics must be very different from the singlet decay constant to achieve net polarization. With the spectroscopic parameters reported for the photosynthetic reaction center, such an ideal matching of kinetic constants was not achieved except for specific orientations, and yet numerical simulations indicate that we can explain polarizations of up to 104 × Boltzmann polarization (single turnover) and up to 105 × Boltzmann polarization (steady-state case) with this mechanism. The anisotropy associated with the g and hyperfine tensors resulted in a highly anisotropic steady state polarization; the largest effect is expected for orientations for which anisotropic zero-quantum electron frequency (the difference of the g tensors of BChl and BPheo) is the smallest.