Time-resolved X-, K-, and W-band EPR of the radical pair state P₇₀₀^•+A₁^•- of photosystem I in comparison with P₈₆₅^•+Q_A^•- in bacterial reaction centers

The spin-polarized EPR spectra at 95 GHz (W-band), 24 GHz (K-band), and 9 GHz (X-band) of the radical pair P₇₀₀^•+A₁^•- in highly purified photosystem I particles are presented. The spectra are analyzed to obtain both the magnetic parameters of the radical pair as well as the relative orientation of the two species. From the analysis, the g-tensor of A₁^•- is found to be g_xx = 2.0062, g_yy = 2.0051, and g_zz = 2.0022, and it is shown that A₁ is oriented such that the carbonyl bonds are parallel to the vector joining the centers of P₇₀₀^•+ and A₁^•-. The anisotropy of the g-tensor is considerably larger than that obtained for chemically reduced phylloquinone in frozen 2-propanol solution. Possible reasons for this difference and their implications for the A₁ binding site are discussed. The relative orientation of P₇₀₀^•+ and A^1•- is compared with earlier estimates obtained using less accurate g-values for A₁^•1. A comparison with the spectra of P₈₆₅^•+Q_A^•- in bacterial reaction centers (bRCs) of Rhodobacter sphaeroides R-26 in which the nonheme iron has been replaced by zinc (Zn-bRCs) allows the structural and magnetic properties of the charge-separated state in the two systems to be compared. From the similarity of the two W-band spectra in the region around the free electron g-value it is clear that the dipolar vector, Z_d, between P^•+ and Q_A^•-/A₁^•- has a similar orientation relative to P₇₀₀ in PS I and P₈₆₅ in bRCs. This is compatible with the similar overall structural arrangement of P₇₀₀ and P₈₆₅. In contrast, the low-field parts of the two spectra are very different as a result of differences in the orientation of A₁ and Q_A with respect to z_d.