Angular orientation of the stable tyrosyl radical within photosystem II by high-field 245-GHz electron paramagnetic resonance.

Abstract

The 4 K 245-GHz/8.7-T electron paramagnetic resonance spectrum of the stable tyrosyl radical in photosystem I, known as TyrD, has been measured. Illumination at 200 K enhances the signal Intensity of TyrD" by a factor of >40 compared to the signal obtained from darkadapted samples. This signal ehncement and the unusual ine shape of the TyrD' resonance result from the magnetic dipolar coupling of the radical to the manganese duster involved in oxygen evolution. The relative alar orientation of the m _ cluster with respect to TyrD-has been determined from line-shape analysis. The resonance arising from TyrD' in Tris-washed nsfree photosystem U sample is also distorted. This effect probably originates from the influence of the nonheme iron on the spin aon of the tyrosyl radical.The relative angular orientation of the nonheme iron has also been determined. Oriented samples were used to determine the angular orientation of TyrD with respect to the membrane plane. Combining angular data with pubilshed distances, we have constructed a three-dimensional picture of the relative positions of TyrD', the manganese cluster, and the noneme iron. The data suggest a more symmetrical placement of the manganese relative to TyrD-and TyrZ, the tyrosine involved in eleco transfer, than is usually assumed in current models of photosystem H.At low temperatures, the electron paramagnetic resonance (EPR) signal of the stable tyrosyl radical, known as TyrD- (1, 2), from plant photosystem II (PSIT) is known to be "relaxation enhanced" when the sample is illuminated at 200 K (3, 4). This enhancement has been attributed to the presence of a second paramagnetic species that is capable of increasing the relaxation rates of the tyrosyl radical through a dipolar interaction. The second paramagnet is thought to be the manganese cluster that is involved in charge storage and probably acts as the active site for oxygen evolution (5, 6). The dipolar spin-relaxation mechanism has been thoroughly studied (7-9). The strength of the dipolar interaction is determined by the distance between the two spins and the angle between the interspin vector and the applied magnetic field. Previous work on PSII has concentrated on the determination of the interspin distance (10-13). No attempts have been made to obtain the angular orientation of the two spins.At magnetic field strengths used in conventional EPR spectroscopy, the spectrum of the tyrosyl radical is dominated by the hyperfine interactions between the electron spin and its neighboring protons. The inhomogeneous line width of the spectrum is determined by the anisotropic parts of these hyperfine interactions and the g anisotropy, each of which has its own particular orientation dependence relative to the molecular frame. A good approximation is that all possible orientations ofthe radical with respect to the applied magnetic field contribute to a given field position on the TyrD spectrum. Thus, the determination ofthe orientation of an external relaxer is difficult. However, at magnetic fields m...

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“…2, Top) exhibit excellent resolution, reflecting the very good homogeneity of the magnetic field and the absence of g and hyperfine strain in the samples. The g principal values obtained from simulations (error ⌬g Յ 2⅐10 Ϫ5 ) are compared with those reported in the literature (20,25,(34)(35)(36)(37) in Table 1.…”

Radicals and Radical Pairs in Photosynthesis

“…anism between the tyrosine radicals and both the nonheme iron and the Mn cluster (324). By analysis of the EPR lineshapes in both Mn-containing and Mn-depleted PS I1 particles, the angular orientation of Tyr, with respect to both the Fe(I1) and the Mn complex was derived with an eight-fold uncertainty due to different symmetry-related solutions.…”

“…Furthermore, the analysis of the EPR spectra of dried, oriented membranes showed that the C-0 bond is aligned with the membrane normal, which is in satisfactory agreement with similar results by Mino and Kawamori (328) and the assignment of the angular orientation of the tyrosine radicals with respect to the Mn-cluster and the iron atom by Un et at. (324).…”

“…By analysis of the EPR lineshapes in both Mn-containing and Mn-depleted PS I1 particles, the angular orientation of Tyr, with respect to both the Fe(I1) and the Mn complex was derived with an eight-fold uncertainty due to different symmetry-related solutions. Un et al (324) chose a set of angles that resembled most closely the structure predicted from comparison with the bacterial RC. This allowed them to draw a model relating the molecular planes of the tyrosine radicals (Tyg was assumed to be related to Tyr, by a C2 symmetry operation).…”

X-ray absorption spectroscopy on the calcium cofactor to the manganese cluster in photosynthetic oxygen evolution

“…Through work on high-field EPR, it is now possible to simulate EPR spectra on oriented samples, using known gand hyperfine tensors, while incorporating a disorder parameter. [134][135][136][137] A single spectrum can be calculated and its disorder parameter found without the need for EPR measurements at multiple angles. 21 This polarized EXAFS study on oriented Sr-reactivated PS II has detected at least one Sr-Mn vector, at 3.5 Å distance, whose orientation (φ) is 23 ± 4° relative to the membrane normal.…”