Ligated chlorophyll cation radicals: Their function in photosystem II of plant photosynthesis

Abstract

Magnesium tetraphenylchlorin, a synthetic model for chlorophyll, exhibits significant variations in the unpaired spin densities of its cation radicals with concomitant changes in oxidation potentials as a function of solvent and axial ligand. Similar effects are observed for chlorophyll (Chl) a and its cation radicals. Oxidation potentials for Chl -Chl+' as high as +0.9 V (against a normal hydrogen electrode) are observed in nonaqueous solvents, with linewidths of the electron spin resonance signals of monomeric Chl+' ranging between 9.2 and 7.8 G in solution. These changes in electronic configuration and ease of oxidation are attributed to mixing of two nearly degenerate ground states of the radicals theoretically predicted by molecular orbital calculations. Comparison of the properties of chlorophyll in vitro with the optical, redox, and magnetic characteristics attributed to P680, the primary donor of photosystem II which mediates oxygen evolution in plant photosynthesis, leads us to suggest that P480 may be a ligated chlorophyll monomer whose function as a phototrap is determined by interactions with its (protein?) environment. Photosynthesis in algae and green plants functions via two chlorophyll (Chl)-mediated systems that cooperatively oxidize water (photosystem II, PS II) and reduce carbon dioxide (PS I) (1-14). The primary chemical products generated by the light harvested by the antenna network and funnelled to the reaction centers are believed to be oxidized Chls, P-700+ in PS I and P-680+-in PS II1(1-14), and reduced Chl or pheophytin (11,(15)(16)(17). The optical changes associated with the oxidations of P-700 and P-680 (18-24), particularly an enhanced absorption at -820 nm, and the electron spin resonance (ESR) linewidths and g-values (10, [24][25][26][27][28][29][30][31][32] are characteristic of Chl ir cation radicals (33). ESR (10,(25)(26), ENDOR (electron-nuclear double resonance) (34-35), optical, and circular dichroism (36) results have further established the dimeric or "special pair" nature of P-700+ . ESR data show (25) that the 7 G (1 G = 1o-4 tesla) peak-to-peak linewidth (AH) of P-700+ is narrower by V-than that of Chl+-(AH = 9 G) in dichloromethane and dichloromethane/methanol solutions, as expected if the unpaired electron of the radical is equally shared between two Chl molecules. Additional support for delocalization of spin over two Chls in P-700+-derives from ENDOR spectra of oxidized chloroplasts (34,35) in which several proton hyperfine splittings are found to be approximately half those of Chl+-in vitro (37,38). The nature of P-680+' in PS II is considerably more ambiguous: linewidths of ESR signals attributed to P-680+-range from 7 to 9 G (27-32), resulting in proposals of monomer (28), dimer (29-31), and even trimer (32) configurations. Even more puzzling are the large differences in oxidation potentials of P-700 and P-680. Titrations of P-700 yield a midpoint potential (39-41) ranging between +0.4 and +0.5 V against the normal hydrogen electrode, whereas the minimu...

Electron nuclear double resonance evidence supporting a monomeric nature for P700 + in spinach chloroplasts

“…Our in vitro spectrum (Fig. 2b) is also an immobilized spectrum and, similar to Davis et al (10), a 7-8 band at =10.6 MHz is not observed. A distinct broadening of the As band is noticeable, however, giving rise to a partially resolved A6 band at 8.0 MHz.…”

“…This difference is principally due to the broad bands observed in both spectra, which render an accurate value difficult to obtain. The behavior of the band due to the 7-8 proton couplings on immobilization is best illustrated by the spectra of Davis et al (10). When the radical is rapidly tumbling in solution (Figure 4a in ref.…”

“…When the radical is rapidly tumbling in solution (Figure 4a in ref. 10), a distinct band due to the 7-8 protons is observed at 10.64 MHz. On immobilization this band is not resolved, due to severe broadening over a wide spectral range.…”

“…2b are not resolved in ref. 10), we attribute the A6 band to the broadened 7-8 band. This might suggest a lowering in the coupling value of these protons on immobilization; however, this is not borne out by the linewidth value, which remains at 9.0 G (10). As the P700' spectrum is also an immobilized spectrum we assign the broad A6 band of Fig.…”

Identification of the special pair of photosystem II in a chlorophyll d -dominated cyanobacterium

“…P680, the special pair of PSII in oxygenic photosynthetic organisms, was initially detected by a flash photolysis technique (9) and was later spectrally characterized in detail (10,11). Isolation of a D1-D2-cyt b 559 complex [PSII reaction center (RC)] from spinach (12) accelerated detailed characterization.…”