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The integral membrane protein complex of the photosystem I reaction center from the cyanobacterium Synechococcussp. has been isolated and highly purified. At extremely low protein complex concentrations (50 μg/ml), crystallization occurred. The crystals are dichroic, photoactive and show an X-ray powder diffraction pattern.

The number and chemical nature of the electron carrier(s) between Chl aIIand the water-oxidizing enzyme, S, were analyzed through flash-induced absorption changes in the UV with nanosecond time resolution. (i) At all wavelengths where the reaction of the donor with Chl a+IIhas been characterized, this donor is oxidized in the nanosecond time range in exact accordance with the reduction kinetics of Chl a+II. The donor is in turn re-reduced with t12> 10,μs, i.e. in the range where S is oxidized. From this time course it is concluded that there exists only one electron carrier between Chl a+IIand S. (ii) The UV-diference spectrum due to the electron transfer from the immediate donor to Chl a+IIin the nanosecond time range in O 2-evolving PS II complexes is characterized by a maximum around 260 nm and smaller minimum around 310 nm. This spectrum is identical with that observed for the reaction of the donor with Chl a+IIin the microsecond time range in Tris-treated PS II. Therefore, the donors in both reactions must be of the same chemical nature. (iii) This result, together with the well-established similarity of EPR signal II fof the oxidized donor in Tris-treated PS II to the EPR signal III s, recently assigned to Tyr-160 of the D2 protein of PS II [(1988) Proc. Natl. Acad. Sci. USA 85, 427–430], provides strong evidence that the immediate donor to Chl a+IIin water-oxidizing PS II is also a tyrosine. (iv) It is shown that the UV-difference spectra of the oxidation of the immediate donor in O 2-evolving as well as that of Tris-treated PS II complexes are similar to the in vitro difference spectrum of the oxidation of tyrosine in water. This independent result supports the conclusion that the donor is a tyrosine.

Flash‐induced absorption changes in the UV have been monitored with nanosecond time resolution in oxygen‐evolving photosystem II complexes from Synechococcussp. At 260 nm, the oxidation of the immediate donor to Chl‐a+II(P‐680+) in the 10–500 ns range could be observed directly. The difference extinction coefficient for its oxidation is estimated to be 6000–8000 M−1·cm−1at 260 nm. The observed kinetics for the oxidation of the donor is identical with the reduction kinetics of Chl‐a+IIas monitored at 824 nm. At 320 nm, the reduction of QAwas found to be faster than 1 ns.

Flash-induced absorption changes in the UV have been monitored with nanosecond time resolution in oxygen-evolving photosystem II complexes from Synechococcussp. At 260 nm, the oxidation of the immediate donor to Chl- a+II(P-680 +) in the 10–500 ns range could be observed directly. The difference extinction coefficient for its oxidation is estimated to be 6000–8000 M −1·cm −1at 260 nm. The observed kinetics for the oxidation of the donor is identical with the reduction kinetics of Chl- a+IIas monitored at 824 nm. At 320 nm, the reduction of Q Awas found to be faster than 1 ns.

The number and chemical nature of the electron carrier(s) between Chl aIIand the water‐oxidizing enzyme, S, were analyzed through flash‐induced absorption changes in the UV with nanosecond time resolution. (i) At all wavelengths where the reaction of the donor with Chl a+IIhas been characterized, this donor is oxidized in the nanosecond time range in exact accordance with the reduction kinetics of Chl a+II. The donor is in turn re‐reduced with t> 10,μs, i.e. in the range where S is oxidized. From this time course it is concluded that there exists only one electron carrier between Chl a+IIand S. (ii) The UV‐diference spectrum due to the electron transfer from the immediate donor to Chl a+IIin the nanosecond time range in O2‐evolving PS II complexes is characterized by a maximum around 260 nm and smaller minimum around 310 nm. This spectrum is identical with that observed for the reaction of the donor with Chl a+IIin the microsecond time range in Tris‐treated PS II. Therefore, the donors in both reactions must be of the same chemical nature. (iii) This result, together with the well‐established similarity of EPR signal IIfof the oxidized donor in Tris‐treated PS II to the EPR signal IIIs, recently assigned to Tyr‐160 of the D2 protein of PS II [(1988) Proc. Natl. Acad. Sci. USA 85, 427–430], provides strong evidence that the immediate donor to Chl a+IIin water‐oxidizing PS II is also a tyrosine. (iv) It is shown that the UV‐difference spectra of the oxidation of the immediate donor in O2‐evolving as well as that of Tris‐treated PS II complexes are similar to the in vitro difference spectrum of the oxidation of tyrosine in water. This independent result supports the conclusion that the donor is a tyrosine.