PHOTOCHEMISTRY AND SPECTROSCOPY OF A 5-CHLOROPHYLL REACTION-CENTER OFPHOTOSYSTEM-II ISOLATED BY USING A CU AFFINITY COLUMN

A reaction center of photosystem II was isolated from Pisum sativum by using immobilized metal affinity chromatography. This reaction center is photochemically active and has a room temperature Qy chlorophyll ( Chl) absorption band peaking at 677.5 nm, From HPLC analysis, the pigm ent stoichiometry was suggested to be 5 Chls per 1 beta-carotene per 2 pheophytins, Low-temperature absorption measurements at 77 K were con sistent with the removal of one of the Chls associated with the usual form of the reaction center isolated by using ion-exchange chromatogra phy. Transient absorption spectroscopy on the picosecond time scale in dicated that the Chi removed belongs to a pool of Chl absorbing at app roximate to 670 nm (C670(II)) that transfers energy relatively slowly to the primary donor P680 in support of our recently proposed model, T he results also support the previous conclusion that radical pair form ation is largely associated with a 21-ps time constant when P680 is di rectly excited and that the identity of C670(II) is likely to be perip herally bound Chls possibly ligated to conserved His residues at posit ions 118 on the D1 and D2 proteins.