GAUSSIAN BAND ANALYSIS OF ABSORPTION, FLUORESCENCE AND PHOTOBLEACHINGDIFFERENCE SPECTRA OF D1 D2/CYT B-559 COMPLEX/

A study of the absorption and fluorescence characteristics of the D1/D 2/cytb-559 reaction centre complex of Photosystem II has been carried out by gaussian decomposition of absorption spectra both at room tempe rature and 72 K and of the room temperature fluorescence spectrum. A f ive component fit was found in which the absorption and fluorescence s ub-bands could be connected by the Stepanov relation. The photobleachi ng and light-activated degradation in the dark of long wavelength pigm ents permitted a further characterisation of the absorption bands. The absorption (fluorescence) maxima of the five bands al room temperatur e are 660 nm (670 nm), 669 nm (675 nm), 675 nm (681 nm), 680 nm (683 n m), 681 nm (689 nm). A novel feature of this analysis is the presence of two approximately isoenergetic absorption bands near 680 nm at room temperature. The narrower one (FWHM = 12.5 nm) is attributed to pheop hytin while the broader band (FWHM = 23 nm) is thought to be P680. The P680 band width is discussed in terms of homogeneous and site inhomog eous band broadening. The P680 fluorescence has a large Stokes shift ( approximate to 9 nm) and most fluorescence in the 690-700 nm range is associated with this chromophore. The three accessory pigment bands ar e broad (FWHM = 17-24 nm) and the 660 nm gaussian is largely temperatu re insensitive thus indicating significant site inhomogeneous broadeni ng. The very slight narrowing of the D1/D2/cytb-559 Q(y) absorption at crytogenic temperatures is discussed in terms of the coarse spectral inhomogeneity associated with the spectral forms and the apparently la rge site inhomogeneous broadening of short wavelength accessory pigmen ts.