EXCITED-STATE EQUILIBRATION IN THE PHOTOSYSTEM-I LIGHT-HARVESTING-I COMPLEX - P700 IS ALMOST ISOENERGETIC WITH ITS ANTENNA

Photosystem I with its full antenna complement (PSI-LHCI) has been pre pared by mild detergent solubilization with octyl beta-D-glucopyranosi de from maize thylakoids. A preliminary polypeptide analysis is presen ted. At room temperature, the steady-state fluorescence derives from a n almost perfectly thermalized State, as demonstrated by a Stepanov an alysis, in which about 90% of the excited states are associated with t he red chlorophyll spectral forms absorbing above 700 nm. Equilibratio n is temperature-sensitive and Is lost at T < 200 K, A careful analysi s of fluorescence between 75 and 280 K clearly demonstrates the presen ce of at least three red chlorophyll spectral forms with emission maxi ms at 720, 730, and 742, nm, the absorption origin bands of which have been calculated al 714, 725, and 738 nm. On the basis of a minor devi ation from thermal equilibration around 695 nm, it is suggested that a t least 3-4 antenna chlorophylls, with an average absorption near 695 nm, are strongly coupled to P700. Thermodynamic analysis of absorption and fluorescence spectra indicates that the equilibrium, absorption-w eighted excited state population of the P700 dimer is around 0.013 ass uming that the low-energy exciton stale possesses all the oscillator s trength. The average free energy for excitation transfer from antenna to P700 is thus calculated to be -0.26 kT at room temperature. This in dicates that P700 is almost isoenergetic with its antenna at room temp erature when the red forms are taken fully into account. From the calc ulated excited stale population of P700, we estimate that the primary charge separation rate in PSI is 1-2 ps(-1).