SPECTRAL AND KINETIC CHARACTERIZATION OF ELECTRON-ACCEPTOR A(1) IN A PHOTOSYSTEM-I CORE DEVOID OF IRON-SULFUR CENTERS F-X, F-B AND F-A

The kinetic and spectroscopic properties of the secondary electron acc eptor A(l) were determined by flash absorption spectroscopy at room an d cryogenic temperatures in a Photosystem I (PS I) core devoid of the iron-sulfur clusters F-x, F-B and F-A. It was shown earlier (Warren, P .V., Golbeck, J.H. and Warden, J.T. (1993) Biochemistry 32: 849-857) t hat the majority of the flash-induced absorbance increase at 820 nm, r eflecting formation of P700(+), decays with a t(1/2) of 10 mu s due to charge recombination between P700(+) and A(l)(-). Following A(l)(-) d irectly around 380 nm, where absorbance changes due to the formation o f P700(+) are negligible, two major decay components were resolved in this study with t(1/2) of similar to 10 mu s and 110 mu s at an amplit ude ratio of similar to 2.5:1. The difference spectra between 340 and 490 nm of the two kinetic phases are highly similar, showing absorbanc e increases from 340 to 400 nm characteristic of the one-electron redu ction of the phylloquinone A(l). When measured at 10 K, the flash-indu ced absorbance changes around 380 nm can be fitted with two decay phas es of t(1/2) similar to 15 mu s and 150 mu s at an amplitude ratio of similar to 1:1. The difference spectra of both kinetic phases from 340 to 400 nm are similar to those determined at 298 K and are therefore attributed to charge recombination in the pair P700(+) A(l)(-). These results indicate that the backreaction between P700(+) and A(l)(-) is multiphasic when F-x, F-B and F-A are removed, and only slightly tempe rature dependent in the range of 298 K to 10 K.