Nanosecond flash studies of the absorption spectrum of the Photosystem I primary acceptor Ao.

Photosystem I particles devoid of the secondary electron acceptor A1 were studied by nanosecond flash absorption. The primary radical pair (P-700(+), A0 (-)) decays with a half-time of 35 ns. The difference spectrum was measured (400-870 nm). After subtraction of the P-700(+)/P-700 difference spectrum, the A0 (-)/A0 was obtained. It includes bleachings centered at 690 and 430 nm, and broad positive bands in the near infra-red and the blue-green. This spectrum is consistent with A0 being chlorophyll a absorbing at 690 nm.