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.