Four way cuvette holder

Absorbance spectra were recorded on a Varian Cary60 spectrophotometer and emission spectra on a Varian Cary Eclipse Fluorimeter in nitrogen-gassed, freshly prepared Tris/DDM.
Photooxidation of the RC P870 primary electron donor was measured using an optical fibre attachment for the Cary60 and a four-way cuvette holder (Ocean Optics, Inc.). For excitation, light from an HL-2000 light source (Ocean Optics, Inc.) was passed through an optical fibre and a 25-nm band-pass filter centred at 650 nm (Edmund Optics Ltd). Incident light intensity was approximately 0.1 mW cm⁻², which excited ~15% of the RC population. Light-on/off was controlled using the electronic shutter on the light source triggered by a TGP110 pulse generator (Aim-TTi Ltd, UK). After incubation with a 5-fold excess of ubiquinone-0 (UQ₀) in the dark for 10 min, samples at an RC concentration of 0.5 μM (0.25 μM with LHCI-Td) were housed in a 3 mm path length, four-sided microcuvette (110-15-QS, Hellma^® Analytics). Each measurement was repeated five times and the traces were fitted to a model assuming a simple interconversion between the ground and photo-oxidised state: $\mathrm{P}870\begin{array}{c}\hfill \underset{k_{\mathrm{r}}}{\overset{k_{\mathrm{f}}}{\leftrightarrow }}\hfill \end{array}\mathrm{P}870^{+}$ Parameters k_f and k_r from these fits are shown in Supplementary Table 2. All control samples had equimolar LHC and RC except a WT RC/Td-LHCI-Td mix where the molar ratio of RC to LHC was two.