Extinction coefficients and critical solubilisation concentrations of photosystems I and II from Thermosynechococcus elongatus

The absorption properties of chlorophyll a (Chla) in active core complexes of photosystems I (PSI) and II (PSII) isolated in high purity from the thermophilic cyanobacterium Thermosynechococcus elongatus were correlated with those of extracts in 80% acetone to determine effective extinction coefficients of protein-bound Chla and molar extinction coefficients of core complexes and reaction centers (RC). These coefficients allow a quick determination of Chla and protein concentrations from steady-state absorption spectra of intact samples without the need for pigment extraction and protein destruction. In the visible range, ε680p = 57 mM−1 cm−1 for trimeric PSI (PSIt) and ε674p = 70 mM−1 cm−1 for dimeric (PSIId) and monomeric (PSIIm) PSII (error ±6%; superscript “p” refers to Chla bound to intact protein, subscripts are the peak maxima in nm). The integral extinction coefficient ϕp = 2.8 nm μM−1 cm−1 for the wavelength interval between 550 and 800 nm and the extinction coefficient εBp = 14 mM−1 cm−1 for the smaller absorption maximum (B = 632 nm for PSI and 627 nm for PSII) were found to be essentially the same for both types of PS. The coefficients of PSIt are shown to remain unaltered when 65% (v/v) of the buffer is replaced with glycerol. Molar extinction coefficients of core complexes were determined using Chla/RC ratios of 96±1 for PSI and 35±2 for PSII based on X-ray data. In addition, the critical solubilisation concentration of n-dodecyl-β-d-maltoside (βDM), necessary to keep the core complexes in solution, was determined by turbidimetric titrations. It was found that at least ∼500 βDM molecules per PSIt (∼2 βDM per Chla) and 190 βDM molecules per PSIIm (∼5 βDM per Chla, also for PSIId) in excess of the critical micelle concentration of 0.16 ± 0.03 mM are necessary for a complete solubilisation of the core complexes.