Thermodynamic properties of the photochemical reaction center of Heliobacterium chlorum

The thermodynamic and spectral properties of the photochemical reaction center components of Heliobacterium chlorum have been examined. The primary electron donor bacteriochlorophyll has E_m,7 = +225 mV, and the ‘primary acceptor’ E_m,10 = ?510 mV. The former has an EPR signal in its oxidised form near G = 2.0025, ΔH = 0.95 mT, reminiscent of the properties of the primary donor in bacteria containing bacteriochlorophyll a. The ‘primary acceptor’ has properties similar to those of the iron-sulfur cluster acceptors of green sulfur bacteria. H. chlorum contains a c-type cytochrome (E_m,7 = +170 mV) that donates electrons to the photooxidised primary donor with . The reaction center of H. chlorum is thus very similar to that found in representative green sulfur bacteria, but the cellular architecture and photopigments of this group are quite distinct from those of H. chlorum.