Probing native-like orientation of pigments in modified reaction centers from Rhodobacter sphaeroides R26 by linear dichroism

Site-specific pigment modifications are useful to investigate structure-function relationships in photosynthesis. In reaction centers bearing modified (bacterio)pheophytins, changed electron transfer kinetics have been related to the changed redox potentials of the pigments introduced (Huber, H. et al. (1995) Chemical Physics, Special Issue, vol 197 (Hochstrasser, R.M. and Hofacker, G.L. eds.) pp. 297–305; [1]). In order to analyze potentially interfering structural changes induced in these reaction centers by the exchange procedure, in particular mispositioning or misorientation of the pigments, low-temperature linear dichroism spectra have been measured for reaction centers from Rhodobacter sphaeroides containing modified bacteriopheophytins and bacteriochlorophylls at the sites HAB and BA,B, respectively. They show that all modified pigments are oriented similar to the native ones, and that they do not affect significantly the linear dichroism of the monomeric bacteriochlorophylls and bacteriopheophytins or of the primary donor.