
%0 Journal Article
%A Heike, Witt 
%A Schlodder, Eberhard 
%A Teutloff, Christian 
%A Bordignon, Enrica 
%A Carbonera, Donatella 
%A Niklas, Jens 
%A Lubitz, Wolfgang 
%T Site-directed mutagenesis of Thr A739 of photosystem I in Chlamydomonas reinhardtii alters significantly the excitonic and electronic coupling of the primary electron donor P700
%J Science Access
%D 2001/01/01
%V 3
%N 1
%P -
%K 
%R https://doi.org/10.1071/SA0403173
%U https://www.publish.csiro.au/paper/SA0403173 
%X The primary electron donor P700 of photosystem I is a dimer comprised of one chlorophyll (Chl) a (PB) and one Chl a´ (PA). To investigate the influence of protein-cofactor interactions on the properties of P700, we constructed a series of site-directed mutants in the surrounding of P700. The most interesting effects were obtained for the replacement of Thr A739, a possible hydrogen bond donor to the 9-keto group of PA, against Val. The low-energy exciton absorption band of P700 observed as a shoulder at about 700 nm in the absorption spectrum of PS I complexes from wild type is not visible in the mutant TV A739. The main bleaching band in the (P700+-P700) and (3P700-P700) absorption difference spectra is blue shifted by 9 nm. Both results imply that the excitonic coupling of P700 is severely disturbed. A similar blue shift is observed for the main bleaching in the Soret region. Redox titrations yielded a decrease of the midpoint potential for the oxidation of P700 by 32 mV. ENDOR spectroscopy revealed a change of the electron spin densitiy distribution of P700+. The data provide evidence that P700 is a Chl-dimer with an asymmetric spin/charge density distribution. 
