PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Asymmetric hydrogen bonding between the quinone electron acceptor and the protein in Photosystem I as studied by various EPR methods. Yulia Pushkar^*,1, Herbert Zimmermann², Dietmar Stehlik¹, ¹ Institut für Experimentalphysik, Freie Universität Berlin, Berlin, Germany² Max-Planck Institut für Medizinische Forschung, Jahnstrasse 29, Heidelberg, Germany

ABSTRACT- During the past several years the hydrogen bond interaction between the quinone cofactors in different types of photosynthetic reaction centers (RC) has been recognized as important both for the binding of the quinones to the protein complexes and for their functions. Hydrogen bonds have a direct influence on the spin density distribution in the quinone radicals formed during the electron transfer process. The asymmetry in the spin density distribution over the quinone anion radical in the RC as well as the hyperfine splitting parameters of the proton(s) involved in the H-bond(s) can be directly measured by EPR and ENDOR techniques. The results obtained from such experiments should correlate with the number, asymmetry and strength of a particular H-bond between the quinone and the protein. The asymmetry in the spin density distribution over quinone acceptor in PS I has been studied using ¹³C and ¹⁷O isotope labels in different positions of quinone ring. The Hfs of the H-bond proton have been characterized by pulsed Q-band ENDOR and the back bone nitrogen which provides the proton for the formation of the H-bond has been detected in nitrogen ESEEM experiments. The magnitude of the dipolar tensor corresponds to an unusually short H-bond which can be estimated from the point-dipole approximation (∼1.5 ± 0.1 ). All obtained results are in good agreement with high asymmetry in spin density distribution over quinone ring in PS I. Experimental evidence for an unusual backbone H-bond strength are analysed by comparing the quinone binding site and the observed H-bond hfc tensors for related quinone radical anions in frozen protic solution as well as in the Q_A site of type II reaction centers. Possible mechanisms of increased H-bond strength are considered.

KEY WORDS: Phylloquinone, Photosystem I, EPR