P-E1-21 Muitifrequency EPR of site-selectively 13C-labelled quinone and tyrosine electron transport compounds in vivo and in vitro: R.J. Hulsebosch,a J.S. van den Brink,a P. Gast,a T.N. Kropacheva,b R.I. Samoilova,c J. Raap,d J. Lugtenburg,d Yu.D. Tsvetkovc and A.J. Hoff.a a Department of Biophysics, Huygens Laboratory, Leiden University, Leiden, The Netherlands. b Department of Chemistry, Udmurt University, Izhevsk, Udmurtia, Russia. c Institute of Chemical Kinetics and Combustion, Academy of Sciences, Novosibirsk, Russia. d Department of Chemistry, Gorlaeus Laboratory, Leiden University, Leiden, The Netherlands