Proton-electron hyperfine coupling constants of the chlorophyll a cation radical by ENDOR spectroscopy

In this paper we describe the assignment of the major coupling constants in monomer chlorophyll a cation free radical ENDOR spectroscopy. To facilitate chemical manipulation methylpyrochlorophyllide a has been used as a stand-in, a suite of six selectively deuterated derivatives have been subjected to ENDOR investigation. Details of the synthesis of compounds are described. To study the effect of structural features on the spin distribution in the free radicals, six additional derivatives have been studied. Five coupling constants have been assigned, which account for about 80% of observed electron spin resonance line width in the chlorophyll a monomer cation radical. The spin distribution appears to highly asymmetric.