High-resolution absorbance-difference spectra of the triplet state of the primary donor P-700 in Photosystem I subchloroplast particles measured with absorbance-detected magnetic resonance at 1.2 K. Evidence that P-700 is a dimeric chlorophyll complex

Abstract

ESR transitions in zero field of the triplet state of the primary donor (P-700 T) of Photosystem (PS) I subchloroplast particles of spinach have been measured by monitoring the absorbance at 697 nm at 1.2 K (absorbance-detected magnetic resonance (ADMR) spectra). Using the ADMR technique, we have recorded the triplet absorbance-difference spectrum (triplet-minus-singlet spectrum) of P-700 T minus P-700 of the primary donor of PS I at 1.2 K. This method is considerably more sensitive and selective than conventional flash absorption spectroscopy. The ADMR-monitored triplet-minus-singlet spectra of fresh and aged PS I particles are compared to published triplet-minus-singlet spectra of monomeric and dimeric chlorophyll (Chl) a in vitro. The triplet-minus-singlet spectrum of P-700 can be explained as a superposition of the triplet-minus-singlet spectrum of a dimeric Chl a complex and an additional red shift at about 689 nm, attributed to an adjacent Chl a molecule. We conclude that the primary donor of PS I is a dimeric Chl a complex with its long-wavelength absorption band peaking at 697 nm.