High‐resolution triplet‐minus‐singlet absorbance difference spectrum of photosystem II particles

The triplet state of the primary donor of photosystem II particles prepared from a mutant of Chlamydomonas reinhardtii has been studied at 1.2 K with absorbance‐detected ESR in zero‐magnetic field (ADMR). Two sets of resonances with slightly different zero‐field splitting parameters | D | and | E | were observed, | D | = 285.5, | E | = 38.8 and | D | = 288.8 × 10 −4 cm −1 , | E | = 42.2 × 10 −4 cm −1 , respectively. Both sets of | D | and | E | values are close to those found for P T ‐700, as are the sublevel decay rates k x = 930 ± 40, k y = 1088 ± 50 and k z = 110 ± 5 s −1 . The AMDR‐detected triplet‐minus‐singlet absorbance difference spectrum of P T ‐680 is very similar to that of P T ‐700 and closely resembles that of covalently connected Chl a dimers in vitro. We conclude that P‐680 is a Chl a dimer whose general structure is similar to that of P‐700.