The monomeric chlorophyll, Chl
D1, which is located between the P
D1P
D2 chlorophyll pair and the pheophytin, Pheo
D1, is the longest wavelength chlorophyll in the heart of Photosystem II and is thought to be the primary electron donor. Its central Mg
2+ is liganded to a water molecule that is H-bonded to D1/T179. Here, two site-directed mutants, D1/T179H and D1/T179V, were made in the thermophilic cyanobacterium,
Thermosynechococcus elongatus, and characterized by a range of biophysical techniques. The Mn
4CaO
5 cluster in the water-splitting site is fully active in both mutants. Changes in thermoluminescence indicate that
i) radiative recombination occurs
via the repopulation of *Chl
D1 itself;
ii) non-radiative charge recombination reactions appeared to be faster in the T179H-PSII; and
iii) the properties of P
D1P
D2 were unaffected by this mutation, and consequently
iv) the immediate precursor state of the radiative excited state is the Chl
D1+Pheo
D1? radical pair. Chlorophyll bleaching due to high intensity illumination correlated with the amount of
1O
2 generated. Comparison of the bleaching spectra with the electrochromic shifts attributed to Chl
D1 upon Q
A? formation, indicates that in the T179H-PSII and in the WT*3-PSII, the Chl
D1 itself is the chlorophyll that is first damaged by
1O
2, whereas in the T179V-PSII a more red chlorophyll is damaged, the identity of which is discussed. Thus, Chl
D1 appears to be one of the primary damage site in recombination-mediated photoinhibition. Finally, changes in the absorption of Chl
D1 very likely contribute to the well-known electrochromic shifts observed at ~430?nm during the S-state cycle.
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