The monomeric chlorophyll, Chl D1 , which is located between the P D1 P 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 4 CaO 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 D1 P 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 1 O 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 1 O 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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