When is Pheophytin used in electron transfer to the q cycle in plants?
Not sure if there is a definite answer. However, from structural studies, its proposed that essentially the Chlorophyll a and pheophytin(Pheo) are positioned ~10A away, and the transfer of a radical electron from the P680 center to Pheophytin does occur. The exact mechanism is possibly mediated by a tyrosine


Durant J.R et al (1995). Proceedings of the National Academy of Sciences of the United States of America, 92, 4798–4802
Oleg Victorovich (2006). Retrived:5/25/16. Site: http://samlib.ru/o/oleg_w_m/cdocumentsandsettingsolegmoidokumentythefunctionandthestructureofporphyrinmoleculesrtf.shtml


3.Zouni A et al (2001). Nature, 409, 739-743