An NMR study elucidating the binding of Mg(II) and Mn(II) to spinach plastocyanin. Regulation of the binding of plastocyanin to subunit PsaF of photosystem I

► Dissociation constants for the binding of Mg(II) and the luminal domain of PsaF to plastocyanin. ► Competitive binding between Mg(II) and PsaF to the acidic patches of plastocyanin. ► Exploration of Mn(II) as a paramagnetic analogue of Mg(II) binding to plastocyanin. ► PRE-derived structure of the complex between plastocyanin and hexaquo Mn(II). ► Mg(II), a plausible modulator of electron donation to photosystem 1.