Metal cation assisted protonation of retinylidene Schiff base in aqueous acetonitrile and in reverse micelles

Abstract

Metal cations (Ca²⁺, Mg²⁺, Zn²⁺) cause water to protonate all-trans-N-retinylidenebutylamine when solubilised in (i) acetonitrile containing aqueous MgBr₂ and (ii) CaA₂, MgA₂ and ZnA₂[where A = bis(2-ethylhexyl) sulfosuccinate anion] solubilised water pools in heptane. The results are discussed in terms of the role of metal cations and water in wavelength regulation in the opsin family of proteins.