EFFECT OF BINDING OF LANTHANIDE IONS ON THE BACTERIORHODOPSIN HEXAGONAL STRUCTURE - AN X-RAY STUDY

The effect of the bind;ng of trivalent lanthanide metal cations (Eu3+, Ho3+, and Dy3+) on the hexagonal structure of bacteriorhodopsin (bR) is investigated at different pH using x-Ray diffraction to examine fil ms made by slow evaporation of the corresponding regenerated bR. It is observed that the lanthanide-regenerated bR (at a ratio of 2:1 metal ion to bR) does not form a 2D structure isomorphous to that of native bR or Ca2+-regenerated samples at low sample pH. The native bR hexagon al structure is recovered by titration of the sample with sodium hydro xide. The pH at which the hexagonal structure is recovered depends on the charge density of the lanthanide ion used for the regeneration. Th e higher the charge density of the ion, the higher the pH at which an isomorphous lattice is formed. A model is proposed in which at normal or low pH a complex bidentate and monodentate type binding (which disr upts the lattice hexagonal structure) exists between a lanthanide ion, the O- of PO2- groups, and/or the amino acid residues. Ar high pH, co mplexation with OH- takes place, which converts this binding to a simp le monodentate type complete that leads to the recovery of the lattice structure. An equation is derived for the pH at which this conversion takes place and is found to be proportional to the binding constant o f the lanthanide ions to the O- of the PO2- groups or the amino acid r esidues and inversely proportional to the binding constant of the lant hanide ion to the OH- groups. This predicts an increase of conversion pH with the charge-density of the lanthanide ion, as observed.