MONODENTATE VS BIDENTATE BINDING OF LANTHANIDE CATIONS TO PO2- IN BACTERIORHODOPSIN

The frequency difference between the symmetric and antisymmetric stret ching vibration of PO2- in phosphatidylglycerol phosphate (PGP) is use d to differentiate between monodentate and bidentate binding of these groups to metal cations in the membrane of bacteriorhodopsin (bR) and phosphatidylglycerol phosphate. The binding of Ca2+ to PGP is found to have a frequency difference corresponding to monodentate binding. The symmetric and antisymmetric PO2- bands in bR show similar frequency s hifts upon Ca2+ binding, which is independent of pH. This suggests tha t Ca2+ has a monodentate type binding with the PO2- in bR. In contrast , the PO2- symmetric and antisymmetric frequencies of PGP complexes wi th trivalent lanthanide cations with higher charge density (Ho3+ and D y3+) are observed to have smaller separations and to increase their se paration with increasing pH toward the value observed for Ca2+ binding . Lanthanide cations (Ho3+ Dy3+, Eu3+;, Nd3+, and La3+) binding in bR at pH 4 show small frequency separations that are observed to have sim ilar frequency shifts with pH, the magnitude of which is dependent on the cation. It is proposed that at low pH the lanthanide cations with higher charge density have bidentate binding to bR, while at high pH, complexation with the OH- competes with one of the oxygens of the PO2- for the binding of the lanthanide ion thus changing the bidentate to monodentate type binding.