Monodentate vs Bidentate Binding of Lanthanide Cations to PO₂^- in Bacteriorhodopsin

Abstract

The frequency difference between the symmetric and antisymmetric stretching vibration of PO2 - in phosphatidylglycerol phospate (PGP) is used to differentiate between monodentate and bidentate binding of these groups to metal cations in the membrane of bacteriorhodopsin (bR) and phosphatidylglycerol phospate. 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 shifts upon Ca2+ binding, which is independent of pH. This suggests that Ca2+ has a monodentate type binding with the PO2 - in bR. In contrast, the PO2 - symmetric and antisymmetric frequencies of PGP complexes with trivalent lanthanide cations with higher charge density (Ho3+ and Dy3+) are observed to have smaller separations and to increase their separation 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 similar 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.

The Effect of Metal Cation Binding on the Protein, Lipid and Retinal Isomeric Ratio in Regenerated Bacteriorhodopsin of Purple Membrane¶

“…Larger changes in the case of Ho 3ϩ than Ca 2ϩ indicates the difference in the nature of cation binding between trivalent and divalent cation. This suggests that Ca 2ϩ might not prefer binding to the lipid head groups as does Ho 3ϩ as concluded previously (48).…”

“…The two negative bands at 1218 and 1050 cm Ϫ1 resulting from the POO Ϫ asymmetric and symmetric stretching of phatidylglycerol phosphate lipids (44,46), which is one of the major charged lipid components in the deionized blue membrane (47), shift to 1183 and 1114 cm Ϫ1 , respectively, upon Ho 3ϩ binding. Such a frequency shifting pattern in which the asymmetric stretching of POO Ϫ shifts to a lower frequency and the symmetric shifts to a higher frequency, therefore resulting in a smaller frequency separation between them suggests a bidentate type of cation binding in nature (48). Full titrations of metal cations (Ho 3ϩ and Ca 2ϩ ) with the blue state of bR have been carried out and the integrated band intensity versus M nϩ /bR molar ratio for a few selected vibrational modes has been shown in Fig.…”

Studies of cation binding in ZnCl2-regenerated bacteriorhodopsin by x-ray absorption fine structures: effects of removing water molecules and adding Cl- ions

“…In addition, it was reported that the second shell observed in Fe3+-regenerated bR (Englehard et al, 1987) was missing in Mn2+-regenerated bR (Sepulcre et al, 1996). Recent publications on the binding of rare earth ions to deionized bR suggest that the binding site differs from that for Ca2 , in that it has strong affinity for bidentate binding to the PO2 groups of the lipids (Griffiths et al, 1996b). From the near-infrared vibronic side band spectroscopy of Yb3+, it was concluded that in addition to binding to the PO2 groups of the phospholipids, the rare earth Ybb3+ also binds to the protein residues at the same time (Roselli et al, 1996).…”

Effect of Binding of Lanthanide Ions on the Bacteriorhodopsin Hexagonal Structure: An X-ray Study

“…Our previous study on the pH dependence of the FTIR spectrum of Ca 2+ and lanthanides in the PO 2 -sM n+ binding has shown that at low pH the lanthanides form bidentate binding to PO 2 -groups of the phospholipids which is not observed for Ca 2+ binding. 44 More recently, using the emission for Yb 3+ , it was found that the binding site of the unquenched lanthanide was involved in bidentate binding to the PO 2 -groups and binding to the protein residues as well. 45 EXAFS studies on Mn 2+ has recently shown 46 that its binding seems to differ from the results of EXAFS studies 27 on Fe 3+ .…”

“…This would explain the absence of detecting bidentate binding in the FTIR spectra of Nd 3+ -regenerated bR at neutral pH. 44 It thus appears that at least one strong binding site for lanthanide metal ions (and transition metal ions with 3+ charge) involves the bidentate complexation with the PO 2 -groups of the phospholipids (and some protein residues) and thus surfacelike sites. Since lanthanides are found to radiate strongly, this binding site might not involve water molecules that are known to quench the lanthanide emission.…”