SPECTRAL STUDIES OF LANTHANIDE INTERACTIONS WITH MEMBRANE SURFACES

We have monitored the interactions of the series of trivalent lanthani de cations with the thylakoid membrane surface of spinach chloroplasts using two complementary spectral techniques. Measurements of the fluo rescence emission of the extrinsic probe 2-p-toluidinonaphthalene-6-su lfonate (TNS) and the absorbance of the intrinsic chromophore chloroph yll provide two sensitive means of characterizing the dependence of th e cation-membrane interaction on the nature of the cation. In these sy stems, added lanthanide cations adsorb onto the membrane surface to ne utralize exposed segments of membrane-embedded protein complexes. The lanthanide-induced charge neutralization increases the proximity of ad ded TNS anion to the membrane surface as evidenced by variations in th e TNS fluorescence level and wavelength of maximum emission. Our resul ts reveal a strong dependence of TNS fluorescence parameters on both l anthanide size and total orbital angular momentum L value. Lanthanides with greater charge density (small size and/or low L value) enhance t he TNS fluorescence level to a greater extent. A possible origin for t he lanthanide-dependent TNS fluorescence levels is suggested in terms of a heterogeneity in the number and type of TNS binding sites. In the absence of the probe TNS, lanthanide-induced changes in the chlorophy ll absorption spectrum reflect the shrinkage of chloroplasts accompany ing thylakoid membrane stacking. Absorbance increases in the 500-660 n m region, attributed to increases in light scattering arising from the membrane structural reorganization, reveal a dependence on lanthanide identity. The data are consistent with the proposal that larger lanth anides with smaller enthalpies of hydration induce more significant me mbrane appression. These investigations illustrate a novel utilization of lanthanides in cation binding studies by employing their chemical and physical differences rather than their similarities in luminescenc e properties.