Dielectric spectroscopy of energy coupling membranes: Chloroplast thylakoids

(1) The dielectric properties (complex permittivity) of suspensions of chloroplast thylakoids have been determined in the range 10 Hz to 13 MHz. In common with other charged membrane vesicles, thylakoids exhibit two broad dielectric dispersions corresponding to the classical α- and β-dispersions. (2) Heat treatment of thylakoids, to produce blebs of a greater average radius, increases, as expected, the magnitude of the β-dispersion, but not that of the α-dispersion. This result suggests that the α-dispersion is not caused solely by the unrestricted tangential relaxation of the ions of the diffuse double layer. (3) The magnitude of the α-dispersion is strongly pH-dependent, and is negligible at the pI (ca. pH 4.3-4.4) of thylakoids. The α-dispersion is much less sensitive to pH. Measurements of the magnitude of the a dispersion may therefore be used to obtained the pI of charged membrane vesicles. (4) Both α- and β-dispersions are significantly decreased by treatment of the thylakoids with the cross-linking reagent glutaraldehyde, suggesting that the (lateral) motions (in particular) of charged membrane components contribute to the dielectric properties in this frequency range. The relaxation times observed, however, are not consistent with the view that such motions are restricted by hydrodynamic forces alone. The breadth of the dispersion remains very large, however, suggesting also a variable restriction on the genuinely tangential motions of double ions.