Equilibrium behavior of an isotropic polyelectrolyte gel model of the tectorial membrane: effect of pH.

An isotropic polyelectrolyte gel model (Weiss and Freeman, 1996a) of the tectorial membrane (TM) was extended to incorporate the effect of pH. The effect of pH is analyzed in order to interpret measurements of the effect of pH on the dimensions of the TM (Freeman et al., 1996a). The pH dependence of the model results from the binding of hydrogen ions to TM macromolecules--to both neutral sites with basic pK and negatively charged sites with acidic pK. Parameters of the model can be based on estimates of the concentration in the TM of collagen and glycosaminoglycans (GAGs), two identified constituents of the TM that account for approximately 40% of its dry weight. The resulting model shows swelling responses at both high and low pH that are qualitatively similar to the measurement on the TM but that differ quantitatively. Alternatively, parameters of the model can be chosen in an ad hoc fashion to closely fit the measurements of TM swelling as a function of pH. Taken together these results suggest that either estimates of the collagen and GAG content of the TM are in error or constituents of the TM, which have not yet been identified, contain appreciable pH-dependent fixed charge and contribute to the swelling behavior of the TM.