Proton-Metal Cation Exchange in the Cell Wall of Nitella flexilis.

When protons are exchanging for bivalent cations (Cu(2+), Zn(2+), or Ca(2+)) on the carboxylic groups of Nitella flexilis cell wall, the values of the respective global equilibrium constants do not change up to a protonation degree of 80%. These values drastically increase at higher proton concentrations and tend to approximately 3.4, which is the intrinsic pK value of the constitutive alpha-d-galacturonic acid monomer. These data suggest that the electric field in the matricial polymer and the cation bridges between pairs of negative sites have disappeared.