Ion exchange properties of isolated tomato fruit cuticular membrane: Exchange capacity, nature of fixed charges and cation selectivity.

Isolated tomato fruit cuticular membrane, free of extractable materials, was titrated potentiometrically using various bases. Three dissociable groups were observed in the pH ranges 3-6 (0.2 meq g(-1)), 6-9 (0.3 meq g(-1)) and 9-12 (0.55 meq g(-1)). The first group was tentatively assigned to-COOH groups of pectic materials and protein embedded in the membrane, the second to nonesterified-COOH groups of the cutin polymer and the third to phenolic-OH groups, such as non-extractable flavenoids present in the membrane, and to a small amount of-NH 3 (+) groups of proteins. The cuticular membrane exhibited a behavior typical of highly cross-linked, high-capacity ion exchange resins of the weak-acid type. Ion exchange capacity increased with increasing pH and neutral salt concentration. At constant pH and salt concentration, the exchange capacity increased with increasing counter ion valence and decreasing crystal radius, e.g. [tris (ethylenediamine) Co](3+)≥Ca(2+)>Ba(2+)>Li(+)>Na(+)>Rb(+)>N(CH3) 4 (+) . The cutin polymer exhibited a pronounced selectivity for Ca(2+) over Na(+) which increased with increasing neutralization of fixed charges. The large trivalent [Co(en)3](3+) was preferred only at low equivalent ionic fractions in the polymer. These results are discussed in relation to the structure and function of cuticular membranes.