Penicillamine disulfide (PNS) and alkaline cations

D-penicillamine disulfide (PNS) shows protolytic properties and is able to form complexes with cations, because it has two aminic groups and two carboxylic groups. The four protonation constants of its deprotonated species were determined by means of electromotive force (e.m.f.) measurements of a galvanic cell involving a glass electrode at 25degreesC and in a constant ionic medium constituted by N(CH3)(4)Cl 3.00 or 1.00 mol dm(-3). At 25degreesC and in 3.00 mol dm(-3) N(CH3)(4)Cl as ionic medium, equilibria taking place between PNS and lithium, sodium and potassium ions were investigated. Experimental data, again obtained from e.m.f. measurements, were explained by assuming the formation of species of the type MH2PNS ed M2H2PNS, where M indicates a cation. Stability constants for each proposed species were calculated. A comparison with cystine is discussed.