Melanosomes have the capacity to bind significant concentrations of calcium, suggesting there are surface binding sites that enable cations to access the interior of fully pigmented melanosomes. The surface of melanosomes is known to contain significant concentrations of carboxylate groups which likely are the initial biding sites for calcium, but their arrangement on the surface of the melanosome is not known. In various calcium proteins, a bidentate coordination by two carboxylate groups is the most common structure. In this study, we determine the distance between neighboring surface carboxylic acid groups by examining the binding of a series of diamines +H3N(CH2)mNH(3)(+) (m = 1-5) to melanosomes isolated from the ink sacs of Sepia officinalis and bovine choroid tissue. Of these amines, ethylenediamine (m = 2) shows optimal bidentate binding, revealing a narrow distribution of distances between neighboring carboxylic acid groups, similar to 480 pm, similar to that found in proteins for calcium binding motifs involving two carboxylate groups.