Melanin isolated from the ink sac ofSepia officinalis(Sepiamelanin) has been proposed as a standard for natural eumelanin, and a standard mild isolation and purification protocol forSepiamelanin has been developed (Zeise, doctoral dissertation, Johns Hopkins University, 1991). The goal of the present work, developed usingSepiamelanin, was to quantify the bioavailable carboxylic acid groups present in melanin particles. Bioavailability is governed by the accessibility of carboxy groups to the surrounding biological milieu, and is expressed as microequivalents of carboxy group per gram of melanin. The present work was carried out using an heterogeneous slurry of melanin in a nonaqueous system. A standard acidic titrant, and an automatic titrator operating in an equilibrium titration mode were used to characterize and quantify the carboxy group content ofSepiamelanins and several other commonly used melanins purified by a standard method (Zeise et al.,Pigment Cell Res. Suppl 2:48–53, 1992
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