Selective demineralization of hard tissues in organic solvents: retention or extraction of proteoglycan?

SUMMARY Ideally, demineralization of tissues, fossils, etc., should not result in removal of other chemical species along with the mineral components. Organic solvents containing alkylammonium salts of EDTA are good demineralizing media (Scott & Kyffin, 1978), offering great flexibility and a variety of solvent properties which can be chosen either to extract or to retain tissue polymers in situ. The properties of the alkylammonium cation are crucial in determining whether a given solvent will extract tissue polyanions (i.e. nucleic acids, proteoglycans, acid glycoproteins, etc.) or not. The situation is analysed for the general case, using the fact that a simple ion‐exchange reaction determines the outcome. Some solvent‐alkylammonium combinations, e.g. ethanol‐trimethylammonium, are very poor solvents for proteoglycans, leaving them behind in the tissue, whereas by increasing the size of the organic portion of the alkylammonium salt (e.g. to tri‐n‐butylammonium) or changing the solvents to e.g. dimethyl sulphoxide, the proteoglycan can be extracted from cartilage, etc. The aninic half of the alkylammonium salt plays a relatively minor role. The exception is EDTA itself, which sequestrates inorganic ions (e.g. Na+) in organic solvents, thus driving the ion‐exchange towards completion, with efficient conversion to alkylammonium salts of the polyanion. These results, and the chemical background to them, constitute a general framework within which various strategies for demineralization (with or without extraction of tissue polyanion such as proteoglycans) can be formulated.