Hydroxyapatite crystal growth on modified collagen I-templates in a model dual membrane diffusion system

The finding of proteoglycans (PG) in mineralized tissues confirms their role in the biomineralization process, but fails to give answers regarding possible detailed mechanisms of interaction. As deglycosilation of PG (especially decorin) is known to be a prerequisite for biomineralisation in vivo, glucuronic acid (GA), as a monomer common to different proteoglycans and glycosaminoglycans; and decorin were studied in mineralization experiments together with collagen type I using the dual membrane diffusion method. It was shown that under fibrillogenesis conditions glucuronic acid and decorin bind to a different extent to collagen. Binding of GA, in contrast to decorin, results in changes in both the fibril morphology and the banding pattern. The mineralization of unmodified collagen templates and of those modified with GA and decorin results in the formation of different calcium phosphate phases. Hydroxyapatite (HAP), as the thermodynamically stable product under the chosen experimental conditions, is formed only in the case of collagen modified with GA. To aid interpretation of this effect, a possible mechanism of glucuronic acid interaction with collagen by a non-enzymatic glycation pathway is proposed and discussed. Stimulation of HAP formation is explained by the introduction of acidic GA-carboxylate groups that allow an increase of the electrostatic binding of calcium ions compared to unmodified collagen molecules. Substrates modified in this way should subsequently serve as model templates for the mineralization process in order to make deductions about the mechanism by which acidic groups affect the biomineralization of collagen via the structure of the monomers as well as the architecture of the fibrils.