The effect of hydrophilic adhesive monomers on the stability of type I collagen

The adsorption effects of adhesive monomers on the structural stability of type I collagen were studied at an acid pH condition for two monomers: 2-hydroxyethyl methacrylate (HEMA), a neutral monomer and N-methacryloyl glycine (NMGly), an acidic monomer. Differential scanning calorimetry (DSC) measurements were done to assess the denaturation temperature (Td), which is a measure of the structural stability of the proteins, including the bovine tendon collagen (BTC). While HEMA lowered the Td of the BTC linearly with HEMA concentrations, NMGly exhibited a two-step decrease of the Td. The rate of decrease in the Td by the NMGly was by far greater than the rate of decrease with the HEMA. The first step had a larger slope than the second step in the Td vs. CNMGly plot. The degree of adsorption of these two monomers to the BTC was estimated from infrared absorption measurements on the monomer solutions of various concentrations, before and after the immersion of the BTC. Both the adsorption of HEMA to the BTC and the Td of the BTC were linearly dependent on HEMA concentrations. Conversely, NMGly was adsorbed to the BTC, again, in a two-step decrease similar to the Td vs. CNMGLy profile. An enhanced adsorption of NMGly, which might be attributed to a strong electrostatic interaction, was observed below 0.013 mol%. Circular dichroism measurements of the collagen of the same type as the BTC, in the absence and in the presence of the monomers, revealed that the native collagen helix structure was scarcely affected by the monomers. From these observations, it was concluded that (1) both of the monomers were adsorbed onto the BTC, which thus destabilized the triple helical collagen structure, and that (2) the effect was higher for NMGly in which the electrostatic attraction with the oppositely charged collagen might be effective at a pH of 3. If compared to HEMA, an acidic NMGly is a potential monomer that binds strongly to collagen and one that is hardly hydrolyzed.