Characterization of Acid-Neutralizing Basic Monomers in Co-Solvent Systems by NMR

Abstract

Metabolic activity of the oral microbiota leads to acidification of the microenvironment and promotes demineralization of tooth structure at the margin of composite restorations. The pathogenic impact of the biofilm at the margin of the composite restoration could be reduced by engineering novel dentin adhesives that neutralize the acidic microenvironment. Integrating basic moieties into methacrylate derivatives has the potential to buffer against acid-induced degradation, and we are investigating basic monomers for this purpose. These monomers must be compatible with existing formulations, which are hydrophobic and marginally miscible with water. As such, co-solvent systems may be required to enable analysis of monomer function and chemical properties. Here the authors present an approach for examining the neutralizing capacity of basic methacrylate monomers in a water/ethanol co-solvent system using nuclear magnetic resonance (NMR) spectroscopy. NMR is an excellent tool for monitoring the impact of co-solvent effects on pKa and buffering capacity of basic monomers because chemical shift is extremely sensitive to small changes that most other methods cannot detect. Because lactic acid (LA) is produced by oral bacteria and is prevalent in this microenvironment, it was used to analyze the effectiveness of basic monomers to neutralize acid. The 13C chemical shift of the carbonyl in LA was monitored as a function of ethanol and monomer concentration and each was correlated with pH to determine the functional buffering range. This study shows that the buffering capacity of even very poorly water-soluble monomers can be analyzed using NMR.