Steric Effects of N-Acyl Group in O-Methacryloyl-N-Acyl Tyrosines on the Adhesiveness of Unetched Human Dentin

We have prepared various O-methacryloyl-N-acyl tyrosines (MAATY) to reveal the relationship between molecular structure near carboxylic acid and adhesive strength of MAATY-HRMA type adhesive resin to unetched dentin. In this study, we attempted to change the steric hindrance effect without changing the HLB value, i.e., introducing an iso-acyl group instead of n-acyl group into MAATY. O-methacryloyl-N-ethylbutyryl tyrosine (MIHTY) showed significantly lower adhesive strength when compared with O-methacryloyl-N-hexanoyl tyrosine even though both MAATY have the same HLB value. The possible explanation of the significantly different adhesive strength was that the 2-ethylbutyryl group in MIHTY was bulky, resulting in inhibition of the hydrogen bonding of the carboxylic group. The HLB value is independent of the steric effect of molecular structure, and thus the steric factor should be taken into consideration for the explanation of different adhesive strengths within the adhesive monomers having the same HLB value but different molecular structures.