Interaction of water-soluble collagen with poly(acrylic acid)

Interactions between poly(acrylic acid) labeled with pyrene (PAA-Py) and succinylated calfskin collagen (type I) (SCSC) were studied by fluorescence spectroscopy. PAA-Py exhibits a strong emission from pyrene monomer (intensity, I(M)) when it exists in an extended conformation. It exhibits another broad emission from pyrene excimer (intensity, I(E)) when it adopts a collapsed globule conformation. At pH 3, a value that is lower than the isoelectric point of SCSC, the ratio I(E)/I(M) value decreased cooperatively with increasing concentration of SCSC at constant PAA-Py concentration, under salt-free condition. On the other hand, this effect was not observed in the presence of 0.1 M NaCl. At pH 7, a value higher than the isoelectric point of SCSC, the ratio I(E)/I(M) was not affected by the presence of SCSC in the absence and presence of salt. From electrophoretic light scattering experiments, it was found that at pH 3 PAA-Py was negatively charged, while SCSC had a positive charge. Thus it is strongly suggested that the two polymers interact by electrostatic attraction at low pH where they are oppositely charged, and that PAA-Py adopts an extended conformation in the complex formed with SCSC. Similar interactions are believed to occur between dentinal collagen and the polycarboxylate component of glass-ionomer cements.