Interactions between homopolypeptides and lightly cross-linked microgels.

The relative importance of electrostatic and nonelectrostatic interactions in peptide-microgel systems was evaluated by micromanipulator-assisted light microscopy, confocal microscopy, and circular dichroism. For this purpose, the interaction of various homopolypeptides with lightly cross-linked polyelectrolyte gel particles ( approximately 70 microm in diameter) was studied with focus on peptide-induced microgel deswelling and its relation to peptide distribution within the microgel particles. Negatively charged poly-l-glutamic acid (pGlu) and poly-l-aspartic acid (pAsp), as well as uncharged poly-l-proline (pPro) and poly-l-threonine (pThr), were found to not bind to negatively charged poly(acrylic acid) microgels under the conditions investigated, but were instead depleted from the microgel particles. Positively charged poly-l-arginine (pArg), poly-l-histidine (pHis), and poly-l-lysine (pLys), on the other hand, interacted strongly with the oppositely charged microgel particles and caused significant deswelling of these. In parallel, cationic acrylamidopropyltriethylammoniumchloride (APTAC) microgels bound negatively charged polypeptides to a much higher extent than positively charged and uncharged ones. These findings suggest that electrostatic interactions dominate peptide binding and resulting microgel deswelling in these systems. Nevertheless, although the amount of cationic peptide bound to the anionic microgel particles was similar for cationic pLys, pArg, and pHis, peptide-induced gel deswelling differed significantly, as did the change in peptide conformation after microgel binding and the peptide distribution within the microgels. These effects, as well as pH dependent binding and release of titrable pHis, are discussed in terms of the effects of the charge density of, and structural differences between, the cationic homopolypeptides on the interaction with the oppositely charged microgel particles.