Reversible pKa shifts of Brønsted acids such as carboxylic acids, alcohols, ammonium ions, and imidazolium ions in proteins play crucial roles in enzymatic reactions and efficient molecular/ion transport. To vary the pKa values of acids, the proteins change the dielectric constants around the acids and/or reversibly stabilize/destabilize protons on the acids by hydrogen bonding and/or coulombic interaction through conformational changes induced by external stimuli. Synthetic materials that alter their binding affinity to protons in response to external stimuli have gained considerable attention as substitutes for proteins. We revealed that pNIPAm-based NPs that show large and reversible pKa shifts can be prepared by the “proton imprinting” and “microenvironment imprinting” strategy: When acidic monomers such as acrylic acids were protonated in the polymerization process, the protonated acids were easily incorporated into the less polar domains of growing collapsed-pNIPAm NPs, and were subsequently stabilized by a cross-linker. As a result, high pKa acids (pKa ~8) were imprinted in the NPs. When the degree of cross-linking was low, the structure of the high pKa acidic sites was reversibly denatured by temperature-induced conformational changes of the polymer chains, resulting in a large reversible pKa shift as proteins. The range of pKa shift was in the neutral pH around 6-8. The pKa variation range of carboxylic acids in the NPs can be tuned by designing structure of monomers containing carboxylic acids and imprinting cationic group in the NPs. The pKa variation range can be lowered/raised to be 4.3/8.7 by stabilizing/destabilizing carboxylate anions by modifying the acids with electron withdrawing/donating group. The pKa of acids can also be lowered dramatically by imprinting cationic functional group such as guanidium group around the carboxylate anions.

Tuning pKa of Brønsted acids in temperature-responsive hydroel particles by proton- and ion-imprinting strategy

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.