Pectin is often used with acidifiers (typically citric acid). It’s not technically a “synergistic” effect, but the acid inverts part of the sugars, and since invert sugar is hygroscopic it binds some of the free water, making it easier for the pectin gel to contain the rest. In cooking fruit jellies, it’s common to add the pectin near the beginning of the cooking, but to hold onto the acid until the end. As soon as the acid is added, the gelling effect is dramatic.