This study was designed to test whether the single appended phosphonate group in GdDOTA-1AmP is sufficient for catalyzing the exchange of proton from the single inner-sphere water-exchanging molecule. Unlike the other phosphonate derivatives in this series, GdDOTA-1AmP showed a surprisingly smooth increase in r(1) relaxivity from 3.0 to 6.3 mM^(-1) s^(-1) at 20 MHz as the pH was lowered from 9 to 2.5. In comparison to the bis-, tris-, and tetrakis-phosphonate analogues, which all show a …