MnPs were 0.1 mM and ascorbate was 5 mM in 0.05 M phosphate buffer, pH 7.8, (25±1)0C. The T1 relaxivity measurements were carried out immediately after ascorbate addition. (A) T1 relaxivity values in the absence and presence of ascorbate, Asc, are plotted for different MnPs. (B) T1 relaxivity values in the presence and absence of ascorbate are plotted versus metal-centered reduction potential, E1/2. (C) T1 relaxivity values are plotted versus E1/2 and pKa(ax). The following porphyrins are plotted in (B) and (C): (#1) MnTE-2-PyPhP5+, (#2) MnTE-3-PyP5+, (#3) MnTE-2-PyP5+, (#4) MnTPhE-2-PyP5+, (#5), MnTnHexOE-2-PyP5+, and (#6) MnTnOct-2-PyP5+.