Field- and Temperature-Dependent 13C NMR Studies of the EDTA Zn2+ Complex: Insight into Structure and Dynamics via Relaxation Measurements

The relaxation rates for the three different carbon types in EDTA (carbonyl, CH2 central, and CH2 lateral) were measured with and without Zn2+ as a function of field strength and temperature. The use of different field strengths in combination with NOE measurements allowed for the contribution of each relaxation mechanism (chemical shift anisotropy; spin rotation; dipoledipole) to the total relaxation rate for each carbon to be determined. Temperature studies allowed for determination of the activation energy (E-a) for the motions of each carbon type. The most surprising result was the observation that the tc decreases significantly for the lateral carbon upon addition of Zn2+ at neutral pH, going from 54 to 8.6 ps at 298 K. This appears to be a pH-dependent phenomenon as other reports indicate that tau(c) increases for the lateral carbon upon addition of Zn2+ under strongly basic conditions.