Difficulties in Determining Accurate Molecular Motion Parameters from Proton Relaxation Enhancement Measurements as Illustrated by the Immunoglobulin G · Gd(III) System

Longitudinal and traverse proton magnetic relaxation rates for water in the hydration sphere of Gd(III) bound to non‐immune rabbit IgG (immunoglobulin G) have been determined over a wide range of frequencies (4–84 MHz) at constant temperature (19 °C) using pulsed nuclear magnetic resonance spectrometry. The rates have also been determined at temperatures between 0 and 40 °C for two frequencies (61 and 84 MHz). The rates were fitted to existing theory using a computer leastsquares procedure. Further computer analysis was then carried out to determine the sensitivity of the best‐fit error to variation in the variable parameters in the theoretical expressions used. These include the water co‐ordination number (q) for which it was found large variations could occur (between approximately 2 and 8) for only small changes in the error of best‐fit. It was concluded that a slow exchange contribution to the relaxation rates was important in deciding which parameters are poorly determined. A rotational correlation time (τR) was obtained which suggested there might be considerable internal motion of the Fe region (C‐terminal half of heavy‐chain dimer) of the IgG molecule. However the possibility of large errors in this value prevented unequivocal conclusions being drawn.