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

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Abstract
Longitudinal and traverse PMR rates for water in the hydration sphere of Gd(III) bound to non-immune rabbit IgG (immunoglobulin G) were determined over a wide range of frequencies (4-84 MHz) at constant temperature (19.degree. C) using pulsed NMB spectrometry. The rates were also determined at temperatures between 0 and 40.degree. C for 2 frequencies (61 and 84 MHz). The rates were fitted to existing theory using a computer least-squares 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. A slow exchange contribution to the relaxation rates was important in deciding which parameters are poorly determined. A rotational correlation time (.tau.R) was obtained which suggested there might be considerable internal motion of the Fc region (C-terminal half of heavy-chain dimer) of the IgG molecule. The possibility of large errors in this value prevented unequivocal conclusions being drawn.

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