A PAC study of the binding of 111In to a monoclonal antibody via the macrocyclic molecule 1,4,7-triazacyclononanetriacetic acid

Time integral and time differential PAC measurements have been made from samples of 111In bound to the functionalized 9N3 macrocycle, and 111In bound to the monoclonal antibody AUA1 via the macrocycle, over the temperature range 80-350K. Values obtained for the integrated perturbation coefficient G22(∞) clearly illustrate the effects which the antibody has on the angular correlation of the γ-rays emitted by the 111In decay. Measurements of the quadrupole frequency in the 111In-9N3-AUA1 samples show a transition temperature between 250 and 275 K which was not detected in the 111In-9N3. An arrhenius plot of the temperature dependence of the correlation time for the 111In-9N3 yielded a value of 0.10 ± 0.01 eV for the activation energy associated with molecular re-orientation, whilst a Debye plot indicated an effective volume of (10 ± 1)·10-27 m3 for the same sample. Extrapolation of the Debye plot suggests a high relaxation constant which may be attributed to internal vibrational modes in the macrocycle. Comparisons are made with similar work on the plasma protein transferrin. (author)