Abstract
Consideration is given to the use of the time differential perturbed directional correlation (TDPDC) technique in biomedical applications such as structural studies of the selenoproteins. Mathematical models of the perturbed directional correlations show that the application of the TDPDC technique to the study of the electric quadrupole interactions experienced by an ensemble of probe nuclei bound to a protein is capable of yielding the fraction of probe nuclei occupying each of a number of distinct types of binding site in the protein, the strength, symmetry and inhomogeneity of the nuclear quadrupole interaction occurring at each different type of site and, if the environment of the protein is a viscous solution, the diffusion constant for the protein in the solution, from which the associated correlation time and hence the radius of the protein may be derived. A time spectrometer employing a pair of barium fluoride (BaF2) scintillation counters was designed and constructed to allow TDPDC experiments to be performed. The optimum time resolution of the spectrometer was found to be 354+/-10 ps for the 1.173 and 1.332 MeV gamma-rays emitted in the decay of 60Co. The characteristics of the spectrometer were inferior to those of similar systems which are described elsewhere. A number of improvements to the spectrometer that was used in this work have been suggested. The manner in which various deviations from the ideal experimental arrangement affect both the observed perturbed directional correlation and the optimum methods of conducting TDPDC experiments has been carefully considered. Of particular importance was the effect of source decentring, since the employed apparatus does not allow the radioactive source to be precisely centred. It was found that the implications of source decentring are that the values of the parameters describing the perturbation of the directional correlation must be derived from a single TDPDC spectrum, as must those of a number of additional parameters which are of little interest. The viability of this procedure was investigated by carrying out a TDPDC study of the 356-81 keV gamma-gamma cascade in 133Cs, taking account of the interference from sum-coincidence effects and competing cascades. From the TDPDC spectrum that was obtained, the smearing effects of the finite time resolution of the spectrometer were partially removed by deconvolution, resulting in an unfolded time spectrum to which attempts were made to fit a suitable model. Although it was not possible to achieve a satisfactory fit, a number of potential remedies for the poor fit have been proposed. A TDPDC study of the 121-280 keV gamma-gamma cascade in 75As was performed for the purpose of assessing the feasibility of using the TDPDC technique in future investigations employing 75Se-labelled selenoproteins. Account was again taken of the interference from sum-coincidence effects and competing cascades. A bid to deconvolve the smeared TDPDC spectrum that was acquired was not successful, and so attempts were made to fit to the smeared spectrum a model which allowed for the finite time resolution of the spectrometer. The fitting proved to be problematic, indicating that it would be difficult to apply the TDPDC technique to an investigation of 75Se-labelled selenoproteins. However, this conclusion is perhaps unduly pessimistic because some of the problems that arose in the feasibility study will not be present in practice when a 75Se-labelled selenoprotein is used. The available evidence suggests that 73Se offers advantages over 75Se as a radiolabel in TDPDC studies of the selenoproteins.