MEASUREMENT OF ELECTRIC FIELD GRADIENT AT THE METAL SITES IN PROTEINS

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The structure around the metal site of mavicyanin, a protein molecule with a copper site, was investigated by using time-differential perturbed angular correlation of γ-rays related to a metastable state of ^<117>In. Its parent nuclei of ^<117>Cd are subjected to β^- decay to populate the 749 keV excited state of ^<117>In with a half-life t_<1/2>=2.49 h, decaying to the 315 keV excited state through the 660 keV intermediate state having a spin I= 3/2, t_<1/2>=53.6 ns, and an electric quadrupole moment Q=(-)0.59(1) b. The parent nuclei ^<117>Cd were obtained by irradiating enriched ^<116>CdO (96.53%) for about 30 min in thermal neutron flux of 1.93×10^<13> cm^<-2>s^<-1> or 2.34×10^<13> cm^<-2>s^<-1> at Kyoto University Research Reactor Institute. The Cd ions from solution of the irradiated sample were substituted in the metal sites of mavicyanin, from which Cu ions were extracted beforehand. The sample solution was subjected to PAC measurements.The field gradient values were determined … More from the PAC spectra of wild-type mavicyanin for pH=6.0, 7.5 and 8.0 to be 1.48×10^<22> to 2.08×10^<22> Vm^<-2>, while those of a mutant-type mavicyanin, Thr15Ala-Mav, were 0.43×10^<22> to 1.49×10^<22> Vm^<-2>. Compared with the value, 1.71×10^<22> Vm^<-2> reported on stellacyanin, the results obtained in this work seem to be reasonable. It was also found that there is a steep change of the gradient between 6.0 and 7.5 in pH for both of the proteins. In order to clarify the relevant change of the proteins in structure, several metal chelates were subjected to the PAC measurement as described above for comparison. In addition to that, additional parent nuclides of Cd-111m and In-111 decaying to PAC probes were used for the experiment for comparison as well. The latter experiment demonstrated that there is clear difference between proteins and metal chelates in gradient fields probably due to coordination of relevant ligands and that there is dependence of the used probes in measurement of the gradients. Less