MEASUREMENT OF ELECTRIC FIELD GRADIENT AT THE METAL SITES IN PROTEINS

利用γ射线在亚稳态^<117>In下的微分摄动角相关，研究了具有铜位点的蛋白质分子mavicyanin金属位点周围的结构。^<117>Cd的母核经过β^-衰变进入^<117>In的749kev激发态，半衰期t_<1/2>=2.49 h，通过自旋I= 3/2, t_<1/2>=53.6 ns的660 keV中间态衰至315 keV激发态。在京都大学反应堆研究所的热中子通量为1.93×10^<13> cm^<-2>s^<-1>或2.34×10^<13> cm^<-2>s^<-1>的条件下，对富集的^<116>CdO（96.53%）辐照约30 min，得到母核^<117>Cd。辐照样品溶液中的Cd离子取代了马维杨宁的金属位，并预先提取了Cu离子。样品溶液进行PAC测量。在pH=6.0、7.5和8.0时，野生型马维杨素的PAC谱为1.48×10^<22> ~ 2.08×10^<22> Vm^<-2>，而突变型Thr15Ala-Mav的PAC谱为0.43×10^<22> ~ 1.49×10^<22> Vm^<-2>。与星青花苷报道的值1.71×10^<22> Vm^<-2>相比，本文所得结果似乎是合理的。还发现，这两种蛋白质的pH值在6.0和7.5之间有一个陡峭的梯度变化。为了阐明蛋白质在结构上的相关变化，我们对几种金属螯合物进行如上所述的PAC测量进行比较。此外，实验中还使用了Cd-111m和In-111衰变为PAC探针的额外亲本核素进行比较。后一个实验证明了蛋白质和金属螯合物在梯度场中存在明显的差异，这可能是由于相关配体的配位，并且在测量梯度时使用的探针存在依赖性。少

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