SQUID measurement of metalloprotein magnetization. New methods applied to the nitrogenase proteins.

New techniques have been developed to exploit the sensitivity of a commercial SQUID susceptometer in the study of the magnetization of metalloproteins. Previous studies have ignored both the slow relaxation (hours) of spin I = 1/2 nuclei and residual ferromagnetic impurities in sample holders. These potential sources of noise were at or below the sensitivity of previous instruments. With these noise sources under control, one can now decrease the protein concentration by a factor of ten. In addition careful characterization of the frozen magnetization sample, including the use of a multi-instrument holder for combined study of the magnetization sample with Mössbauer spectroscopy, is required for reliable interpretation of the data in the face of paramagnetic impurities common to metalloprotein samples. Many previous magnetic studies of metalloproteins have been carried out in the Curie region. Saturation magnetization studies down to 1.8 K and up to 5 T can determine zero-field splitting parameters in addition to the spin and exchange coupling parameters measured in previous studies at lower fields and higher temperatures. Applications of these techniques to the study of the nitrogenase proteins of Azotobacter vinelandii are presented as examples.