MOSSBAUER STUDIES OF SUPERPARAMAGNETISM IN ESCHERICHIA-COLI

Proteins containing magnetic cores and combining the properties of cytochrome b1 with those of ferritin have been isolated from bacterial sources such as Azotobacter vinelandii and Escherichia coli. The similarity with ferritin is supported by electron microscopy in which the imaged molecules appear spherical with an electron opaque core and an electron transparent shell. The small particle sizes of such iron containing cores normally leads to the observation of superparamagnetism with blocking temperatures in the range 20-50 K when investigated by Mossbauer spectroscopy. However, earlier work on E. coli indicated that in these samples no superparamagnetism is observed. Instead they found a magnetic ordering temperature of only 3.5 K with quadrupole doublets observed at higher temperatures. In this Fe-57 Mossbauer investigation of E. coli, however, we do observe superparamagnetic behaviour consistent with that found in other forms of ferritin and these new results will be presented and discussed.