Biomineralization of iron: Moessbauer spectroscopy and electron microscopy of ferritin cores from the chiton Acanthopleura hirtosa and the limpet Patella laticostata

Ferritins isolated from the hemolymph of the chiton Acanthopleura hirtosa and the limpet Patella laticostata have been studied by 57Fe Mössbauer spectroscopy, electron diffraction, transmission electron microscopy (TEM), and inductively coupled plasma (ICP) spectrometry. Mössbauer spectra of the samples at 78 K were quadrupole-split doublets with similar quadrupole splittings and chemical isomer shifts, characteristic of octahedral high-spin iron(III), while at 4.2 K the spectra were magnetically split sextets. The spectra yield mean superparamagnetic blocking temperatures of about 32 and 30 K for the A. hirtosa and P. laticostata ferritins, respectively, and indicate magnetic ordering temperatures of about 37 and 34 K. Core size distributions were measured for both the A. hirtosa and P. laticostata ferritins by using TEM and gave mean core size ranges of 8.0-8.5 and 7.5-8.0 nm, respectively. Diffuse lines in the electron diffraction patterns of the ferritin cores indicated the presence of ferrihydrite (5Fe2-O3·9H2O) of limited crystallinity. Phosphorus to iron atomic ratios were measured by ICP spectrometry. The phosphorus levels were close to the limit of detection, giving approximate mean values of P:Fe of 1:44 for the P. laticostata ferritin and 1:36 for the A. hirtosa ferritin. These levels of phosphate are significantly less than those for the crystalline cores of mammalian ferritins and considerably less than those for the noncrystalline core of bacterioferritins.