Nuclear Resonance Spectroscopy of Engineered Ferritin Samples

Ferritin consists of a protein shell encapsulating an iron biomineral core¹. The shell is composed of 24 amino acid chains of two types, H and L². Mӧssbauer spectroscopy³ based on the recoil-free emission and absorption of nuclear γ-rays in solids can uniquely characterize the structure of the core⁴. It uses radioactive ⁵⁷Co that decays into ⁵⁷Fe at an excited energy state with subsequent emission of a 14.4 KeV γ-ray. The lifetime of the 14.4 KeV excited state is 10^-7 s making the γ-ray extremely sharp. Resonant absorption of the γ-ray by the ferritin sample is achieved by using the Döppler Effect. The absorption spectrum reports on the electronic charge density, the iron coordination symmetry and magnetic order in the material. The Mӧssbauer spectra of L-rich and H-rich ferritins reconstituted to 1000⁵⁷Fe/protein in the presence of phosphate were recorded at RT. The spectra were analyzed in terms of the core/shell model of the iron core⁵. The results show the H-rich cores to be more crystalline than the L-rich, while TEM studies indicate that the L-rich proteins form somewhat larger cores than the H-rich under similar iron reconstitution conditions. These observations can be traced to differences in H- and L-chain molecular structures and their influence on core growth ^6,7.