Dehydration Mossbauer spectrum

The system FeCl2/NH4-Y turned out to be a very interesting example of the application of Mossbauer spectroscopy in the field of solid-state ion exchange in zeolites [149,150]. The spectra (cf. Fig. 44) were decomposed (vide infra cf., as an example. Fig. 45), and the oxidation states and coordination of incorporated iron were deduced on the basis of assignments reported earlier [ 154]. Results are presented in Table 10. When the above mixture was investigated as prepared, the Mossbauer spectrum provided evidence that 57 % of the iron was oxidized to the trivalent state (cf. Table 10, RI = 57). From the Mossbauer parameters it was concluded that the Fe(III) species were almost perfectly octahedrally coordinated. The remaining non-oxidized iron occurred as three different species, viz., (i) partially dehydrated Fe(II) chloride (21%) (ii) also partially dehydrated. 

The beef heart enzyme (M, = 80,000) (117) is a component of the citric acid cycle. Its active form contains one [4Fe-4S] cluster. Although such a cluster is normally associated with electron transfer, the enzyme catalyzes the nonredox reaction of citrate-isocitrate interconversion via a dehydration-hydration pathway. The current state of understanding of cluster structures and reactions of beef heart aconitase has been thoroughly reviewed by Emptage (130). When isolated aerobically, aconitase is inactive and contains one [3Fe-4S] cluster. Upon incubation of the reduced protein with Fe(ll), the fully active enzyme is generated. When a 3-Fe center is reduced to [3Fe-4S]°, Reaction 10 builds a 4-Fe cluster in a nonredox process. The Mossbauer spectra in Fig. 8 address the question of subsite specificity in this reaction of aconitase (124). If the externally supplied iron is Fe, the resultant spectrum reveals the intrinsic (original) Fe atoms... 

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