Membranes Mossbauer spectroscopy

In connection with the problem of oscillations discussed by previous speakers and other types of dynamical behavior of membranes, it would probably be timely to mention here in some more detail the experiments with vision rhodopsin that were performed in our institute by using the Mossbauer spectroscopy method [G. R. Kalamkarov et al., Doklady Biophys., 219, 126 (1974)]. These experiments manifested the existence of reversible photo-induced conformational changes in the photoreceptor membrane even at such low temperature as 77°K. We have labeled various samples of solubilized rhodopsin and of photoreceptor membranes by iron ascorbate enriched with Fe57 and looked for the change of Mossbauer spectra caused by the illumination of our samples. 

Another advantage of Mossbauer spectroscopy that should benefit future research is that it can be used in a non-destructive way. This advantage should allow the investigation of membranes and monoliths without them being damaged, even... 

Pinerii and coworkers, and a few other groups, have used ESR and Mossbauer spectroscopy as well as SANS, extended x-ray absorption fine structure (EX.AFS), and magnetization and susceptibility data to analyze local. struct.ure in perfluorinated ionomer membranes and the distribution of water within them isee, for inst,ance, (61-65) 1. The application of the KNDOR (electron nuclear double resonance) technique to deuteriated methanol-swollen Scunples of these membranes has been reportesd i-ecentiy (66). Photophysical methods have also tef n applied in hydration. si.udies of these membranes (67-69). Finally, some NMR results on the same hydrated perfluorinat,ed ionomer.s well as on hydrated... 

On the other hand, the spectroscopic techniques probe individual ionic species which build up the ionic aggregates. These techniques permit the investigation of the immediate chemical environments, the mobility of cations and water-ions Interactions. Metal nuclear magnetic resonance and Mossbauer spectroscopy are sensitive probes of counter cations and provide valuable information on the cations and their environment. Infrared spectroscopy is complementary to the above methods and addresses itself to the bound SO3" anions or water and the interaction of water molecules with the various species with which it is in contact. A common conclusion that is reached in the above mentioned studies is that four or five water molecules are needed to complete the hydration process. Reducing the level of moisture content (which surrounds the ionic species) below four water molecules per unit SOj site enhances the Coulombic interaction between the ionic species. This eventually leads to the formation of ion pairs in the dry membranes. These ion pairs do not necessarily disperse homogeneously in the fluorocarbon matrix but tend to form aggregates, phase separated from the matrix materials as demonstrated in the scattering studies. 

We have completed experiments label the ionic domains. We have found evidence of a precipitation phenomenon of particles of iron oxide or hydroxide when an iron form of membrane was exchanged by different other ions like K+, Na+, etc. We therefore have analyzed these particles by different techniques -like X rays, Mossbauer spectroscopy, magnetic measurements and electron microscopy- with two goals in mind. First of all the formation of ultra thin particles is very important in different domains and especially in catalysis when these membranes are used in the solid polymer electrolyte process. Second, we expect some correlation between the sizes and distribution of precipitates with the starting ionic domains. 

The structure and properties of perfluorocarbon cation exchange membranes have been actively studied using various analytical methods SAXS, SANS, NMR, ESR, electrical conductance and IR and Mossbauer spectroscopy.109 The phase separation of perfluorocarbon ion exchange membranes by transmission electron microscopy has also been shown in detail.110... 

A detailed investigation, involving elastic and inelastic neutron scattering and Mossbauer spectroscopy has been carried out on Nation membranes as a function of water content. Nafion membranes consist of a perfluoroethylene backbone with side chains having a terminal sulphonic acid group. 

To determine the structure of the active site iron center we attempted to over-express various membrane bound enzymes for spectroscopic analysis. While the membrane bound desaturases accumulated to very low levels, the alkane hydroxylase accumulated to very high levels. Membranes containing Fe-enriched alkane hydroxylase were isolated and examined by Mossbauer spectroscopy. Preliminary spectra (Fig. 2) shows the major fraction of the iron in these preparations has Mossbauer parameters similar to those of the soluble castor A desaturase. 

It is also interesting to speculate about the feed back system in which the low iron stress causes an organism to produce much larger than normal quantities of a siderochrome. This question and the question of how a siderochrome binds to or passes through a cellular membrane are undoubtedly related to the molecular conformation which can be studied via the electronic states using Mossbauer and ESR Spectroscopy. 

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