Biomembranes hydrogenation

Sakai, N., Mareda, J., Matile, S., Rigid rod molecules in biomembrane models From hydrogen-bonded chains to synthetic... 

Ground-state dioxygen ( 2) can be activated by photolytic energy transfer to the singlet state ( 2) (eqnation 129) and this may be dnplicated at the snrface of biomembranes by solar radiation. In the laboratory, singlet dioxygen is conveniently prodnced by the stoichiometric oxidation of hydrogen peroxide by hypochlorous acid (eqnation 130). 

Kayalar, C., A model for proton translocation in biomembranes based on keto-enol shifts in hydrogen bonded peptide groups, J. Membrane Biol. 45, 37-42 (1979). 

Often, experimental studies of lipid systems are based on spectroscopic approaches, which in turn frequently employ probes for enhancement of sensitivity and resolution. For example, in NMR, hydrogen atoms of lipids are replaced with deuterium, and in fluorescence spectroscopy and imaging, native lipid molecules are replaced with lipids in which one of the hydrocarbon chains is linked covalently to a fluorescent marker such as pyrene or diphenylhexatriene. Fluorescent markers allow one to follow numerous cellular processes in real time, such as intracellular trafficking of molecules and formation of domains within a biomembrane, see Fig. 3. The downside is that the probes tend to perturb their environment and affect the thermodynamic state of the system. Experiments have shown, for example, that probes may change the main transition temperature of a lipid membrane, and that the dynamics of probes may deviate considerably from the dynamics of corresponding native molecules (see discussion in Reference 27). Therefore, we wish to pose several questions. What is the range of perturbations induced by the probe How significant are these perturbations actually ... 

Short hydrogen-bonded chains are characterized by a large proton polarizability, and they play a role of typical proton channels in biomembranes. That is why it is reasonable to assume that coherent proton transitions in the chain can be involved in the mechanism of real proton transport along the chain [327]. 

A recent study has confirmed that the Fenton process [Eq. (4-18)] occurs in pyridine/acetic add (2 1 molar ratio) when bis(picolinato)iron(II) [Fe (PA)2l and hydrogen peroxide are combined in 1 1 molar ratio (the rate constant for the Fe RPA)2/HOOH reaction is JxlO M- s-l). In the presence of hydrocarbon substrates (RH) the HO- flux produces carbon radicals (R-), which can be trapped by PhSeSePh to give RSePh. This chemistry, which is outlined in Scheme 4-1 for cyclohexane (C-C6H12), represents a reasonable model for HO- radical generation within biomembranes. 

One of the objectives in performing hydrogenation reactions in situ was to modulate the fluidity of biomembranes and to examine the role of membrane lipid fluidity in biochemical and physiological functions. Secondly, because membrane lipids with six or more unsaturated double bonds were found to be significant components of some membranes, such as the retinal rod membranes of the eye, the hydrogenation of these lipids was thought to be a useful tool to identify their role in these membranes. 

Chemical catalysis is often performed under conditions of temperature, etc., that are well outside the physiological range. In biological applications, the catalyst complex must be stable under the conditions required to preserve stability of biomembranes or viability of living organisms. At the same time reasonable reaction rates must be sustained under these physiological conditions. Ideally the presence of the catalyst in the system should not affect the properties of the membrane other than in the response of the membrane to the altered level of saturation of the constituent lipids. This can be achieved by removal of the catalyst complex at the completion of the hydrogenation reaction. 

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