Hydrogenation of biomembranes

As can be seen from this short overview of the hydrogenation of biomembranes, one has to be very careful in the experimental work (choice of catalyst, conditions, reaction time, etc.) and even more careful in the interpretation of the changes such a manipulations bring in the properties of live cells. Nevertheless, the results outweigh the problems, and catalytic hydrogenation has become a powerful tool in membrane biochemistry [331], A selection of the various systems which have already been studied by this method is given in Table 3.11 Further details can be obtained from the references in the Table. 

For two recent reports and pertinent references, see (a) Nadasdi, L., J06, F, Horvath, 1. and Vigh, L., Colloidal metal dispersions as catalysts for selective surface hydrogenation of biomembranes. 21. Preparation of nanosize platinum metal catalysts and characterization in hydrogenation of water-soluble olefins and synthetic biomembrane models, AppL Catal A Gen., 1997, 162, 57 (b) Csabai, P. and J06, R, Reactivity of the individual lipid classes in homogeneous catalytic hydrogenation of model and biomembranes detected by MALDl-TOF mass spectrometry, Catal Commun., 2003, 4, 275. 

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). 

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. 

Table 3.11. Selected examples of hydrogenation of liposomes and biomembranes...

The basic structural unit of biomembranes is the phospholipid bllayer. Acetic acid and ethanol are composed each of two carbons, hydrogen and oxygen, and both enter cells by passive diffusion. At pH 7, one is much more membrane permeant than the other. Which is the more permeable, and why Predict how the permeability of each is altered when pH is reduced to 1.0, a value typical of the stomach. 

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 ... 

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