Thu, 21 Aug 2008 07:14:10 BST CiteULike: sobolevnrms Clarke http://www.citeulike.org/user/sobolevnrm/author/Clarke CiteULike.org en-gb Copyright © 2004-2008 citeulike.org http://www.citeulike.org/user/sobolevnrm/article/2658171 <i>Langmuir (11 April 2008)</i><br /><br />Abstract: The mixing behavior of binary combinations of perfluoroalkanes in the bulk and in solid monolayers adsorbed at the graphite/liquid interface, determined by calorimetry and powder diffraction, is reported. The perfluoroalkanes are found to generally have a smaller excess enthalpy of mixing on the surface than in the bulk, and their relative size ratio is a good parameter to predict the mixing behavior. The excess enthalpy of mixing for perfluoroalkanes is found to be significantly smaller than that of the closely related hydrocarbons. The preferential adsorption of longer homologues over shorter ones is observed. Interestingly, the extent of preferential adsorption with relative size ratio is very similar to that of the hydrocarbons. These results can be understood in terms of the increased compressibility and lower polarizability of the perfluoroalkanes compared to hydrocarbons. Mixing in Adsorbed Monolayers: Perfluorinated Alkanes JE Parker SM Clarke doi:10.1021/la703995u Langmuir (11 April 2008) 2008-04-11T15:26:32-00:00 2008 Langmuir biomembranes perfluorocarbons http://www.citeulike.org/user/sobolevnrm/article/1332290 <i>Physical Review Letters, Vol. 98 (2007)</i><br /><br />We report the measurement of the surface conductivity of biological macromolecules by dielectrophoretic trapping at the tip of a glass nanopipet. We find that the threshold voltage for trapping is a function of salt concentration and can be directly linked to the effective conductivity of the biomolecule and its solvation shell. The surface conductivities obtained for 20-mer single-stranded DNA, 40-mer double-stranded DNA, and yellow fluorescent protein are 7.9$±$1.9 nS, 5.3$±$0.7 nS, and 21.5$±$1.6 nS, respectively. Surface Conductivity of Biological Macromolecules Measured by Nanopipette Dielectrophoresis RW Clarke JD Piper L Ying D Klenerman Physical Review Letters, Vol. 98 (2007) 2007-05-25T13:38:05-00:00 2007 Physical Review Letters 98 diffusion electrostatics http://www.citeulike.org/user/sobolevnrm/article/1331994 <i>Biochimica et Biophysica Acta (BBA) - Biomembranes, Vol. 1768, No. 3. (2007), pp. 562-70.</i><br /><br />Here we present a fluorescence method based on the Stokes shift of the voltage-sensitive dye di-8-ANEPPS to quantify the orientational polarisability of lipid membrane surfaces, i.e. the polarisability due to molecular reorientation. Di-8-ANEPPS is already an established probe of membrane dipole potential. Its use, therefore, as a probe of both the dipole potential and orientational polarisability allows a direct comparison of these two properties in an identical region of the lipid bilayer. We applied the new technique on phosphatidylcholine vesicles to study the effects of different degrees of hydrocarbon saturation and of the incorporation of cholesterol and some of its oxidized derivatives. We found that lipids with unsaturated chains had a lower orientational polarisability than those with saturated chains. This could be explained by a reduction in membrane dipole potential as a result of a decrease in lipid packing density. Cholesterol derivatives were found to either increase or decrease the orientational polarisability depending on their molecular structure. The varying effects could be explained by antagonistic effects of the dipole potential and membrane order, which are both changed to varying degrees by the cholesterol derivatives and which lead to increases and decreases in orientational polarisability, respectively. Orientational polarisability of lipid membrane surfaces G Le Goff MF Vitha RJ Clarke Biochimica et Biophysica Acta (BBA) - Biomembranes, Vol. 1768, No. 3. (2007), pp. 562-70. 2007-05-25T13:36:28-00:00 2007 Biochimica et Biophysica Acta (BBA) - Biomembranes 1768 3 562 70 biomembranes electrostatics http://www.citeulike.org/user/sobolevnrm/article/1331807 <i>Biochimica et Biophysica Acta (BBA) - Biomembranes, Vol. 1768, No. 1. (2007), pp. 107-14.</i><br /><br />We are interested in developing fluorescence methods for quantifying lateral variations in the dipole potential across cell surfaces. Previous work in this laboratory showed that the ratio of fluorescence intensities of the voltage-sensitive dye di-8-ANEPPS using excitation wavelengths at 420 and 520 nm correlates well with measurements of the dipole potential. In the present work we evaluate the use of di-8-ANEPPS and an emission ratiometric method for measuring dipole potentials, as Bullen and Saggau (Biophys. J. 65 (1999) 2272â2287) have done to follow changes in the membrane potential in the presence of an externally applied field. Emission ratiometric methods have distinct advantages over excitation methods when applied to fluorescence microscopy because only a single wavelength is needed for excitation. We found that unlike the excitation ratio, the emission ratio does not correlate with the dipole potential of vesicles made from different lipids. A difference in the behaviour of the emission ratio in saturated compared to unsaturated lipid vesicles was noted. Furthermore, the emission ratio did not respond in the same way as the excitation ratio when cholesterol, 6-ketocholestanol, 7-ketocholesterol, and phloretin were added to dimyristoylphosphatidylcholine (DMPC) vesicles. We attribute the lack of correlation between the emission ratio and the dipole potential to simultaneous changes in membrane fluidity caused by changes in membrane composition, which do not occur when the electric field is externally applied as in the work of Bullen and Saggau. Di-8-ANEPPS can, thus, only be used via an excitation ratiometric method to quantify the dipole potential. Comparison of excitation and emission ratiometric fluorescence methods for quantifying the membrane dipole potential MF Vitha RJ Clarke Biochimica et Biophysica Acta (BBA) - Biomembranes, Vol. 1768, No. 1. (2007), pp. 107-14. 2007-05-25T13:35:46-00:00 2007 Biochimica et Biophysica Acta (BBA) - Biomembranes 1768 1 107 14 biomembranes http://www.citeulike.org/user/sobolevnrm/article/1331527 <i>Biochimica et Biophysica Acta (BBA) - Biomembranes (in press)</i><br /><br />We are interested in developing fluorescence methods for quantifying lateral variations in the dipole potential across cell surfaces. Previous work in this laboratory showed that the ratio of fluorescence intensities of the voltage-sensitive dye di-8-ANEPPS using excitation wavelengths at 420 and 520 nm correlates well with measurements of the dipole potential. In the present work we evaluate the use of di-8-ANEPPS and an emission ratiometric method for measuring dipole potentials, as Bullen and Saggau (Biophys. J. 65 (1999) 2272â2287) have done to follow changes in the membrane potential in the presence of an externally applied field. Emission ratiometric methods have distinct advantages over excitation methods when applied to fluorescence microscopy because only a single wavelength is needed for excitation. We found that unlike the excitation ratio, the emission ratio does not correlate with the dipole potential of vesicles made from different lipids. A difference in the behaviour of the emission ratio in saturated compared to unsaturated lipid vesicles was noted. Furthermore, the emission ratio did not respond in the same way as the excitation ratio when cholesterol, 6-ketocholestanol, 7-ketocholesterol, and phloretin were added to dimyristoylphosphatidylcholine (DMPC) vesicles. We attribute the lack of correlation between the emission ratio and the dipole potential to simultaneous changes in membrane fluidity caused by changes in membrane composition, which do not occur when the electric field is externally applied as in the work of Bullen and Saggau. Di-8-ANEPPS can, thus, only be used via an excitation ratiometric method to quantify the dipole potential. Comparison of excitation and emission ratiometric fluorescence methods for quantifying the membrane dipole potential MF Vitha RJ Clarke Biochimica et Biophysica Acta (BBA) - Biomembranes (in press) 2007-05-25T13:23:15-00:00 Biochimica et Biophysica Acta (BBA) - Biomembranes biomembranes http://www.citeulike.org/user/sobolevnrm/article/1331210 <i>Advances in Colloid and Interface Science, Vol. 89-90 (2001), pp. 263-81.</i><br /><br />The dipole potential is an electrical potential within phospholipid membranes, which arises because of the alignment of dipolar residues of the lipids and/or water dipoles in the region between the aqueous phases and the hydrocarbon-like interior of the membrane. For a fully saturated phosphatidylcholine membrane, its value is believed to be in the range 220-280 mV, positive in the membrane interior. This results in an enormous electric field strength within the membrane of 10(8)-10(9) Vm(-1). The dipole potential is thus likely to have great significance in controlling the conformation of ion-translocating membrane proteins and so in regulating enzyme function. Because of its location within the membrane, quantification of the dipole potential is extremely difficult and presents a great challenge to the experimentalist and theoretician alike. Both electrical and spectroscopic methods developed for the determination of the dipole potential on lipid bilayers and monolayers are presented and possible causes for differences in the values derived are discussed. The dipole potential of phospholipid membranes and methods for its detection. RJ Clarke Advances in Colloid and Interface Science, Vol. 89-90 (2001), pp. 263-81. 2007-05-25T13:22:44-00:00 2001 Advances in Colloid and Interface Science 89-90 263 81 biomembranes electrostatics http://www.citeulike.org/user/sobolevnrm/article/1331103 <i>Biophysical Journal, Vol. 90, No. 11. (2006), pp. 4060-70.</i><br /><br />The effect of cholesterol removal by methyl-{beta}-cyclodextrin on the dipole potential, {psi}d, of membrane vesicles composed of natural membrane lipids extracted from the kidney and brain of eight vertebrate species was investigated using the voltage-sensitive fluorescent probe di-8-ANEPPS. Cyclodextrin treatment reduced cholesterol levels by on average 80\% and this was associated with an average reduction in {psi}d of 50 mV. Measurements of the effect of a range of cholesterol derivatives on the {psi}d of DMPC lipid vesicles showed that the magnitude of the effect correlated with the component of the sterol's dipole moment perpendicular to the membrane surface. The changes in {psi}d observed could not be accounted for solely by the electric field originating from the sterols' dipole moments. Additional factors must arise from sterol-induced changes in lipid packing, which changes the density of dipoles in the membrane, and changes in water penetration into the membrane, which changes the effective dielectric constant of the interfacial region. In DMPC membranes the cholesterol-induced change in {psi}d was biphasic, i.e. a maximum in {psi}d was observed at 35-45 mol\%, after which {psi}d started to decrease. We suggest that this could be associated with a maximum in the strength of DMPC-cholesterol intermolecular forces at this composition. Cholesterol Effect on the Dipole Potential of Lipid Membranes Starke Peterkovic N Turner MF Vitha MP Waller DE Hibbs RJ Clarke Biophysical Journal, Vol. 90, No. 11. (2006), pp. 4060-70. 2007-05-25T13:22:42-00:00 2006 Biophysical Journal 90 11 4060 70 biomembranes electrostatics http://www.citeulike.org/user/sobolevnrm/article/1331037 <i>Biophysical Journal, Vol. 82, No. 6. (2002), pp. 3081-8.</i><br /><br />The magnitude of the dipole potential of lipid membranes is often estimated from the difference in conductance between the hydrophobic ions, tetraphenylborate, and tetraphenylarsonium or tetraphenylphosphonium. The calculation is based on the tetraphenylarsonium-tetraphenylborate hypothesis that the magnitude of the hydration energies of the anions and cations are equal (i.e., charge independent), so that their different rates of transport across the membrane are solely due to differential interactions with the membrane phase. Here we investigate the validity of this assumption by quantum mechanical calculations of the hydration energies. Tetraphenylborate (Delta Ghydr = -168 kJ mol-1) was found to have a significantly stronger interaction with water than either tetraphenylarsonium (Delta Ghydr = -145 kJ mol-1) or tetraphenylphosphonium (Delta Ghydr = -157 kJ mol-1). Taking these differences into account, literature conductance data were recalculated to yield values of the dipole potential 57 to 119 mV more positive in the membrane interior than previous estimates. This may partly account for the discrepancy of at least 100 mV generally observed between dipole potential values calculated from lipid monolayers and those determined on bilayers. Hydrophobic Ion Hydration and the Magnitude of the Dipole Potential J Schamberger RJ Clarke Biophysical Journal, Vol. 82, No. 6. (2002), pp. 3081-8. 2007-05-25T13:22:42-00:00 2002 Biophysical Journal 82 6 3081 8 biomembranes electrostatics solvation http://www.citeulike.org/user/sobolevnrm/article/1329157 <i>Journal of the American Chemical Society, Vol. 126, No. 25. (2004), pp. 7734-5.</i><br /><br />Previous work by Wand et al. (Nature 2001, 411, 501-504) showed that the NMR order parameters characterizing the amplitude of motion of protein side chains seemed to form a multimodal distribution. At the time, no detailed explanation of this at the molecular level was offered, yet three "classes" of motion were inferred. We have analyzed a larger published data set and found that, although the distribution is multimodal, the evidence for three classes is weak. More significantly, we have been able to provide a simple physical explanation for the distributions based on the results of molecular dynamics simulations. This result will aid in the interpretation of data from NMR dynamics experiments. The Origin of Protein Sidechain Order Parameter Distributions RB Best J Clarke M Karplus Journal of the American Chemical Society, Vol. 126, No. 25. (2004), pp. 7734-5. 2007-05-25T13:10:27-00:00 2004 Journal of the American Chemical Society 126 25 7734 5 experimental_methodology molecular_simulation http://www.citeulike.org/user/sobolevnrm/article/1328042 <i>Biophysical Journal, Vol. 76 (1999), pp. 2614-2624.</i><br /><br />Anions and cations have long been recognized to be capable of modifying the functioning of various membrane-related physiological processes. Here, a fluorescent ratio method using the styrylpyridinium dyes, RH421 and di-8-ANEPPS, was applied to determine the effect of a range of anions and cations on the intramembrane dipole potential of dimyristoylphosphatidylcholine vesicles. It was found that certain anions cause a decrease in the dipole potential. This could be explained by binding within the membrane, in support of a hypothesis originally put forward by A. L. Hodgkin and P. Horowicz $[$1960, J. Physiol. (Lond.) 153:404-412.$]$ The effectiveness of the anions in reducing the dipole potential was found to be ClO4 > SCN > I > NO3 > Br > Cl > F > SO42. This order could be modeled by a partitioning of ions between the membrane and the aqueous phase, which is controlled predominantly by the Gibbs free energy of hydration. Cations were also found to be capable of reducing the dipole potential, although much less efficiently than can anions. The effects of the cations was found to be trivalent > divalent > monovalent. The cation effects were attributed to binding to a specific polar site on the surface of the membrane. The results presented provide a molecular basis for the interpretation of the Hofmeister effect of lyotropic anions on ion transport proteins. Influence of anions and cations on the dipole potential of phosphatidylcholine vesicles: a basis for the Hofmeister effect RJ Clarke C Lupfert Biophysical Journal, Vol. 76 (1999), pp. 2614-2624. 2007-05-25T12:55:21-00:00 1999 Biophysical Journal 76 2614 2624 biomembranes electrostatics solvation