Biomedical subjects

Vladimir L Shapovalov

Publications and source records attributed to Vladimir L Shapovalov.

3 recordsLinked to original sources

PubMed2006

Breakdown of the Gouy-Chapman model for highly charged Langmuir monolayers: counterion size effect.

Deviations from the classic Gouy-Chapman (GC) model due to the finite size of hydrated counterions were tested for negatively charged Langmuir monolayers with different surface charge densities. Monolayers with the largest charge density (>0.6 C.m(-2)) show an increase of the surface potential for a series of alkali metal cations from Li(+) to Cs(+) by 200-250 mV. The increase is similar for different monolayers and suggests that this effect is independent of the particular type of headgroup. The magnitude of variation is comparable with model estimations of the electrical double layer (EDL) potential implying that the deviation from the GC model is drastic. Deviations from the GC model rapidly vanish with decreasing monolayer charge density and become hardly observable below 0.3 C.m(-2). For monolayers with a high charge density on subphases containing different sized counterions, preferential participation of the smallest ions in the EDL should be favorable in terms of electrostatic free energy because of packing density limitations. This effect was demonstrated for behenyl sulfate (BS) monolayers (0.64 C.m(-2)) with the X-ray reflectivity technique. For the Cs(+)-Li(+) system, the fraction of Cs(+) in the EDL is 50-60% compared with only 10% of Cs(+) in the subphase. Providing high surface charge density, a small univalent Cs(+) is capable to compete even with a bulky divalent Mg(2+). For equal concentrations of Cs(+) and Mg(2+) in the subphase, the Cs(+)/Mg(2+) ratio in EDL of BS monolayer is 1.3 to 2.0 (in contrast to 0.04, predicted by the GC model). All experimental results of this study are described in terms of packing density limitations for hydrated counterions in the EDL.

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PubMed2006

Ionization state and structure of l-1,2-dipalmitoylphosphatidylglycerol monolayers at the liquid/air interface.

Phosphatidylglycerols are components of biological membranes. The phase behavior of these phospholipids was extensively investigated. However, there is still no definite picture about the dependence of the ionization state and monolayer structure on subphase composition. The major problem of previous investigations is that none of the methods used allow obtaining the ionization degree directly. In the present work we apply techniques developed in the past decades for Langmuir monolayers: infrared reflection absorption spectroscopy (IRRAS) as well as X-ray diffraction and reflectivity techniques, which provide straightforward information about structure and ionization state of a L-1,2-dipalmitoylphosphatidylglycerol (DPPG) monolayer. The Gouy-Chapman model is applied to evaluate the intrinsic pKa. Therewith, the ionization degree can be determined even at low pH values. The experimental titration curves are in good agreement with theoretical curves based on the Gouy-Chapman model. The obtained instrinic pKa amounts to 1. The ionization degree of a DPPG monolayer is independent of the monovalent cation size. In contrast, the structure of a DPPG monolayer is strongly affected by the type of divalent cations.

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PubMed2002

Tuning the membrane surface potential for efficient toxin import.

Membrane surface electrostatic interactions impose structural constraints on imported proteins. An unprecedented sensitive dependence on these constraints was seen in the voltage-gated import and channel formation by the C-terminal pore-forming domain of the bacteriocin, colicin E1. At physiological ionic strengths, significant channel current was observed only in a narrow interval of anionic lipid content ([L-]), with the maximum current (I(max)) at 25-30 mol% (dioleoyl)-phosphatidylglycerol ([L-]max) corresponding to a surface potential of the lipid bilayer in the absence of protein, psi(o)max = -60 +/- 5 mV. Higher ionic strength shifted [L-]max to larger values, but psi(o)max remained approximately constant. It is proposed that the channel current (i) increases and (ii) decreases at /psi(o)/ values <55 mV and >65 mV, because of (i) electrostatic interactions needed for effective insertion of the channel polypeptide and (ii) constraints due to electrostatic forces on the flexibility needed for cooperative insertion into the membrane. The loss of flexibility for /psi(o)/ 65 mV was demonstrated by the absence of thermally induced intraprotein distance changes of the bound polypeptide. The anionic lipid content, 25-30 mol%, corresponding to the channel current maxima, is similar to that of the target Escherichia coli cytoplasmic membrane and membranes of mesophilic microorganisms. This suggests that one reason the membrane surface potential is tuned in vivo is to facilitate protein import.

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