Alberto Martín-Molina

Simulation of electric double layers with multivalent counterions: Ion size effect

In this paper, the structure of the electric double layer in the presence of (mostly) multivalent counterions is investigated through Monte Carlo simulations. Unlike previous similar studies addressing this matter, the difference of this study lies in the use of realistic hydrated ion sizes. Additionally, two different methods for calculating energies in the Metropolis algorithm are applied. The obtained results show that the conclusions of preceding papers must be revised. In particular, our simulations suggest the existence of certain ion layering effects at high surface charge densities, which are not accounted for by integral equation theories in the case of divalent counterions. These layering effects could justify why the overcharging phenomena due to ion size correlations are hardly observable in real colloids with divalent counterions. The existence of charge inversion due to ion size correlations (and without requiring specific counterion adsorption) is probed for trivalent counterions. Moreover, the hypernetted-chain/mean-spherical-approximation is tested under conditions not studied yet.

Effect of Calcium and Magnesium on Phosphatidylserine Membranes: Experiments and All-Atomic Simulations

It is known that phosphatidylserine (PS(-)) lipids have a very similar affinity for Ca(2+) and Mg(2+) cations, as revealed by electrokinetic and stability experiments. However, despite this similar affinity, experimental evidence shows that the presence of Ca(2+) or Mg(2+) induces very different aggregation behavior for PS(-) liposomes as characterized by their fractal dimensions. Also, turbidity measurements confirm substantial differences in aggregation behavior depending on the presence of Ca(2+) or Mg(2+) cations. These puzzling results suggest that although these two cations have a similar affinity for PS(-) lipids, they induce substantial structural differences in lipid bilayers containing each of these cations. In other words, these cations have strong ion-specific effects on the structure of PS(-) membranes. This interpretation is supported by all-atomic molecular-dynamics simulations showing that Ca(2+) and Mg(2+) cations have different binding sites and induce different membrane hydration. We show that although both ions are incorporated deep into the hydrophilic region of the membrane, they have different positions and configurations at the membrane. Absorbed Ca(2+) cations present a peak at a distance ~2 nm from the center of the lipid bilayer, and their most probable binding configuration involves two oxygen atoms from each of the charged moieties of the PS molecule (phosphate and carboxyl groups). In contrast, the distribution of absorbed Mg(2+) cations has two different peaks, located a few angstroms before and after the Ca(2+) peak. The most probable configurations (corresponding to these two peaks) involve binding to two oxygen atoms from carboxyl groups (the most superficial binding peak) or two oxygen atoms from phosphate groups (the most internal peak). Moreover, simulations also show differences in the hydration structure of the membrane: we obtained a hydration of 7.5 and 9 water molecules per lipid in simulations with Ca(2+) and Mg(2+), respectively.

Computer simulations of thermo-sensitive microgels: quantitative comparison with experimental swelling data.

In this work, a quantitative comparison between experimental swelling data of thermo-sensitive microgels and computer simulation results obtained from a coarse-grained model of polyelectrolyte network and the primitive model of electrolyte is carried out. Polymer-polymer hydrophobic forces are considered in the model through a solvent-mediated interaction potential whose depth increases with temperature. The qualitative agreement between simulation and experiment is very good. In particular, our simulations predict a gradual shrinkage with temperature, which is actually observed for the microgels studied in this survey. In addition, the model can explain the swelling behavior for different contents of ionizable groups without requiring changes in the hydrophobic parameters. Our work also reveals that the abruptness of the shrinkage of charged gels is considerably conditioned by the number of monomeric units per chain. The swelling data are also analyzed with the Flory-Rhener theory, confirming some limitations of this classical formalism.

A Theoretical and Experimental Approach to the Compaction Process of DNA by Dioctadecyldimethylammonium Bromide/Zwitterionic Mixed Liposomes

The compaction of DNA by cationic liposomes constituted by a mixture of a cationic lipid, dioctadecyldimethylammonium bromide (DODAB), and a zwitterionic lipid, 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) or 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), has been evaluated by means of experimental studies (electrophoretic mobility, conductometry, cryogenic electron transmission microscopy or cryo-TEM, and fluorescence spectroscopy) as well as theoretical calculations. This information reveals that DODAB/DOPE and DODAB/DLPC liposomes are mostly spherical and unilamellar, with a mean diameter of around 70 and 61 nm, respectively, a bilayer thickness of 4.5 nm, and gel-to-fluid transition temperatures, T(m), of around 19 and 28 degrees C, respectively. Their positively charged surfaces efficiently compact the negatively charged DNA by means of a strong entropically driven surface interaction that yields DODAB/DOPE-DNA and DODAB/DLPC-DNA lipoplexes as confirmed by zeta potential and ethidium bromide fluorescence intercalation assays. These experiments have permitted as well the evaluation of the different microenvironments of varying polarity of the DNA helix, liposomes, and/or lipoplexes. DODAB/DOPE-DNA and DODAB/DLPC-DNA lipoplexes have been characterized by isoneutrality ratios (L/D)(phi) of around 4.7 and 4.8, respectively, a more fluid membrane than that of the parent liposomes, and T(m) around 24 and 28 degrees C, respectively, as revealed by fluorescence anisotropy. Cryo-TEM micrographs reveal a rich scenario of nanostructures and morphologies, from unilamellar DNA-coated liposomes to multilamellar lipoplexes passing through cluster-like structures. Phase diagrams (aggregation and re-entrant condensation phenomena), calculated by means of a phenomenological theory, have confirmed the experimental concentration domains and the isoneutrality conditions. The influence of helper lipid in the compaction process, as well as the optimum choice among those herein chosen, has been analyzed.

Looking into overcharging in model colloids through electrophoresis: Asymmetric electrolytes

Some theories claim that the Poisson–Boltzmann approach could fail to describe the electric double layer of colloids under certain conditions as a result of neglecting ion size correlations. For instance, if the surface charge density and/or the electrolyte concentration are high enough, the counterion local density in the vicinity of charged surface could become so large that the particle charge would be overcompensated. This phenomenon is theoretically known as overcharging and, sometimes, should involve a ζ-potential reversal. Accordingly, this work looks into overcharging through electrophoresis experiments. The electrophoretic mobility has been measured for latex particles with moderate and large surface charge density in solutions of asymmetric electrolytes z:1 (symmetric electrolytes have been studied in a previous work). In order to find out the relevance of ion size correlations, results are analyzed within the so-called hypernetted-chain/mean-spherical approximation (HNC/MSA) as well as a Poisso...

Probing charge inversion in model colloids: electrolyte mixtures of multi- and monovalent counterions

Under certain conditions ion?ion correlations play a crucial role in the description of the electrical double layer of colloidal particles. In fact, in many instances, the inclusion of the short range correlations between ions in the study of the ionic distribution leads to quite different results with respect to the classical treatment (where ions are assumed to be points). In particular, these discrepancies become more noticeable for highly charged particles in the presence of moderate or highly multivalent counterion concentrations. Moreover, it can be shown that the existence of an electrolyte mixture consisting of multi-?and monovalent counterions may cause that system to become overcharged, a feature that cannot be predicted from a classical point of view based on the Boltzmann distribution function. Precisely this aspect has recently produced an enormous interest in the field of biophysics since small variations in the physiological conditions of biocolloidal systems (e.g.?the addition of a multivalent salt) can induce important changes in their behaviour. In order to determine the relevance of ion correlations in electrolyte mixtures, we present some experimental results on the electrophoretic mobility of latex particles in the presence of different 1:1 and 3:1 salt mixtures. Likewise, these results are analysed within the so-called hypernetted-chain/mean spherical approximation where ion size correlations are taken into account.

Computer simulations of thermo-shrinking polyelectrolyte gels

In this work, thermo-responsive polyelectrolyte gels have been simulated using polymer networks of diamond-like topology in the framework of the primitive model. Monte Carlo simulations were performed in the canonical ensemble and a wide collection of situations has been systematically analysed. Unlike previous studies, our model includes an effective solvent-mediated potential for the hydrophobic interaction between non-bonded polymer beads. This model predicts that the strength of the attractive hydrophobic forces increases with temperature, which plays a key role in the explanation of the thermo-shrinking behaviour of many real gels. Although this hydrophobic model is simple (and it could overestimate the interactions at high temperature), our simulation results qualitatively reproduce several features of the swelling behaviour of real gels and microgels reported by experimentalists. This agreement suggests that the effective solvent-mediated polymer-polymer interaction used here is a good candidate for hydrophobic interaction. In addition, our work shows that the functional form of the hydrophobic interaction has a profound influence on the swelling behaviour of polyelectrolyte gels. In particular, systems with weak hydrophobic forces exhibit discontinuous volume changes, whereas gels with strong hydrophobic forces do not show hallmarks of phase transitions, even for highly charged polyelectrolyte chains.

Ribbon-type and cluster-type lipoplexes constituted by a chiral lysine based cationic gemini lipid and plasmid DNA

Lipoplexes constituted by plasmid DNA pEGFP-C3 (pDNA) or linear double-stranded calf thymus DNA (ctDNA) and mixed cationic liposomes consisting of several percentages of the cationic lysine derived lipid C6(LL)2 and the zwitterionic lipid 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) have been analyzed by both experimental and theoretical approaches. Experimental studies, consisting of electrophoretic mobility/zeta potential, small angle X-ray scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM), negatively stained transmission electron microscopy (NS-TEM), and GelRed f1uorescence intercalation assays, have been carried out at several liposome and lipoplex compositions, defined in terms of cationic lipid molar fraction and either the mass or charge ratios of the lipoplex. The electrochemical study confirms that, in the presence of the mixed lipids and in contrast with what has usually been found for linear DNA, the plasmid DNA is compacted with a large number of its Na+ counterions, thus yielding a much lower effective negative charge (q−pDNA) than that for ctDNA (q−ctDNA), as reported recently by us (J. Am. Chem. Soc., 2011) for other lipoplexes. This finding is revealed as crucial for an optimum and efficient lipoplex preparation, since a lower effective negative charge implies a lower quantity of cationic lipid and, accordingly, a potential lower cytotoxicity. TEM experiments reveal a complex scenario of multilamellar nanostructures, from ribbon-type (typically present for chiral lipids) to cluster-type structures (usually found in cationic lipid/DOPE systems), the composition of the mixed liposome playing an important role in the final morphology of the lipoplex. SAXS diffractograms confirm the existence of these two types of multilamellar structures through a deconvolution process of the first peak of diffractograms into two overlapping bands. On the other hand, a theoretical complexation model is employed to determine the net charge of the lipoplexes studied in this work. The model allows analysis and comparison of the electrochemical behaviour of lipoplexes containing linear DNA vs. those constituted by a supercoiled DNA, confirming the experimental findings.

Effect of Surface Charge on Colloidal Charge Reversal

The objective of this research work is to understand the effect of the surface charge density on the charge reversal phenomenon. To this end, we use experimental results and computer simulations. In particular, we measure the electrophoretic mobility of latex particles (macroions) in the presence of a multivalent electrolyte. We have focused on the electrolyte concentration range at which a reversal in the electrophoretic mobility is expected to happen. In particular, the role of the surface charge on the charge reversal process is looked into from several latexes with the same functional group but different surface charge densities. Although the mechanism responsible for the colloidal charge reversal is still a controversial issue, it is proved that ionic correlations are behind the appearance of such phenomenon (especially near the macroion surface). This conclusion can be inferred from a great variety of theoretical models. According to them, one of the factors that determine the charge reversal is the surface charge density of the macroions. However, this feature has been rarely analyzed in experiments. Our results appear therefore as a demanded survey to test the validity of the theoretical predictions. Moreover, we have also performed Monte Carlo simulations that take the ion size into account. The correlation found between experiments and simulations is fairly good. The combination of these techniques provides new insight into the colloidal charge reversal phenomena showing the effect of surface charge.

Gene vectors based on DOEPC/DOPE mixed cationic liposomes: a physicochemical study

A double approach, experimental and theoretical, has been followed to characterize from a physicochemical standpoint the compaction process of DNA by means of cationic colloidal aggregates. The colloidal vectors are cationic liposomes constituted by a mixture of a novel cationic lipid, 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine (chloride salt) (DOEPC) and a zwitterionic lipid, the 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE). A wide variety of high precision experimental techniques have been used to carry out the analysis: electrophoretic mobility, small-angle X-ray scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM) and fluorescence spectroscopy (ethidium bromide intercalation assays). On the other hand, a theoretical model that considers the renormalization of charges of both the polyelectrolyte and the colloidal aggregates sheds light as well on the characteristics of the compaction process. This global information reveals that the compaction of DNA by the cationic liposomes is mostly driven by the strong electrostatic interaction among the positively charged surfaces of the colloidal aggregates and the negatively charged DNA, with a potent entropic component. DOEPC/DOPE liposomes are mostly spherical, with a mean diameter of around 100 nm and a bilayer thickness of 4.4 nm. From a morphological viewpoint, an appreciable amount of multilamellar structures has been found not only on the lipoplexes but also on the parent liposomes. The isoneutrality of the lipoplexes is found at liposome/DNA mass ratios that decrease with the molar fraction of cationic lipid in the mixed liposome (α). This liposome composition has a clear effect as well on the lipoplex structure, which goes from an inverted hexagonal phase (HII), usually related to improved cell transfection efficiency, at low cationic lipid molar fraction (α ≈ 0.2), to a lamellar structure (Lα) when the cationic lipid content in the mixed liposomes increases (α ≥ 0.4), irrespective of the lipoplex charge ratio. On the other hand, a theoretical complexation model is employed to determine the net charge of the lipoplexes studied in this work, by using renormalized charges. The model allows us to confirm and predict the experimental isoneutrality conditions as well as to determine the maximum magnitude of this charge as a function of the composition of the resulting lipoplexes.