Zlatica Kozarac

Interaction of proteins with lipid monolayers at the air-solution interface studied by reflection spectroscopy

The interaction of a fluorescein-labelled insulin and of cytochrome C with the air-solution interface and with lipid monolayers at the air-solution interface has been studied by measuring the change in surface pressure at constant area and by reflection spectroscopy. Chromophores at the interface only give rise to enhanced light reflection without contribution to the signal from chromophores in the bulk. The accumulation of labelled insulin at the solution surface is very weak as concluded from the shape of the spectrum and reflection intensity. No interaction with a monolayer of dipalmitoyl-phosphatidylcholine at initial surface pressure of 5mN/m was detected. In contrast, the interaction with monolayers of dioctadecyl-dimethyl-ammonium bromide at initial surface pressures between 5 and 40 mN/m is much stronger, leading to a remarkable increase of surface pressure at constant area and strong reflection signal. The technique was also used to detect cytochrome C at the air-solution interface.

Application of Brewster angle microscopy and fractal analysis in investigations of compressibility of Langmuir monolayers

The aim of this study was to investigate the connection between the lipid/amphiphile monolayer structure at the interface and its macroscopic/rheological properties, in particular, to establish the link between the fractality of the monolayer structure and its compressibility modulus. To that purpose we have used fractal analysis of images obtained by Brewster angle microscopy to infer the fractal dimension of the monolayer structure and relate its change to the corresponding changes in compressibility derived from a simultaneously measured π-A isotherm. The results of the study confirmed the starting assumption based on theoretical considerations that the fractal dimension of an amphiphilic monolayer and its compressibility should be correlated. We have shown that there exists a strong correlation between the fractal dimension and the corresponding compressibility modulus of different amphiphilic materials. Thus, confirming the link between the short ordered structure on the molecular level and the macroscopic property-compressibility of the monolayer. The established correlation between the fractal dynamics and compressibility modulus of the monolayer enabled identification of onset of percolation-a second-order phase transition that is otherwise not easy and unambiguously detectable. We have found that the signature of percolation in a monolayer, regardless of its composition, is the occurrence of a sharp increase (a jump) of compressibility modulus (at macroscopic level) at the characteristic value of the corresponding fractal dimension D = 1.89. This is the result of the abrupt establishment of a connected structure on the molecular level, consequently involving a change in the elastic properties of the monolayer on a macroscopic scale. The results of this investigation provide means for unambiguous identification of the onset of percolation in the Langmuir layer and should facilitate a more efficient application of the percolation theory in further study of processes and structures at the interface during the monolayer compression.

Surface active substances (SAS) and dissolved organic matter (DOC) in atmospheric precipitation of urban area of Croatia (Zagreb)

We report measurements of surface active substances (SAS) in atmospheric bulk precipitation on the basis of adsorption characteristics on the mercury electrode by using phase sensitive alternating current voltammetry (out-of-phase measurement). Samples collected during the 1998–1999 at an urban area of Croatia. The obtained surfactant activity values were within the range of 0.09–0.37 mg dm−3 eq. Triton X-100. Concentrations of dissolved organic carbon (DOC) in the same samples were in the range of 0.78–4.39 mg dm−3. In this paper, we also used Pb2+ ion as an electrochemical probe for additional characterization of the properties of the organic matter in precipitation, which may be important in various atmospheric processes.

Surface-active substances in atmospheric aerosol: an electrochemical approach

ABSTRACT We characterised surface-active substances (SASs) in aqueous extract of atmospheric aerosols by using phase sensitive alternating current voltammetry. The electrochemical method has mainly been used for the quantification of surfactants in sea water but has not been applied to atmospheric aerosols yet. The advantage of the method is its simplicity and sensitivity that enables direct analysis of aerosol extracts without the need for sample concentration. Aerosol samples were collected at Middle Adriatic Martinska station influenced by different air masses as well as from urban (Zagreb, Croatia) and rural (K-puszta, Hungary) areas from late spring to early autumn in 2010. The highest SAS concentrations, expressed in equivalents of T-X-100, ranging from 0.34 to 0.91 µg m−3 were detected in urban samples. The SAS concentrations obtained for marine, regional and continental samples ranged from 0.14 to 0.31, 0.18 to 0.42 and 0.07 to 0.28 µg m−3, respectively. The SAS concentrations in K-puszta aerosols ranged from 0.13 to 0.46 µg m−3. Investigation of humic-like substances isolated from K-puszta samples (2008) confirmed their significant surfactant nature. Different SAS chemistry was noticed for urban and non-urban samples. Investigations at different pH revealed anionic character of SASs in aerosol samples.

Interaction of insulin with lipid monolayers at the air/solution interface

Abstract Interactions of insulin with lipid monolayers at the air/solution interface were studied by observing the kinetics of surface pressure change in the lipid monolayers at constant area and by spectroscopic techniques. The strong binding of insulin to positively-charged lipid monolayer resulted in increased surface pressure values, as well as in enhanced absorption and reflection signals in comparison to all other investigated lipid systems. This indicates that electrostatic interactions dominate these interactions.

Physicochemical Signatures of Natural Sea Films from Middle Adriatic Stations

Monolayer studies and a force-area quantification approach, in combination with electrochemical methods, are applied for physicochemical characterization of surface active substances (SAS) of the sea surface microlayers (MLs) from Middle Adriatic stations. Higher primary production during late spring-early autumn was reflected in the presence of MLs of higher surfactant activity containing on average molecules of lower molecular masses (M(w) = 0.65 ± 0.27 kDa) and higher miscibility (y = 6.46 ± 1.33) and elasticity (E(isoth) = 18.33 ± 2.02 mN m(-1)) modulus in comparison to structural parameters (average M(w) = 2.15 ± 1.58 kDa; y = 3.51 ± 1.46; E(isoth) = 6.41 ± 1.97 mN m(-1)) obtained for MLs from a period of lower production. A higher inhibition effect on the reduction process of Cd(2+) was observed for SAS abundant MLs from a more productive period. This kind of distribution is explained as the consequence of competitive adsorption of hydrophobic lipid-like substances of lower M(w) that act as end-members, highly influencing the surface structural properties of the natural air-water interface forming there segregated surface films during more productive period.

The Structure of Percolating Lipid Monolayers

The lattice structure and in plane molecular organization of Langmuir monolayer of amphiphilic material is usually determined from grazing incidence X-ray diffraction (GIXD) or neutron reflectivity. Here we present results of a different approach for determination of monolayer lattice structure based on application of fractal analysis and percolation theory in combination with Brewster angle microscopy. The considerations of compressibility modulus and fractal dimension dynamics provide information on percolation threshold and consequently by application of percolation theory on the lattice structure of a monolayer. We have applied this approach to determine the monolayer lattice structures of single chain and double chain lipids. The compressibility moduli were determined from measured π-A isotherms and fractal dimensions from corresponding BAM images. The monolayer lattice structures of stearic acid, 1-hexadecanol, DPPC and DPPA, obtained in this way conform to the corresponding lattice structures determined previously by other authors using GIXD.

Use of electrochemical impedance spectroscopy to characterise the physical properties of ex situ reconstructed sea surface microlayer

The objective of this work was to develop electrochemical impedance spectroscopy (EIS) to characterise the physical properties of the sea surface microlayer ( ssm ). Samples from Lake Rogoznica in Croatia were extracted by n-hexane and dichloromethane (dcm) respectively and transferred to mercury electrodes. The EIS results were compared with those of a model phospholipid, dioleoyl phosphatidylcholine (DOPC) which forms near defect-free monolayers on a mercury surface. The ssm extracts formed inhomogeneous monolayers on the mercury surface and the dcm ssm extract monolayer showed greater surface roughness than the hexane ssm extract. The hexane ssm extract introduced defects and a greater surface roughness into mixed DOPC- ssm extract monolayers than the dcm ssm extract due to the lower compatibility of the non-polar hexane extract with the DOPC than that of the polar ssm extract. In addition, the dcm ssm component in the mixed DOPC- ssm monolayer showed an association with pyrene added to the solution.