A. Dhathathreyan

Adsorption of cytochrome c to phospholipid monolayers studied by reflection spectroscopy.

The uptake of cytochrome c by charged and neutral lipid monolayers was studied by using reflection spectroscopy. The method was shown to be a very sensitive and useful technique in studies of lipid‐protein interactions. It was found that cytochrome c is preferentially bound to monolayers of negatively charged monolayers in the solid phase. Polarized light under oblique incidence was used to determine the average orientation of chromophores in cytochrome c bound to lipid monolayer. The transition moments of chromophore are oriented parallel to the monolayer plane.

Control of interfacial equilibria by local charge distribution and average surface potential

Abstract The incorporation of cadmium ions in porphyrins embedded in matrix monolayers is dominated by head group interactions as opposed to the effect of the average surface potential on interfacial cation concentrations. Similar phenomena are also observed in the adsorption of water-soluble porphyrin complexes to spread monolayers. The influence of such local interactions can be eliminated when the composition of the head group region is kept constant and the interfacial potential is modified by changing dipole layers at distances of about 2.5 nm from the head group interface. Protonation equilibria can be systematically shifted by planned variation of the average potential via such remote dipole layers.

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.

Interaction of nitrophenols with lipids at the air/water interface

The uptake of ortho and para nitrophenol to charged and neutral lipid monolayers spread at the air/solution interface was studied by reflection spectroscopy. The adsorption characteristics of the two nitrophenols have been studied by measuring the surface pressure and surface potential as a function of molecular area of the different lipid monolayers in the presence of nitrophenols in the subphase. The results have been interpreted in terms of the electrostatic interaction between the negatively charged dissociated phenolate ions and the positively charged head group of dioctadecyldimethylammonium bromide monolayers.

Formation of nickelhydroxy sulfide precursor and nickel sulfide in Langmuir and Langmuir-Blodgett films of a nickel complex of octadecylsuccinic acid

Abstract The influence of parameters on size of NiS particles formed at ambient temperatures by reacting Na 2 S with the nickel salt of octadecylsuccinic acid (ODSA) at the air/water interface and in Langmuir–Blodgett films was investigated by changes in surface pressure–molecular area (π– A ) and surface potential–molecular area (Δ V – A ) isotherms. The contraction in area of ODSA on Ni 2+ subphase is indicative of a strong interaction of the dibasic acid with Ni 2+ leading to a monodentate complex of Ni 2+ with ODSA. The Langmuir monolayers of the nickel complex of ODSA were analysed using Brewster angle microscopy (BAM). In the absence of nickel in the subphase, ODSA exhibited no domains in BAM. The sulfidation reaction on ODSA/Ni 2+ complex at the air/water interface shows two steps as seen in the surface potential vs. time plot corresponding to formation of Ni(OH)S initially which then changes to NiS. The domain size seen with NiS formation increased with time and surface pressure indicating the growth of a two-dimensional solid complex in the monolayer at the air/water interface. The Langmuir–Blodgett (LB) films of ODSA/Ni 2+ on sulfidation transferred onto solid substrates from the two steps in the reaction were characterized using diffuse reflectance UV-visible spectroscopy (DIR-UV-vis) and X-ray photoemission spectroscopy. These experiments confirmed the reduction process involved in the above reaction. Atomic force microscopy experiments on these LB films indicated that there is a variation in the structural organization in the monolayers and in the morphology of the LB film, depending on the initial Ni(OH)S formation which ultimately leads to NiS. The results show that the average particle size which varies between 2 and 3.5 nm depends on the nature of the nickel complexes formed at the air/water interface.

Control of size of PbI2 nanocrystals using Langmuir–Blodgett films of n-octadecyl succinic acid

Lead iodide (PbI2) particulate films, composed of highly oriented nanocrystals have been generated in situ using an oriented inorganic extended solid within a deposited Langmuir–Blodgett film (LB film) of octadecyl lead succinate (ODSAPb). Solid (PbI2) was formed by the reaction of gaseous HI with ODSAPb LB film, which acts as the template for building the inorganic solid. Mixed LB systems with 1:1 mol ratio of ODSAPb with octadecyl amine (ODAM) were also used to form the PbI2. The films were characterized using attenuated total reflection FTIR (ATR-FTIR) to check whether HI had completely protonated the films. The observation of absorption features between 550 and 300 nm region in the UV-Visible spectra of these films strongly suggest sizes of PbI2 ranging between 2.0 and 3.0 nm. The optical band gap estimated from LB films of these two systems showed a marked increase from 0.36 eV for the pure ODSAPb template to 0.47 eV for the ODSAPb+ODAM mixed film. The LB films were examined by small angle XRD and atomic force microscopy (AFM) and indicated that there is a variation in the structural organization in the monolayers and in the size of PbI2 in the pure ODSAPb LB films as compared with the mixed films with ODAM. The average contraction in a axis and elongation in the c axis of the unit cell (a=b=3.9 A and c=7.0 A) for PbI2 in the template, compared with the values for PbI2 in bulk confirm the presence of the nanocrystals in the LB films. The increase in the band gap is also reflected in the increase in the c value of the unit cell parameter that changes from 6.3 to 7.0 A in going from the pure ODSAPb matrix to mixed film of ODSAPb and ODAM.

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.

Local anesthetics—phospholipid interaction. A study of Dibucaine binding to lipid monolayers.

Abstract The interactions of the local anesthetic Dibucaine with some lipid monolayers were studied at the air/water interface by measuring the surface pressure-area and the surface potential-area isotherms at different pH values of 4, 7 and 10 respectively. At any given pH, there was large expansion of film area at low surface pressure for negatively charged phospholipids. This expansion could be interpreted as a preferential binding of this drug to anionic phospholipids. The surface potential value of the lipid monolayers spread on the subphase containing Dibucaine showed a significant decrease with increasing pH. This indicated a depolarization effect of the Dibucaine on the lipid monolayers at high pH. The uptake of the anesthetic molecule by the various lipid monolayers was also studied by measuring the absorption spectra of these monolayers transferred onto quartz with the anesthetic bound to the monolayers. The results obtained have been discussed in terms of the neutral and charged forms of Dibucaine and the interactions of these various forms with the lipid monolayers. Results indicate that the charged form interacts more strongly with the lipids than does the neutral species.

Preparation of copper sulfide clusters in organic-inorganic composites of Langmuir-Blodgett films of amphiphilic Schiff bases.

Copper sulphide clusters were prepared in Langmuir-Blodgett films of copper complexes of amphiphilic Schiff bases-3,4-dimethoxy-N-benzylidene hexadeylamine (I) and 3,4-dimethoxy-N-benzylidene-4’-(hexadecylamino) benzylamine (II) The clusters obtained were analysed using UV-Vis spectroscopy and optical microscopy. Brewster angle microscopic studies on monolayers ofI andII at air/water interface showed formation of needle-like domains which seem to cluster faster inI than inII. Atomic force microscopy (AFM) studies also showed fairly uniform sized clusters inII whereas in the case ofI they seem to show varying sizes. From the results it is concluded that π-elongation in the polar head groups leads to controlled cluster sizes in compoundII as compared to those in compoundI.