The absorption and fluorescence spectra and second harmonic generation (SHG) of the insoluble monolayer of bis-(N-ethyl,N-octadecyl)rhodamine (RhC18) at the air−water interface have been measured. These spectra were affected significantly by compression, and the observed changes were ascribed to the formation and structural rearrangement of aggregated species on the water surface during compression. The spectroscopic behavior of the monolayer was explained in accordance with its rheological properties, and the transition from disordered monomers to dimers, from dimers to aggregates, and from aggregates to two-dimensional arrays was proposed. SHG studies revealed that the RhC18 molecules in the expanded film region are oriented with their C 2-axis tilted away from the surface normal on angle θ distributed in the range of 31−39°. The rotational distribution around the C 2-axis was assumed to be 45−60° according to preferable intermolecular interactions with the water subphase and surrounding molecules. The θ angle distribution became slightly narrow because of the increase of molecular ordering caused by two-dimensional external pressure. The sharp increase of SHG intensity and the phase shift observed at high compression were ascribed to the formation of blue-shifted aggregates with their electronic transition being in resonance with the incident laser frequency. The results of spectroscopic and SHG studies were jointly analyzed, and the structural rearrangement within the monolayer during compression was described.
This study was aimed at understanding the nature of conformational transition(s) that occur in poly(ethylene glycol) chains with molecular weight 5000 (PEG5000) grafted onto phospholipid monolayers. The study was performed with monolayers of PEG−phospholipid, DSPE−PEG5000, at the air/water interface and on solid substrates. Surface pressure and surface potential measurements together with ellipsometry and Brewster angle and atomic force microscopy were used to assess changes in the conformation and hydration of PEG5000 chains with increasing PEG grafting density. A comparative analysis of our experimental data suggests that neither the concept of the first-order pancake-to-brush conformational transition nor the model considering two subsequent transitions, namely, pancake-to-mushroom and mushroom-to-brush transitions, are adequate in describing conformational changes in the DSPE−PEG5000 monolayer at the air/water interface. At low grafting densities, PEG5000 chains did behave as grafted polymeric chains in a good solvent forming true 2D pancakes at the air/water interface. However, the conformation developed at high grafting densities was found to differ significantly from the PEG5000 brush in a good solvent. Its extension into the subphase was in far better agreement with the theoretical predictions for the height of PEG5000 brush in a theta solvent. This finding together with the decreasing absolute value of ψ 0 potential of DSPE−PEG5000 monolayer implies that conformational transition(s) in PEG5000 are accompanied by changes in interactions of PEG monomers with water molecules and by a substantial dehydration of PEG5000 chains. Conformational changes in grafted PEG5000 chains at the air/water interface are therefore interpreted as a transition from pancake to poorly hydrated brush conformation.
Monolayer behavior of a poly(ethylene glycol)-grafted distearoylphosphatidylethanolamine with PEG molecular weight 5000 (DSPE-PEG5000) was studied by a variety of experimental methods. Slope changes in the surface pressure-molecular area (π-A) isotherm suggest two transitions in the DSPE-PEG5000 monolayer upon compression which were previously attributed to conformational changes in the polymeric moiety. However, comparative analysis of our experimental data implies that the second slope change in the isotherm at π ≈ 18 mN/m originates from the monolayer collapse. As visualized by Brewster angle and atomic force (AFM) microscopy, the fluid homogeneous DSPE-PEG5000 monolayer is unable to sustain high pressures and collapses through vesicular structures formed as a result of 2D to 3D relaxation of an overcompressed film. AFM topographic images provided a height value of 9 nm for the collapsed structures on top of the monolayer, which can be referred to as the height of DSPE-PEG5000 lamellar structures. Thus, the collapse at a surprisingly low surface pressure implies that a distinct region where the PEG5000 moiety would adopt a brush conformation is probably never achieved for the DSPE-PEG5000 monolayer. Ellipsometric measurements of monolayer thickness confirm that although an elongated conformation is developed prior to the monolayer collapse, this conformation is better described as an "extended mushroom" rather than a "brush".
Monolayer behavior of the dye 7-nitro-2-1,3-benzoxadiazol-4-yl (NBD)-labeled analogues of phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphatidylethanolamine (DPPE) and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) were studied by a variety of methods. Attachment of the NBD chromophore onto either the phospholipid headgroup or the aliphatic chain significantly changes the parent phospholipid monolayer properties. In contrast to that of the condensed-type DPPE monolayer, the isotherm of the DPPE-NBD derivative with NBD in the headgroup exhibits a liquid-expanded/liquid-condensed phase transition plateau while the isotherm of the acyl-chain-labeled NBD(C12)-PC shows liquid-expanded behavior lacking the plateau observed in the isotherm of DPPC. Surface potential and spectroscopic data revealed that the NBD group on both DPPE-NBD and NBD(C 12)-PC embeds into the phospholipid headgroup region of the monolayer, thus suggesting a looping of the NBD-labeled chain of NBD(C12)-PC toward water. Polarized fluorescence and red-edge excitation shift studies revealed a regular alignment of uniformly oriented NBD chromophores in the DPPE-NBD monolayer controlled by networks of intermolecular hydrogen bonds. By contrast, the NBD group on NBD(C 12)-PC experienced a less-structured environment permitting rotation of the NBD chromophore and fast relaxation of surrounding water dipoles. Organization of the NBD-labeled phospholipids at the air/water interface and intermolecular interactions responsible for the monolayer structuring are discussed.
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