Morphology and molecular conformation in thin films of poly-gamma-methyl-L-glutamate at the air-water interface
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The behavior of poly-gamma-methyl-L-glutamate (pMeE) at the air-water interface has been studied with the surface film balance technique. In addition, Langmuir-Blodgett (LB) films of pMeE deposited on mica and quartz have been studied by atomic force microscopy (AFM) and circular and linear dichroism (CD and LD) spectroscopy. Depending on the spreading solvent, pMeE displays strikingly different compression isotherms. When spread from chloroform or trifluoroacetic acid (TFA) the surface pressure isotherms are consistent with that of a peptide in a-helix conformation. However, the latter solvent gives rise to isotherms with a considerably smaller apparent mean molecular area, A(0). When spread from pyridine, on the other hand, pMeE yields an isotherm that is expanded and inconsistent with the presence of a monolayer consisting entirely of a-helical peptides. Isotherms and AFM images strongly suggest that peptide aggregation and solvent retention are the main factors behind the isotherm differences. When the water-soluble spreading solvent TFA is used, pMeE forms discrete wormlike aggregates embedded in a monolayer matrix. In the pyridine case, aggregation in the spreading solvent and retention of pyridine in the film result in a rough aggregate network coexisting with discrete aggregates. No aggregation takes place when chloroform is used as spreading solvent. CD and LD spectra of the LB films reveal a pronounced lateral orientation of the alpha-helices in films spread from chloroform and TFA, while spectra of films spread from pyridine are consistent with unordered peptide strands in beta-sheet conformation. In conclusion, the results show that if water-soluble and/or low-volatile solvents are used as spreading media, hydrophobic peptides cannot, a priori, be assumed to form proper monolayers.