Vibrational spectroscopic investigations of hydrocarbon chain ordering in phosphatidylcholines (PCs) adsorbed from aqueous solution to a carbon tetrachloride−water interface presented here examine on a molecular level the organization pertinent to the surface characteristics displayed by these films. In a series of saturated symmetric and asymmetric chain PCs, both symmetric PCs with 16 or fewer carbons per acyl chain and highly asymmetric PCs produced relatively disordered films at the liquid−liquid interface. The longest chain PCs studied, 1,2-distearoyl-sn-glycero-3-PC (C18/C18), 1-stearoyl-2-palmitoyl-sn-glycero-3-PC (C18/C16) and 1-palmitoyl-2-stearoyl-sn-glycero-3-PC (C16/C18), formed well-ordered layers at room temperature. The results can be explained in terms of enhanced chain−chain interactions among the longer, nearly symmetric hydrocarbon chains. Properties of the neat liquid−liquid interface that may influence the formation of these well-ordered two-dimensional phases are discussed.

A new optical method intended for interfacial protein studies has surfaced from the observation of near UV second harmonic generation (SHG) from the aromatic amino acids. Here, the first hyperpolarizability, β=4.72±0.52×10−30 esu, of t-butyloxycarbonyl-L-tryptophan is measured for SHG at 266 nm, and its polar orientation at the air–water interface is deduced. The indole ring nitrogen atom is found to point upwards at the interface. From a monolayer surface density of 1.74±0.15 adsorbate molecules/nm2, it is estimated that a minimum of 0.49 Trp residues/nm2 of surface area can be detected above the water background for this wavelength. SHG studies of tryptophan-containing peptides and proteins based on these results have the potential to complement existing linear optical techniques by providing new information on interfacial molecular ordering.

The molecular‐level organization of mixed and pure saturated symmetric chain 1,2‐diacyl‐sn‐glycero‐3‐phosphocholines (PCs) adsorbed at a carbon tetrachloride–aqueous interface is explored by probing the hydrocarbon chain conformation within the adsorbed layer. PCs of the chain lengths found most frequently in biological systems, which in pure form are seen to form either very well‐ordered or disordered layers, are observed in these studies to assemble into interfacial layers ranging from disordered to ordered states when mixed in various proportions. Independently, while C16 and shorter chain PCs tend to form disordered layers, a strong increase in ordering is observed for C18 and longer chain PCs in which the hydrocarbon chains are found to be primarily in an all trans conformation. Pure C17‐PCs adsorbed at the interface produce layers with an intermediate degree of chain ordering. The ability to tune interfacial layer properties in mixed systems as a function of molecular composition, including PC chain length as demonstrated here, is an important mechanism by which surface characteristics of oil–water emulsion systems can be controlled both in vivo and in numerous commercial applications. © 2000 John Wiley & Sons, Inc. Biopolymers (Biospectroscopy) 57: 117–125, 2000

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Surface second harmonic generation (SHO) has recently emerged as a promising new technique capable of probing the molecular organization at liquid interfaces. It provides information on the number of molecules adsorbed, the orientation and local environment of specific residues, and confers the ability to assess the symmetry of the intermolecular arrangement. Previously, SHG had been applied in the study of simple model systems where the major signal contributions originated from dye chromophores. Here we show SHG to be sensitive to submonolayer densities of the aromatic amino acids tryptophan, tyrosine and phenylalanine adsorbed to the air/water interface, with the strongest resonant enhancement at the second harmonic wavelength of 266 nm being found for Iryptophan. This observation opens the door for probing the ordering of peptides at liquid interfaces by exploiting the nonlinear optical properties of the tryptophan residues.