Sensitive Detection of Biomolecular Adsorption by a Low-Density Surfactant Layer Using Sum-Frequency Vibrational Spectroscopy.

Small molecules or proteins interact with a biomembrane in various ways for molecular recognition, structure stabilization, and transmembrane signaling. In this study, 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP), having a choline group, was used to investigate this interaction by using sum-frequency vibrational spectroscopy. The sum-frequency spectrum characteristic of a neat monolayer changed to that of a bare air/water interface at a larger molecular area of the DPTAP molecules due to local laser heating. Upon introduction of the aromatic molecules in the subphase at around 120 Å2 per molecule, the sum-frequency signal suddenly reappeared due to molecular adhesion, and this was utilized to probe the adsorption of the aromatic ring molecules in the water subphase to the choline headgroup of the DPTAP by cation-π interaction. The onset concentrations of this sum-frequency signal change allowed a comparison of the relative interaction strengths between different aromatic molecules. A zwitterionic surfactant molecule (DPPC) was found to interact weakly compared to the cationic DPTAP molecule.

Sum-Frequency Vibrational Spectroscopic Study of the Cation-π Interaction: Amine and Guanidine.

The cation-π interaction is an interaction between a positively charged cation and π electrons in an aromatic group of a molecule. It is considered to play key roles in signal transduction, stabilization of the protein structure, enzyme catalysis in biology, and wet adhesion and biomolecular condensation. In this study, octadecylguanidine hydrochloride (ODG) and octadecylamine (ODA) having guanidine and amine headgroups, respectively, are found to interact with π molecules (phenol or indole) as investigated by sum-frequency vibrational spectroscopy. ODG is unstable and does not form a neat monolayer on the water surface. However, after adding π molecules into subphase water, it becomes more stable against dissolution as evidenced by the appearance of its CHx peaks and a CH peak of the aromatic ring in the sum-frequency spectrum. Unlike ODG, ODA forms a stable monolayer on the water surface at a neutral pH. After adding π molecules into the solution, the amine-π interaction promotes the protonation of the amine headgroup and the penetration of the π molecules makes the ODA monolayer more disordered. Indole is found to be more effective in binding with the ODG as compared to phenol.

Recovery of Fatty Acid Monolayers by Salts Investigated by Sum-Frequency Generation Spectroscopy.

Langmuir monolayers consisting of fatty acids with relatively short alkyl chains (C14H29COOH (pentadecanoic acid), C15H31COOH (palmitic acid), and C16H33COOH (heptadecanoic acid)) are stable at a neutral pH (pH ≈ 6) but become unstable at a high pH (pH ≈ 11). Further addition of a small amount of divalent salt in subphase water was found to recover the monolayer at a high pH because binding of the divalent cations to the carboxylic headgroups renders the molecule more stable against dissolution in subphase water. This revival of the monolayer was observed via a pressure-area isotherm measurement and sum-frequency generation spectrum in the CHx and OH ranges. Fatty acids with longer alkyl chains needed less amount of MgCl2 to recover the monolayer at a high pH. A much lower concentration of Mg2+ as compared to Ca2+ is required to revive fatty acid molecules to the surface. Monovalent and trivalent salts were compared with the above divalent salts on the ability to recover the fatty acid monolayers.

Structure of Electric Double Layer under Cationic Langmuir Monolayer: Charge Condensation Effect.

Langmuir monolayers consisting of mixtures of 1-hexadecanol (HD) and 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP) (having quaternary amine headgroup) at different molar ratios were prepared to investigate the effect of the surface charge density on the structure of an electric double layer. The fatty alcohol molecules worked as passive spacers to widen the distance between the amine groups in the monolayer, to vary the surface charge density of the monolayer, and these mixture monolayer systems were probed by surface-sensitive sum-frequency vibrational spectroscopy. A strong sum-frequency signal in the OH range for a pure DPTAP monolayer (with a surface charge density of ∼0.4 C/m2) hardly decreased as the surface charge density was reduced up to ∼0.12 C/m2 (1 e per 140 Å2) and afterward decreased monotonically as more HD occupied the monolayer. The Gouy-Chapman theory incorporating a charged-condensed layer in which the counterion concentration is limited by a close packing of the counterions could account for the above saturation behavior in the sum-frequency spectra.