In this paper, we employ the surface-specific polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) and sum-frequency generation (SFG) methods with surface pressure and surface potential isotherms to determine the organization of p-tert-butylcalix[6]arene molecules and their interaction with Cd 2+ ions in Langmuir monolayers. The area per molecule was estimated to be 135 Å 2 , which corresponds to the Calix6 axis perpendicular to the air-water interface with most OH groups parallel to the interface. This area is larger than predicted by molecular modeling with quantum chemical calculations with a PM3 Hamiltonian (109 Å 2 ), which is ascribed to the repulsion between Calix6 molecules. The incorporation of Cd 2+ ions in the subphase leads to drastic changes in the dipole moment contribution of the monolayer surface potential. Rather than increasing with incorporation of Cd 2+ ions owing to a decrease in the negative double-layer potential, the measured surface potential decreased monotonically with increasing ion concentration. This unexpected result was ascribed to a strong interaction with Cd 2+ ions that induced the calyx of the molecule to adopt a more open conformation at the air/water interface and affected the orientation of hydration water molecules, according to the SFG data. This finding allows us to understand the reason why the Gouy-Chapman model fails to explain surface potential results for subphases containing divalent or trivalent ions, and may be relevant for the application of calixarenes in sensing.

Expulsion of p-tert-butylcalix[6]arene molecules from a monolayer in a biologically relevant pressure regime, π = 30 mN m−1, correlates with their lack of antibacterial activity.

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One of the major challenges in drug design is to identify compounds with potential toxicity toward target cells, preferably with molecular-level understanding of their mode of action. In this study, the antitumor property of a ruthenium complex, mer-[RuCl 3 (dppb)(VPy)] (dppb =1,4-bis(diphenylphosphine)butane and VPy = 4-vinylpyridine) (RuVPy), was analyzed. Results showed that this compound led to a mortality rate of 50% of HEp-2 cell with 120 ± 10 μmol L −1 , indicating its high toxicity. Then, to prove if its mode of action is associated with its interaction with cell membranes, Langmuir monolayers were used as a membrane model. RuVPy had a strong effect on the surface pressure isotherms, especially on the elastic properties of both the zwitterionic dipalmitoylphosphatidylcholine (DPPC) and the negatively charged dipalmitoylphosphatidylglycerol (DPPG) phospholipids. These data were confirmed by polarization-modulated infrared reflection−absorption spectroscopy (PM-IRRAS). In addition, interactions between the positive group from RuVPy and the phosphate group from the phospholipids were corroborated by density functional theory (DFT) calculations, allowing the determination of the Ru complex orientation at the air−water interface. Although possible contributions from receptors or other cell components cannot be discarded, the results reported here represent evidence for significant effects on the cell membranes which are probably associated with the high toxicity of RuVPy.

We report the first attempt to use a naturally occurring biopolymer, such as humic acid (HA), as modifiers to prepare a new organic-inorganic composite film with the 3-n-propylpyridiniumsilsesquioxane chloride Pt nanoparticles (Pt-SiPy+Cl−). By using layer-by-layer (LbL) technique, we have constructed multilayered films, (HA/Pt-SiPy+Cl−)n and (Pt-SiPy+Cl−/HA)n, with adjustable thickness due to negatively charged macromolecule HA, which assists the electrostatic interaction with the positively charged of the Pt-SiPy+Cl− nanohybrid. The combination of these partially charged polyelectrolytes in the LbL film was explored by spectroscopic and electrochemical techniques. The formation of the film (HA/Pt-SiPy+Cl−)n is more efficient, since the presence of the macromolecule in the inner layer provides a greater number of anchoring sites for the hybrid, ensuring a more linear and uniform growth of the LbL films. This film also presented a better electroanalytical response for oxidation of 17α-ethynylestradiol, EE2, indicating that the Pt nanoparticles strongly influences the electrochemical oxidation of estrogen. The analytical curve for EE2 was linear in the concentration range of 1.37 to 21.4 μmol L−1 (R = 0.998), with a detection limit of 1.10 μmol L−1 and quantification limit of 3.68 μmol L−1. The obtained results showed that the synergism between HA and Pt-SiPy+Cl− improves the electroactive properties of the sensor.

In this paper, the aggregate formation of para-tert-butylcalix[6]arene molecules (Calix6) in dimeric structures was investigated at the water/air interface using experimental and theoretical studies. A specific orientation for such Calix6 molecules was observed with an average area of 133 Å(2), which corresponds to a flat-on orientation with the OH groups parallel to the interface. By varying the pressure on the Calix6 monolayer, the molecules tend to organize at the water/air interface and subsequently, at higher pressures, aggregates were formed atop the monolayer as cluster structures. Morphological characterization by the Brewster Angle Microscopy technique showed the formation of larger domains at lower pressures. Based on such experimental evidence, molecular dynamics (MD) simulations were performed to investigate possible dimeric structures for aggregated Calix6 molecules, which are localized at the water/air interface, where one molecule remains in the water phase and the other remains in the air phase. By increasing surface pressure, experimental and theoretical results corroborate the intermolecular interactions among Calix6 molecules. These results are relevant because a dimeric structure has a molecular cavity, which is a candidate for host-guest chemistry, an ion receptor or a drug-delivery system.

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