Molecular Orientation at the Squalene/Air Interface from Sum Frequency Generation Spectroscopy and Atomistic Modeling

Domaros, Michael von; Liu, Yangdongling; Butman, Jana L; Perlt, Eva; Geiger, Franz M.; Tobias, Douglas J.

doi:10.1021/acs.jpcb.0c11158

Cited by 14 publications

(22 citation statements)

References 73 publications

(131 reference statements)

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“…The MD simulations in refs ( 48 ) and ( 49 ) are in agreement with our earlier study 5 that indicated squalene has a relatively extended conformation in nonpolar solutions. The calculations and Figure 4 of ref ( 48 ) showed that ∼94% of 343 molecules were relatively extended; ∼45% had all- anti methylene bridges, ∼37% had one gauche defect, and ∼12% had two. The percentages in bulk squalene and the interface were essentially the same.…”

Section: Discussionsupporting

confidence: 89%

Diffusion of Squalene in Nonaqueous Solvents

Kowert

2022

ACS Omega

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Capillary flow techniques have been used to determine the translational diffusion constant, D , of squalene in seven alkanes and five cyclohexanes. The alkanes are n -hexane, n -octane, n -decane, n -dodecane, n -tetradecane, 2,2,4,4,6,8,8-heptamethylnonane (isocetane), and 2,6,10,14-tetramethylpentadecane (pristane). The cyclohexanes are cyclohexane, n -butylcyclohexane, n -hexylcyclohexane, n -octylcyclohexane, and n -dodecylcyclohexane. When combined with published data in CD 2 Cl 2 , ethyl acetate, n -hexadecane, squalane, n -octane–squalane mixtures, and supercritical CO 2 , the 35 diffusion constants and viscosities, η, vary by factors of ∼230 and ∼500, respectively. A fit to the modified Stokes–Einstein equation (MSE, D / T = A SE /η p ) gives an average absolute percentage difference (AAPD) of 7.72% between the experimental and calculated D values where p and A SE are constants, T is the absolute temperature, and the AAPD is the average value of (10 2 ) (| D calcd – D exptl |/ D exptl ). Two other MSE fits using subsets of the 35 diffusion constants may be useful for (a) estimating the viscosity of the hydrophobic core of lipid droplets, where squalene is a naturally occurring component, and (b) providing estimates of the D values needed to design extraction processes by which squalene is obtained from plant oils. The Wilke–Chang equation also was considered and found to give larger AAPDs than the corresponding MSE fits.

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Section: Discussionsupporting

confidence: 89%

Diffusion of Squalene in Nonaqueous Solvents

Kowert

2022

ACS Omega

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“…SFG is a second-order nonlinear optical process characterized by second-order nonlinear optical susceptibility χ (2) . ,,,,− SFG theories, experimental setups, and data analyses have been extensively reported ,,,,− and will not be repeated here. Under the electric dipole approximation, the SFG signal cannot be generated from most bulk materials which have inversion symmetry but can be produced from surfaces/interfaces where inversion symmetry is broken. ,,,,− …”

Section: Methodsmentioning

confidence: 99%

Molecular Structure of the Surface-Immobilized Super Uranyl Binding Protein

et al. 2021

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Recently, a super uranyl binding protein (SUP) was developed, which exhibits excellent sensitivity/selectivity to bind uranyl ions. It can be immobilized onto a surface in sensing devices to detect uranyl ions. Here, sum frequency generation (SFG) vibrational spectroscopy was applied to probe the interfacial structures of surface-immobilized SUP. The collected SFG spectra were compared to the calculated orientation-dependent SUP SFG spectra using a one-excitonic Hamiltonian approach based on the SUP crystal structures to deduce the most likely surface-immobilized SUP orientation(s). Furthermore, discrete molecular dynamics (DMD) simulation was applied to refine the surface-immobilized SUP conformations and orientations. The immobilized SUP structures calculated from DMD simulations confirmed the SUP orientations obtained from SFG data analyzed based on the crystal structures and were then used for a new round of SFG orientation analysis to more accurately determine the interfacial orientations and conformations of immobilized SUP before and after uranyl ion binding, providing an in-depth understanding of molecular interactions between SUP and the surface and the effect of uranyl ion binding on the SUP interfacial structures. We believe that the developed method of combining SFG measurements, DMD simulation, and Hamiltonian data analysis approach is widely applicable to study biomolecules at solid/liquid interfaces.

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“…A notable indoor ozone reaction is with squalene. − Squalene is a 30-carbon chain molecule that contains six trans-oriented carbon–carbon double bonds, accounts for 5%–15% of the total human sebum composition, and accounts for approximately 48% of the unsaturated double bonds in skin oil, which makes it one of the most reactive components in human sebum. − The other main components of sebum are fatty acids, wax monoesters, triglycerides, cholesterol, and cholesterol esters. ,,− Squalene is a semivolatile unsaturated compound that is continuously produced by the body’s sebaceous glands and is on the surface of the skin, hair, and clothing, as well as in dust as a component of dead skin cells and in indoor particulate matter. ,,− In addition, squalene is found in fingerprints, which places squalene on most surfaces around the home. , Sebaceous glands produce squalene to beneficially protect the lower layers of the skin’s structure from oxidation by atmospheric oxidants such as free radicals and ozone . The amount of squalene on one’s skin does not vary significantly throughout the day due to continual reactions and replenishment from the sebaceous glands. , Since the SqOz reaction occurs on people’s skin and clothing that contains squalene while being worn, there is high potential for exposure to the reaction products.…”

Section: Introductionmentioning

confidence: 99%

Reactions and Products of Squalene and Ozone: A Review

Coffaro

Weisel

2022

Environ. Sci. Technol.

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This critical review describes the squalene-ozone (SqOz) reaction, or squalene ozonolysis. Ambient ozone penetrates indoors and drives indoor air chemistry. Squalene, a component of human skin oil, contains six carbon–carbon double bonds and is very reactive with ozone. Bioeffluents from people contribute to indoor air chemistry and affect the indoor air quality, resulting in exposures because people spend the majority of their time indoors. The SqOz reaction proceeds through various formation pathways and produces compounds that include aldehydes, ketones, carboxylic acids, and dicarbonyl species, which have a range of volatilities. In this critical review of SqOz chemistry, information on the mechanism of reaction, reaction probability, rate constants, and reaction kinetics are compiled. Characterizations of SqOz reaction products have been done in laboratory experiments and real-world settings. The effect of multiple environmental parameters (ozone concentration, air exchange rate (AER), temperature, and relative humidity (RH)) in indoor settings are summarized. This critical review concludes by identifying the paucity of available exposure, health, and toxicological data for known reaction products. Key knowledge gaps about SqOz reactions leading to indoor exposures and adverse health outcomes are provided as well as an outlook on where the field is headed.

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Molecular Orientation at the Squalene/Air Interface from Sum Frequency Generation Spectroscopy and Atomistic Modeling

Cited by 14 publications

References 73 publications

Diffusion of Squalene in Nonaqueous Solvents

Diffusion of Squalene in Nonaqueous Solvents

Molecular Structure of the Surface-Immobilized Super Uranyl Binding Protein

Reactions and Products of Squalene and Ozone: A Review

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