Unanticipated Stickiness of α‑Pinene

The adsorption of α-pinene to solid surfaces is an important primary step during the chemical conversion of this common terpene over mesoporous materials, as well as during the formation of atmospheric aerosols. We provide evidence of tight and loose physisorbed states of α-pinene bound on amorphous SiO₂ as determined by their adsorption entropy, enthalpy, and binding free energies characterized by computational modeling and vibrational sum frequency generation (SFG) spectroscopy. We find that adsorption is partially (40–60%) irreversible over days at 294–342 K and 1 ATM total pressure of helium, which is supported by molecular dynamics (MD) simulations. The distribution of α-pinene orientation remains invariant with temperature and partial pressure of α-pinene. Using the Redlich–Peterson adsorption model in conjunction with a van’t Hoff analysis of adsorption isotherms recorded for up to 2.6 Torr α-pinene in 1 ATM total pressure of helium, we obtain Δ<i>S</i>°_ads, Δ<i>H</i>°_ads, and Δ<i>G</i>°_ads values of −57 (±7) J mol^–1 K^–1, −39 (±2) kJ mol^–1, and −22 (±5) kJ mol^–1, respectively, associated with the reversibly bound population of α-pinene. These values are in good agreement with density functional theory (DFT)-corrected force field calculations based on configurational sampling from MD simulations. Our findings are expected to have direct implications on the conversion of terpenes by silica-based catalysts and for the synthesis of secondary organic aerosol (SOA) in atmospheric chambers and flow tubes

Cholesterol-Dependent Conformational Exchange of the C‑Terminal Domain of the Influenza A M2 Protein

The C-terminal amphipathic helix of the influenza A M2 protein plays a critical cholesterol-dependent role in viral budding. To provide atomic-level detail on the impact cholesterol has on the conformation of M2 protein, we spin-labeled sites right before and within the C-terminal amphipathic helix of the M2 protein. We studied the spin-labeled M2 proteins in membranes both with and without cholesterol. We used a multipronged site-directed spin-label electron paramagnetic resonance (SDSL-EPR) approach and collected data on line shapes, relaxation rates, accessibility of sites to the membrane, and distances between symmetry-related sites within the tetrameric protein. We demonstrate that the C-terminal amphipathic helix of M2 populates at least two conformations in POPC/POPG 4:1 bilayers. Furthermore, we show that the conformational state that becomes more populated in the presence of cholesterol is less dynamic, less membrane buried, and more tightly packed than the other state. Cholesterol-dependent changes in M2 could be attributed to the changes cholesterol induces in bilayer properties and/or direct binding of cholesterol to the protein. We propose a model consistent with all of our experimental data that suggests that the predominant conformation we observe in the presence of cholesterol is relevant for the understanding of viral budding

Vibrational Mode Assignment of α‑Pinene by Isotope Editing: One Down, Seventy-One To Go

This study aims to reliably assign the vibrational sum frequency generation (SFG) spectrum of α-pinene at the vapor/solid interface using a method involving deuteration of various methyl groups. The synthesis of five deuterated isotopologues of α-pinene is presented to determine the impact that removing contributions from methyl group C–H oscillators has on its SFG response. 0.6 cm^–1 resolution SFG spectra of these isotopologues show varying degrees of differences in the C–H stretching region when compared to the SFG response of unlabeled α-pinene. The largest spectral changes were observed for the isotopologue containing a fully deuterated vinyl methyl group. Noticeable losses in signal intensities allow us to reliably assign the 2860 cm^–1 peak to the vinyl methyl symmetric stretch. Furthermore, upon removing the vinyl methyl group entirely by synthesizing apopinene, the steric influence of the unlabeled C₉H₁₄ fragment on the SFG response of α-pinene SFG can be readily observed. The work presented here brings us one step closer to understanding the vibrational spectroscopy of α-pinene

Assessment of DFT for Computing Sum Frequency Generation Spectra of an Epoxydiol and a Deuterated Isotopologue at Fused Silica/Vapor Interfaces

We assess the capabilities of eight popular density functional theory (DFT) functionals, in combination with several basis sets, as applied to calculations of vibrational sum frequency generation (SFG) spectra of the atmospherically relevant isoprene oxidation product <i>trans</i>-β-isoprene epoxydiol (IEPOX) and one of its deuterated isotopologues at the fused silica/vapor interface. We use sum of squared differences (SSD) and total absolute error (TAE) calculations to estimate the performance of each functional/basis set combination in producing SFG spectra that match experimentally obtained spectra from <i>trans</i>-β-IEPOX and one of its isotopologues. Our joined SSD/TAE analysis shows that while the twist angle of the methyl <i>C</i>_3<i>v</i> symmetry axis of <i>trans</i>-β-IEPOX relative to the surface is sensitive to the choice of DFT functional, the calculated tilt angle relative to the surface normal is largely independent of the functional and basis set. Moreover, we report that hybrid functionals such as B3LYP, ωB97X-D, PBE0, and B97-1 in combination with a modest basis set, such as 6-311G­(d,p), provides good agreement with experimental data and much better performance than pure functionals such as PBE and BP86. However, improving the quality of the basis set only improves agreement with experimental data for calculations based on pure functionals. A conformational analysis, based on comparisons of calculated and experimental SFG spectra, suggests that <i>trans</i>-β-IEPOX points all of its oxygen atoms toward the silica/vapor interface

Sum Frequency Generation Spectroscopy and Molecular Dynamics Simulations Reveal a Rotationally Fluid Adsorption State of α‑Pinene on Silica

A rotationally fluid state of α-pinene at fused silica/vapor interfaces is revealed by computational and experimental vibrational sum frequency generation (SFG) studies. We report the first assignment of the vibrational modes in the notoriously congested C–H stretching region of α-pinene and identify its bridge methylene group on the four-membered ring (“^βCH₂”) as the origin of its dominant spectral feature. We find that the spectra are perfused with Fermi resonances that need to be accounted for explicitly in the computation of vibrational spectra of strained hydrocarbons such as α-pinene. The preferred orientations of α-pinene are consistent with optimization of van der Waals contacts with the silica surface that results in a bimodal distribution of highly fluxional orientations in which the ^βCH₂ group points “towards” or “away from” the surface. Classical molecular dynamics simulations further provide rotational diffusion constants of 49 ± 1 ps and 2580 ± 60 ps, which are attributed to two broad types of adsorption modes on silica. The reported findings are particularly relevant to the exposure of α-pinene to primary oxidants in heterogeneous catalytic pathways that exploit α-pinene as a sustainable feedstock for fine chemicals and polymers

On Surface Order and Disorder of α‑Pinene-Derived Secondary Organic Material

The surfaces of secondary organic aerosol particles are notoriously difficult to access experimentally, even though they are the key location where exchange between the aerosol particle phase and its gas phase occurs. Here, we overcome this difficulty by applying standard and sub- 1 cm^–1 resolution vibrational sum frequency generation (SFG) spectroscopy to detect C–H oscillators at the surfaces of secondary organic material (SOM) prepared from the ozonolysis of α-pinene at Harvard University and at the University of California, Irvine, that were subsequently collected on Teflon filters as well as CaF₂ windows using electrostatic deposition. We find both samples yield comparable SFG spectra featuring an intense peak at 2940 cm^–1 that are independent of spectral resolution and location or method of preparation. We hypothesize that the SFG spectra are due to surface-active C–H oscillators associated with the four-membered ring motif of α-pinene, which produces an unresolvable spectral continuum of approximately 50 cm^–1 width reminiscent of the similar, albeit much broader, O–H stretching continuum observed in the SFG spectra of aqueous surfaces. Upon subjecting the SOM samples to cycles in relative humidity (RH) between <2% RH and ∼95% RH, we observe reversible changes in the SFG signal intensity across the entire spectral range surveyed for a polarization combination probing components of the vibrational transition dipole moments that are oriented parallel to the plane of incidence, but no signal intensity changes for any other polarization combination investigated. These results support the notion that the C–H oscillators at the surfaces of α-pinene-derived SOM deposited on CaF₂ windows shift back and forth between two different molecular orientation distributions as the RH is lowered (more ordered) or raised (less ordered). The findings thus point toward the presence of a reversible surface switch for hindering (more ordered, <2%RH) and promoting (less ordered, ∼95%RH) exchange between the aerosol particle phase and its gas phase

Accurate Line Shapes from Sub‑1 cm^–1 Resolution Sum Frequency Generation Vibrational Spectroscopy of α‑Pinene at Room Temperature

Despite the importance of terpenes in biology, the environment, and catalysis, their vibrational spectra remain unassigned. Here, we present subwavenumber high-resolution broad-band sum frequency generation (HR-BB-SFG) spectra of the common terpene (+)-α-pinene that reveal 10 peaks in the C–H stretching region at room temperature. The high spectral resolution resulted in spectra with more and better resolved spectral features than those of the Fourier transform infrared, femtosecond stimulated Raman spectra in the bulk condensed phase and those of the conventional BB-SFG and scanning SFG spectroscopy of the same molecule on a surface. Experiment and simulation show the spectral line shapes with HR-BB-SFG to be accurate. Homogeneous vibrational decoherence lifetimes of up to 1.7 ps are assigned to specific oscillators and compare favorably to lifetimes computed from density functional tight binding molecular dynamics calculations. Phase-resolved spectra provided their orientational information. We propose the new spectroscopy as an attractive alternative to time domain vibrational spectroscopy or heterodyne detection schemes for studying vibrational energy relaxation and vibrational coherences in molecules at molecular surfaces or interfaces