7 research outputs found
Unanticipated Stickiness of Ī±āPinene
The
adsorption of Ī±-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 Ī±-pinene bound on amorphous SiO<sub>2</sub> 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 (40ā60%)
irreversible over days at 294ā342 K and 1 ATM total pressure
of helium, which is supported by molecular dynamics (MD) simulations.
The distribution of Ī±-pinene orientation remains invariant with
temperature and partial pressure of Ī±-pinene. Using the RedlichāPeterson
adsorption model in conjunction with a vanāt Hoff analysis
of adsorption isotherms recorded for up to 2.6 Torr Ī±-pinene
in 1 ATM total pressure of helium, we obtain Ī<i>S</i>Ā°<sub>ads</sub>, Ī<i>H</i>Ā°<sub>ads</sub>, and Ī<i>G</i>Ā°<sub>ads</sub> values of ā57
(Ā±7) J mol<sup>ā1</sup> K<sup>ā1</sup>, ā39
(Ā±2) kJ mol<sup>ā1</sup>, and ā22 (Ā±5) kJ
mol<sup>ā1</sup>, respectively, associated with the reversibly
bound population of Ī±-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 CāTerminal 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 Ī±āPinene by Isotope Editing: One Down, Seventy-One To Go
This study aims to reliably assign
the vibrational sum frequency
generation (SFG) spectrum of Ī±-pinene at the vapor/solid interface
using a method involving deuteration of various methyl groups. The
synthesis of five deuterated isotopologues of Ī±-pinene is presented
to determine the impact that removing contributions from methyl group
CāH oscillators has on its SFG response. 0.6 cm<sup>ā1</sup> resolution SFG spectra of these isotopologues show varying degrees
of differences in the CāH stretching region when compared to
the SFG response of unlabeled Ī±-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<sup>ā1</sup> peak to the
vinyl methyl symmetric stretch. Furthermore, upon removing the vinyl
methyl group entirely by synthesizing apopinene, the steric influence
of the unlabeled C<sub>9</sub>H<sub>14</sub> fragment on the SFG response
of Ī±-pinene SFG can be readily observed. The work presented
here brings us one step closer to understanding the vibrational spectroscopy
of Ī±-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>-Ī²-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>-Ī²-IEPOX and one of its isotopologues.
Our joined SSD/TAE analysis shows that while the twist angle of the
methyl <i>C</i><sub>3<i>v</i></sub> symmetry axis
of <i>trans</i>-Ī²-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, ĻB97X-D, PBE0, and B97-1 in combination with a modest
basis set, such as 6-311GĀ(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>-Ī²-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 Ī±āPinene on Silica
A rotationally
fluid state of Ī±-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 CāH stretching
region of Ī±-pinene and identify its bridge methylene group on
the four-membered ring (ā<sup>Ī²</sup>CH<sub>2</sub>ā)
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 Ī±-pinene. The preferred orientations of Ī±-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 <sup>Ī²</sup>CH<sub>2</sub> group points
ātowardsā or āaway fromā the surface.
Classical molecular dynamics simulations further provide rotational
diffusion constants of 49 Ā± 1 ps and 2580 Ā± 60 ps, which
are attributed to two broad types of adsorption modes on silica. The
reported findings are particularly relevant to the exposure of Ī±-pinene
to primary oxidants in heterogeneous catalytic pathways that exploit
Ī±-pinene as a sustainable feedstock for fine chemicals and polymers
On Surface Order and Disorder of Ī±āPinene-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<sup>ā1</sup> resolution vibrational sum frequency
generation (SFG) spectroscopy to detect CāH oscillators at
the surfaces of secondary organic material (SOM) prepared from the
ozonolysis of Ī±-pinene at Harvard University and at the University
of California, Irvine, that were subsequently collected on Teflon
filters as well as CaF<sub>2</sub> windows using electrostatic deposition.
We find both samples yield comparable SFG spectra featuring an intense
peak at 2940 cm<sup>ā1</sup> that are independent of spectral
resolution and location or method of preparation. We hypothesize that
the SFG spectra are due to surface-active CāH oscillators associated
with the four-membered ring motif of Ī±-pinene, which produces
an unresolvable spectral continuum of approximately 50 cm<sup>ā1</sup> width reminiscent of the similar, albeit much broader, OāH
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 ā¼95% 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
CāH oscillators at the surfaces of Ī±-pinene-derived SOM
deposited on CaF<sub>2</sub> 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, ā¼95%RH) exchange between
the aerosol particle phase and its gas phase
Accurate Line Shapes from Subā1 cm<sup>ā1</sup> Resolution Sum Frequency Generation Vibrational Spectroscopy of Ī±āPinene 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 (+)-Ī±-pinene
that reveal 10 peaks in the CāH 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