33 research outputs found
Secondary Structures of Ubiquitin Ions Soft-Landed onto Self-Assembled Monolayer Surfaces
The secondary structures of multiply
charged ubiquitin ions soft-landed onto self-assembled monolayer (SAM)
surfaces were studied using in situ infrared reflection–absorption
spectroscopy (IRRAS). Two charge states of ubiquitin, 5+ and 13+,
were mass selected separately from a mixture of different charge states
produced by electrospray ionization (ESI). The low 5+ charge state
represents a nativelike folded state of ubiquitin, while the high
13+ charge state assumes an extended, almost linear conformation.
Each of the two charge states was soft-landed onto a CH<sub>3</sub>- and COOH-terminated SAM of alkanethiols on gold (HSAM and COOH-SAM).
HSAM is a hydrophobic surface known to stabilize helical conformations
of soft-landed protonated peptides, whereas COOH-SAM is a hydrophilic
surface that preferentially stabilizes β-sheet conformations.
IRRAS spectra of the soft-landed ubiquitin ions were acquired as a
function of time during and after ion soft-landing. Similar to smaller
peptide ions, helical conformations of ubiquitin are found to be more
abundant on HSAM, while the relative abundance of β-sheet conformations
increases on COOH-SAM. The initial charge state of ubiquitin also
has a pronounced effect on its conformation on the surface. Specifically,
on both surfaces, a higher relative abundance of helical conformations
and a lower relative abundance of β-sheet conformations are
observed for the 13+ charge state compared to the 5+ charge state.
Time-resolved experiments indicate that the α-helical band in
the spectrum of the 13+ charge state slowly increases with time on
the HSAM surface and decreases in the spectrum of the 13+ charge state
on COOH-SAM. These results further support the preference of the hydrophobic
HSAM surface toward helical conformations and demonstrate that soft-landed
protein ions may undergo slow conformational changes during and after
deposition
Quantitative Extraction and Mass Spectrometry Analysis at a Single-Cell Level
Quantitative live
cell mass spectrometry analysis at a subcellular
level requires the precisely controlled extraction of subpicoliter
volumes of material from the cell, sensitive analysis of the extracted
analytes, and their accurate quantification without prior separation.
In this study, we demonstrate that localized electroosmotic extraction
provides a direct path to addressing this challenge. Specifically,
we demonstrate quantitative mass spectrometry analysis of biomolecules
in picoliter volumes extracted from live cells. Electroosmotic extraction
was performed using two electrodes and a finely pulled nanopipette
with tip diameter of <1 μm containing a hydrophobic electrolyte
compatible with mass spectrometry analysis. The electroosmotic drag
was used to drive analytes out of the cell into the nanopipette. Analyte
molecules extracted both from solutions and cell samples were analyzed
using nanoelectrospray ionization (nanoESI) directly from the nanopipette
into a mass spectrometer. More than 50 metabolites including sugars
and flavonoids were detected in positive mode in 2−5 pL volumes
of the cytoplasmic material extracted from Allium cepa. Quantification of the extracted glucose was performed using sequential
extraction of a known volume of the aqueous solution containing glucose-<i>d</i><sub>2</sub> standard of known concentration. We found
that the ratio of the signal of glucose to glucose-<i>d</i><sub>2</sub> increased linearly with glucose concentration. This
observation indicates that the approach developed in this study enables
quantitative analysis of small volumes of metabolites extracted from
cells. Furthermore, we observed efficient separation of hydrophilic
and hydrophobic analytes through partitioning into the aqueous and
hydrophobic electrolyte phase, respectively, which provides additional
important information on the molecular properties of extracted metabolites
Shotgun Approach for Quantitative Imaging of Phospholipids Using Nanospray Desorption Electrospray Ionization Mass Spectrometry
Mass spectrometry imaging (MSI) has
been extensively used for determining
spatial distributions of molecules in biological samples, and there
is increasing interest in using MSI for quantification. Nanospray
desorption electrospray ionization (nano-DESI) is an ambient MSI technique
where a solvent is used for localized extraction of molecules followed
by nanoelectrospray ionization. Doping the nano-DESI solvent with
carefully selected standards enables online quantification during
MSI experiments. In this proof-of-principle study, we demonstrate
that
this quantification approach can be extended to provide shotgun-like
quantification of phospholipids in thin brain tissue sections. Specifically,
two phosphatidylcholine (PC) standards were added to the nano-DESI
solvent for simultaneous imaging and quantification of 22 endogenous
PC species observed in nano-DESI MSI. Furthermore, by combining the
quantitative data obtained in the individual pixels, we demonstrate
quantification of these PC species in seven different regions of a
rat brain tissue section
Quantitative Extraction and Mass Spectrometry Analysis at a Single-Cell Level
Quantitative live
cell mass spectrometry analysis at a subcellular
level requires the precisely controlled extraction of subpicoliter
volumes of material from the cell, sensitive analysis of the extracted
analytes, and their accurate quantification without prior separation.
In this study, we demonstrate that localized electroosmotic extraction
provides a direct path to addressing this challenge. Specifically,
we demonstrate quantitative mass spectrometry analysis of biomolecules
in picoliter volumes extracted from live cells. Electroosmotic extraction
was performed using two electrodes and a finely pulled nanopipette
with tip diameter of <1 μm containing a hydrophobic electrolyte
compatible with mass spectrometry analysis. The electroosmotic drag
was used to drive analytes out of the cell into the nanopipette. Analyte
molecules extracted both from solutions and cell samples were analyzed
using nanoelectrospray ionization (nanoESI) directly from the nanopipette
into a mass spectrometer. More than 50 metabolites including sugars
and flavonoids were detected in positive mode in 2−5 pL volumes
of the cytoplasmic material extracted from Allium cepa. Quantification of the extracted glucose was performed using sequential
extraction of a known volume of the aqueous solution containing glucose-<i>d</i><sub>2</sub> standard of known concentration. We found
that the ratio of the signal of glucose to glucose-<i>d</i><sub>2</sub> increased linearly with glucose concentration. This
observation indicates that the approach developed in this study enables
quantitative analysis of small volumes of metabolites extracted from
cells. Furthermore, we observed efficient separation of hydrophilic
and hydrophobic analytes through partitioning into the aqueous and
hydrophobic electrolyte phase, respectively, which provides additional
important information on the molecular properties of extracted metabolites
Quantitative Extraction and Mass Spectrometry Analysis at a Single-Cell Level
Quantitative live
cell mass spectrometry analysis at a subcellular
level requires the precisely controlled extraction of subpicoliter
volumes of material from the cell, sensitive analysis of the extracted
analytes, and their accurate quantification without prior separation.
In this study, we demonstrate that localized electroosmotic extraction
provides a direct path to addressing this challenge. Specifically,
we demonstrate quantitative mass spectrometry analysis of biomolecules
in picoliter volumes extracted from live cells. Electroosmotic extraction
was performed using two electrodes and a finely pulled nanopipette
with tip diameter of <1 μm containing a hydrophobic electrolyte
compatible with mass spectrometry analysis. The electroosmotic drag
was used to drive analytes out of the cell into the nanopipette. Analyte
molecules extracted both from solutions and cell samples were analyzed
using nanoelectrospray ionization (nanoESI) directly from the nanopipette
into a mass spectrometer. More than 50 metabolites including sugars
and flavonoids were detected in positive mode in 2−5 pL volumes
of the cytoplasmic material extracted from Allium cepa. Quantification of the extracted glucose was performed using sequential
extraction of a known volume of the aqueous solution containing glucose-<i>d</i><sub>2</sub> standard of known concentration. We found
that the ratio of the signal of glucose to glucose-<i>d</i><sub>2</sub> increased linearly with glucose concentration. This
observation indicates that the approach developed in this study enables
quantitative analysis of small volumes of metabolites extracted from
cells. Furthermore, we observed efficient separation of hydrophilic
and hydrophobic analytes through partitioning into the aqueous and
hydrophobic electrolyte phase, respectively, which provides additional
important information on the molecular properties of extracted metabolites
Revealing Brown Carbon Chromophores Produced in Reactions of Methylglyoxal with Ammonium Sulfate
Atmospheric
brown carbon (BrC) is an important contributor to light
absorption and climate forcing by aerosols. Reactions between small
water-soluble carbonyls and ammonia or amines have been identified
as one of the potential pathways of BrC formation. However, detailed
chemical characterization of BrC chromophores has been challenging
and their formation mechanisms are still poorly understood. Understanding
BrC formation is impeded by the lack of suitable methods which can
unravel the variability and complexity of BrC mixtures. This study
applies high performance liquid chromatography (HPLC) coupled to photodiode
array (PDA) detector and high resolution mass spectrometry (HRMS)
to investigate optical properties and chemical composition of individual
BrC components produced through reactions of methylglyoxal (MG) and
ammonium sulfate (AS), both of which are abundant in the atmospheric
environment. A direct relationship between optical properties and
chemical composition of 30 major BrC chromophores is established.
Nearly all of these chromophores are nitrogen-containing compounds
that account for >70% of the overall light absorption by the MG+AS
system in the 300–500 nm range. These results suggest that
reduced-nitrogen organic compounds formed in reactions between atmospheric
carbonyls and ammonia/amines are important BrC chromophores. It is
also demonstrated that improved separation of BrC chromophores by
HPLC will significantly advance understanding of BrC chemistry
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High-Resolution Mass Spectrometry and Molecular Characterization of Aqueous Photochemistry Products of Common Types of Secondary Organic Aerosols
This work presents a systematic investigation
of the molecular
level composition and the extent of aqueous photochemical processing
in different types of secondary organic aerosol (SOA) from biogenic
and anthropogenic precursors including α-pinene, β-pinene,
β-myrcene, d-limonene, α-humulene, 1,3,5-trimethylbenzene,
and guaiacol, oxidized by ozone (to simulate a remote atmosphere)
or by OH in the presence of NO<sub><i>x</i></sub> (to simulate
an urban atmosphere). Chamber- and flow-tube-generated SOA samples
were collected, extracted in a methanol/water solution, and photolyzed
for 1 h under identical irradiation conditions. In these experiments,
the irradiation was equivalent to about 3–8 h of exposure to
the sun in its zenith. The molecular level composition of the dissolved
SOA was probed before and after photolysis with direct-infusion electrospray
ionization high-resolution mass spectrometry (ESI-HR-MS). The mass
spectra of unphotolyzed SOA generated by ozone oxidation of monoterpenes
showed qualitatively similar features and contained largely overlapping
subsets of identified compounds. The mass spectra of OH/NO<sub><i>x</i></sub>-generated SOA had more unique visual appearance
and indicated a lower extent of product overlap. Furthermore, the
fraction of nitrogen-containing species (organonitrates and nitroaromatics)
was highly sensitive to the SOA precursor. These observations suggest
that attribution of high-resolution mass spectra in field SOA samples
to specific SOA precursors should be more straightforward under OH/NO<sub><i>x</i></sub> oxidation conditions compared to the ozone-driven
oxidation. Comparison of the SOA constituents before and after photolysis
showed the tendency to reduce the average number of atoms in the SOA
compounds without a significant effect on the overall O/C and H/C
ratios. SOA prepared by OH/NO<sub><i>x</i></sub> photooxidation
of 1,3,5-trimethylbenzene and guaiacol were more resilient to photolysis
despite being the most light-absorbing. The composition of SOA prepared
by ozonolysis of monoterpenes changed more significantly as a result
of the photolysis. The results indicate that aqueous photolysis of
dissolved SOA compounds in cloud/fog water can occur in various types
of SOA, and on atmospherically relevant time scales. However, the
extent of the photolysis-driven change in molecular composition depends
on the specific type of SOA
Coverage-Dependent Charge Reduction of Cationic Gold Clusters on Surfaces Prepared Using Soft Landing of Mass-Selected Ions
The ionic charge state of monodisperse multiply charged
cationic
gold clusters on surfaces may be controlled by selecting the coverage
of mass-selected ions soft landed onto a substrate. Polydisperse diphosphine-capped
gold clusters were synthesized in solution and introduced into the
gas phase by electrospray ionization. Mass selection was employed
to isolate a multiply charged cationic cluster species (Au<sub>11</sub>L<sub>5</sub><sup>3+</sup>, <i>m</i>/<i>z</i> = 1409, L = 1,3-bisÂ(diphenylphosphino)Âpropane) which was delivered
to the surfaces of four different self-assembled monolayers on gold
(SAMs) at controlled coverages of 10<sup>11</sup> and 10<sup>12</sup> clusters. Employing the spatial profiling capabilities of <i>in situ</i> time-of-flight secondary ion mass spectrometry (TOF-SIMS),
it is shown that, in addition to the chemical functionality of the
monolayer (as demonstrated previously: ACS Nano 2012, 6, 573), the coverage of cationic gold clusters on
the surface may be used to control the relative abundance of different
charge states of the soft landed multiply charged clusters. In the
case of a 1<i>H</i>,1<i>H</i>,2<i>H</i>,2<i>H</i>-perfluorodecanethiol monolayer (FSAM) almost
complete retention of charge by the deposited Au<sub>11</sub>L<sub>5</sub><sup>3+</sup> clusters was observed at a lower coverage of
10<sup>11</sup> clusters. In contrast, at a higher coverage of 10<sup>12</sup> clusters, pronounced reduction of charge to Au<sub>11</sub>L<sub>5</sub><sup>2+</sup> and Au<sub>11</sub>L<sub>5</sub><sup>+</sup> was observed on the FSAM. When soft landed onto 16- and 11-mercaptohexadecanoic
acid surfaces on gold (16,11-COOH-SAMs), the mass-selected Au<sub>11</sub>L<sub>5</sub><sup>3+</sup> clusters exhibited partial reduction
of charge to Au<sub>11</sub>L<sub>5</sub><sup>2+</sup> at lower coverage
and additional reduction of charge to both Au<sub>11</sub>L<sub>5</sub><sup>2+</sup> and Au<sub>11</sub>L<sub>5</sub><sup>+</sup> at higher
coverage. On the surface of the 1-dodecanethiol (HSAM) monolayer,
the most abundant charge state was found to be Au<sub>11</sub>L<sub>5</sub><sup>2+</sup> at lower coverage and Au<sub>11</sub>L<sub>5</sub><sup>+</sup> at higher coverage, respectively. A coverage-dependent
electron tunneling mechanism is proposed to account for the observed
reduction of charge of mass-selected multiply charged gold clusters
soft landed on SAMs. The results demonstrate that one of the critical
parameters that influence the chemical and physical properties of
supported metal clusters, ionic charge state, may be controlled by
selecting the coverage of charged species soft landed onto surfaces
Excitation–Emission Spectra and Fluorescence Quantum Yields for Fresh and Aged Biogenic Secondary Organic Aerosols
Certain biogenic secondary organic
aerosols (SOA) become absorbent
and fluorescent when exposed to reduced nitrogen compounds such as
ammonia, amines, and their salts. Fluorescent SOA may potentially
be mistaken for biological particles by detection methods relying
on fluorescence. This work quantifies the spectral distribution and
effective quantum yields of fluorescence of water-soluble SOA generated
from two monoterpenes, limonene and α-pinene, and two different
oxidants, ozone (O<sub>3</sub>) and hydroxyl radical (OH). The SOA
was generated in a smog chamber, collected on substrates, and aged
by exposure to ∼100 ppb ammonia in air saturated with water
vapor. Absorption and excitation–emission matrix (EEM) spectra
of aqueous extracts of aged and control SOA samples were measured,
and the effective absorption coefficients and fluorescence quantum
yields (∼0.005 for 349 nm excitation) were determined from
the data. The strongest fluorescence for the limonene-derived SOA
was observed for λ<sub>excitation</sub> = 420 ± 50 nm and
λ<sub>emission</sub> = 475 ± 38 nm. The window of the strongest
fluorescence shifted to λ<sub>excitation</sub> = 320 ±
25 nm and λ<sub>emission</sub> = 425 ± 38 nm for the α-pinene-derived
SOA. Both regions overlap with the EEM spectra of some of the fluorophores
found in primary biological aerosols. Despite the low quantum yield,
the aged SOA particles may have sufficient fluorescence intensities
to interfere with the fluorescence detection of common bioaerosols
Applications of High-Resolution Electrospray Ionization Mass Spectrometry to Measurements of Average Oxygen to Carbon Ratios in Secondary Organic Aerosols
The applicability of high-resolution electrospray ionization
mass
spectrometry (HR ESI-MS) to measurements of the average oxygen to
carbon ratio (O/C) in secondary organic aerosols (SOAs) was investigated.
Solutions with known average O/C containing up to 10 standard compounds
representative of low-molecular-weight SOA constituents were analyzed
and the corresponding electrospray ionization efficiencies were quantified.
The assumption of equal ionization efficiency commonly used in estimating
O/C ratios of SOAs was found to be reasonably accurate. We found that
the accuracy of the measured O/C ratios increases by averaging the
values obtained from both the posive and negative modes. A correlation
was found between the ratio of the ionization efficiencies in the
positive (+) and negative (−) ESI modes and the octanol–water
partition constant and, more importantly, the compound’s O/C.
To demonstrate the utility of this correlation for estimating average
O/C values of unknown mixtures, we analyzed the ESI (+) and ESI (−)
data for SOAs produced by oxidation of limonene and isoprene and compared
them online to O/C measurements using an aerosol mass spectrometer
(AMS). This work demonstrates that the accuracy of the HR ESI-MS method
is comparable to that of the AMS with the added benefit of molecular
identification of the aerosol constituents