Alternative pathway for atmospheric particles growth.

Credible climate change predictions require reliable fundamental scientific knowledge of the underlying processes. Despite extensive observational data accumulated to date, atmospheric aerosols still pose key uncertainties in the understanding of Earth's radiative balance due to direct interaction with radiation and because they modify clouds' properties. Specifically, major gaps exist in the understanding of the physicochemical pathways that lead to aerosol growth in the atmosphere and to changes in their properties while in the atmosphere.

Role of interfacial water in the heterogeneous uptake of glyoxal by mixed glycine and ammonium sulfate aerosols.

This study focuses on the heterogeneous reactions of gas phase glyoxal with aerosols of glycine, the most abundant amino acid in atmospheric aerosols, as well as with a mixture of glycine and ammonium sulfate (AS) at a molar ratio of 1:100 (glycine-AS 1:100). Aerosols were exposed to varying relative humidity (RH) conditions in the presence of gas phase glyoxal for ∼1 h, followed by drying and efflorescence. The changes in size, chemical composition, and optical properties were consequently measured.

Isolated gramicidin peptides probed by IR spectroscopy.

We report double-resonant IR/UV ion-dip spectroscopy of neutral gramicidin peptides in the gas phase. The IR spectra of gramicidin A and C, recorded in both the 1000 cm(-1) to 1800 cm(-1) and the 2700 to 3750 cm(-1) region, allow structural analysis. By studying this broad IR range, various local intramolecular interactions are probed, and complementary IR modes can be accessed.

Conformational structure of tyrosine, tyrosyl-glycine, and tyrosyl-glycyl-glycine by double resonance spectroscopy.

We investigated the variation in conformation for the amino acid tyrosine (Y), alone and in the small peptides tyrosine-glycine (YG) and tyrosine-glycine-glycine (YGG), in the gas phase by using UV-UV and IR-UV double resonance spectroscopy and density functional theory calculations. For tyrosine we found seven different conformations, for YG we found four different conformations, and for YGG we found three different conformations. As the peptides get larger, we observe fewer stable conformers, despite the increasing complexity and number of degrees of freedom.

Changes in the optical properties of benzo[a]pyrene-coated aerosols upon heterogeneous reactions with NO2 and NO3.

Chemical reactions can alter the chemical, physical, and optical properties of aerosols. It has been postulated that nitration of aerosols can account for atmospheric absorbance over urban areas. To study this potentially important process, the change in optical properties of laboratory-generated benzo[a]pyrene (BaP)-coated aerosols following exposure to NO(2) and NO(3) was investigated at 355 nm and 532 nm by three aerosol analysis techniques.

Guanine-aspartic acid interactions probed with IR-UV resonance spectroscopy.

Double resonance spectroscopy of clusters of guanine with aspartic acid reveals geometries similar to patterns exhibited in DNA base pairs. In the spectral region of 32,800 cm(-1) to 35,500 cm(-1) we observe five isomers of guanine-aspartic acid clusters and assign their structures based on IR-UV hole-burning spectra and wave function theory calculations at the MP2/cc-pVDZ and MP2/cc-pVTZ levels. The calculations employed both harmonic and one-dimensional scan anharmonic approximations.

Interaction of internally mixed aerosols with light.

Atmospheric aerosols scatter and absorb solar radiation leading to variable effects on Earth's radiative balance. Aerosols individually comprising mixtures of different components ("internally mixed") interact differently with light than mixtures of aerosols, each comprising a different single component ("externally mixed"), even if the relative fractions of the different components are equal. In climate models, the optical properties of internally mixed aerosols are generally calculated by using electromagnetic "mixing rules", which average the refractive indices of the individual components in different proportions, or by using coated-sphere Mie scattering codes, which solve the full light scattering problem assuming that the components are divided into two distinct layers.

Effect of intrinsic organic carbon on the optical properties of fresh diesel soot.

This study focuses on the retrieval of the normalized mass absorption cross section (MAC) of soot using theoretical calculations that incorporate new measurements of the optical properties of organic carbon (OC) intrinsic to fresh diesel soot. Intrinsic OC was extracted by water and an organic solvent, and the complex refractive index of the extracted OC was derived at 532 and 355-nm wavelengths using cavity ring-down aerosol spectrometry. The extracted OC was found to absorb weakly in the visible wavelengths and moderately at blue wavelengths.

Isomer discrimination of polycyclic aromatic hydrocarbons in the Murchison meteorite by resonant ionization.

We have used two-color resonant two-photon ionization (2C-R2PI) mass spectrometry to discriminate between isomers of polycyclic aromatic hydrocarbons in the Murchison meteorite. We measured the 2C-R2PI spectra of chrysene and triphenylene seeded in a supersonic jet by laser desorption. Since each isomer differs in its R2PI spectrum, we can distinguish between isomers using wavelength dependent ionization and mass spectrometry.

Potential-energy and free-energy surfaces of glycyl-phenylalanyl-alanine (GFA) tripeptide: experiment and theory.

The free-energy surface (FES) of glycyl-phenylalanyl-alanine (GFA) tripeptide was explored by molecular dynamics (MD) simulations in combination with high-level correlated ab initio quantum chemical calculations and metadynamics. Both the MD and metadynamics employed the tight-binding DFT-D method instead of the AMBER force field, which yielded inaccurate results. We classified the minima localised in the FESs as follows: a) the backbone-conformational arrangement; and b) the existence of a COOH.

IR-UV double resonance spectroscopy of xanthine.

We present resonant two-photon ionization (R2PI), UV-UV, and IR-UV double resonance spectra of xanthine seeded in a supersonic jet by laser desorption. We show that there is only one tautomer of xanthine which absorbs in the wavelength range of 36 700 to 37 700 cm(-1). The IR-UV double resonance spectrum shows three strong bands at 3444, 3485, and 3501 cm(-1), all of which we assign as N-H stretching vibrations.

The mid-IR spectra of 9-ethyl guanine, guanosine, and 2-deoxyguanosine.

We present the mid-IR (400-1800 cm(-1)) spectra of 9-ethyl guanine, guanosine, and 2-deoxyguanosine measured by IR-UV double-resonance spectroscopy. We compare the recorded mid-IR spectra with the spectra of the most stable structures obtained from RI-MP2 and RI-DFT-D calculations. The results confirm the enol form for all structures and demonstrate the efficacy of a new approach to DFT calculations that includes dispersion interactions.

Vibrational spectroscopy of the G...C base pair: experiment, harmonic and anharmonic calculations, and the nature of the anharmonic couplings.

The results of harmonic and anharmonic frequency calculations on a guanine-cytosine complex with an enolic structure (a tautomeric form with cytosine in the enol form and with a hydrogen at the 7-position on guanine) are presented and compared to gas-phase IR-UV double resonance spectral data. Harmonic frequencies were obtained at the RI-MP2/cc-pVDZ, RI-MP2/TZVPP, and semiempirical PM3 levels of electronic structure theory. Anharmonic frequencies were obtained by the CC-VSCF method with improved PM3 potential surfaces; the improved PM3 potential surfaces are obtained from standard PM3 theory by coordinate scaling such that the improved PM3 harmonic frequencies are the same as those computed at the RI-MP2/cc-pVDZ level.

IR-UV double resonance spectroscopy of guanine-H2O clusters.

We present the IR-UV double resonance spectrum of guanine monohydrate in the region 3100 cm(-1) to 3800 cm(-1) along with the energies and frequencies of these structures calculated at the non-empirical correlated ab initio RI-MP2/cc-pVDZ level. We assign the structures of guanine-water clusters by comparing the experimental spectra with the ab initio calculations and with the IR spectra of the bare guanine monomer. We find two clusters with guanine in the enol-amino tautomeric form and one structure with guanine in the keto-amino form.

An in situ silver cationization method for hydrocarbon mass spectrometry.

We have developed a novel cationization method for the analysis of long-chain hydrocarbons via UV laser desorption mass spectrometry. In this technique we electrospray a thin coating of AgNO3 over a sample and perform UV laser desorption to produce Ag+ cationization of sample molecules. Use of this technique in our microscope/TOF-MS allows us to determine the spatial distribution of the species we detect in the sample.

Microhydration of Guanine base pairs.

We report spectroscopy of clusters of guanine base pairs with one and two water molecules. We recorded the vibronic spectra of the mass-selected GG(H2O) and GG(H2O)2 clusters using resonant two photon ionization (R2PI) and we used IR-UV double resonance spectroscopy to obtain ground state IR spectra of these clusters. We found that a single water molecule stabilizes one of two structures we had previously found for guanine dimers.

Photochemical selectivity in guanine-cytosine base-pair structures.

Prebiotic chemistry presumably took place before formation of an oxygen-rich atmosphere and thus under conditions of intense short wavelength UV irradiation. Therefore, the UV photochemical stability of the molecular building blocks of life may have been an important selective factor in determining the eventual chemical makeup of critical biomolecules. To investigate the role of UV irradiation in base-pairing we have studied guanine (G) and cytosine (C) base pairs in the absence of the RNA backbone.