Aqueous suspensions of platinum colloidal nanoparticles of varying size and polydispersity have been produced by ablation of a platinum target with a nanosecond Q-switched Nd:YAG laser. Several laser ...wavelengths (1064, 532, and 266 nm) and stabilizing agents (citrate and polymers PEG, PVA, and PVP) were employed. Laser ablation with the infrared and visible wavelengths leads to spherical amorphous nanoparticles with a bimodal distribution of diameters, featuring a global maximum in the range 5–10 nm and a shoulder extending to 25 nm. Such bimodal distributions plausibly arise from thermal and explosive vaporization mechanisms occurring in different time scales, as proposed in earlier studies. Ultraviolet ablation of Pt at 266 nm, reported here for the first time, produces crystalline nanoparticles of small size (1–4 nm diameter), with a weak onset of larger particles (6–8 nm). The ablation at 266 nm also produced an appreciable yield of large rodlike particles of size ∼10 × 70 nm2, especially in the presence of PEG and PVA. The Pt nanoparticles served to fabricate films capable of assisting the laser desorption ionization (LDI) of a model peptide. The best analytical performance for LDI mass spectrometric detection was obtained with Pt nanoparticles surface-functionalized with citrate and PVA.
Algerian crude oil displays a marked propensity for asphaltene precipitation, leading to solid deposits during extraction, transportation, and storage. The relationship between precipitation and ...chemical composition is unclear; in fact, Algerian crude oil actually features a low asphaltene concentration, despite its relatively large rate of deposit formation. The rationalization of the precipitation process and its remediation should benefit from a molecular characterization of the crude oil. In this study, two unstable asphaltene fractions (A1 and A2) from two different deposits, and two resin crude oil fractions (R1 and R2) from the Hassi-Messaoud Algerian field have been characterized at the molecular level by means of high-resolution mass spectrometry with an Atmospheric Pressure Chemical Ionization (APCI) source. Positively and negatively charged compounds with molecular weights 200–1200 m/z were detected. Several thousand molecular stoichiometries were identified and classified for each sample, in terms of heteroatom content and aromaticity, searching for trends characteristic of the two asphaltenes and of the associated resins. The A2 asphaltene, from a downstream storage tank, displays a higher aromaticity and O-heteroatom content, which correlates with an enhanced aggregation propensity, in comparison to the A1 fraction, collected at the well bore. The resin fractions are found to be abundant in aliphatic hydrocarbons and heteroatomic compounds of moderate aromaticity. The more polar resin fraction, R2, is enriched in N-containing species, with respect to the less polar resin fraction R1, which correlates with the stabilizing function observed in previous works. The results stress the view of crude oil fractions as complex mixtures, rather than in terms of average prototypical compounds, when facing the understanding of asphaltene deposition conditions.
Molecular-weight distributions (MWDs) of asphaltenes extracted from coal and petroleum have been measured in laser desorption/ionization (LDI) mass spectrometric experiments. The dried-droplet and ...solvent-free sample preparation methods are compared. The coal asphaltenes have a relatively narrow MWD (full width ≈ 150 amu) with an average molecular weight of ≈340 amu. The petroleum asphaltenes display a broader MWD (full width ≈ 300 amu) and are heavier on average (≈680 amu). The LDI spectra also provide evidence for the formation of noncovalent clusters of the two types of asphaltenes during the desorption process. Petroleum and coal asphaltenes exhibit aggregation as do large model polycyclic aromatic hydrocarbons (PAHs) with five or more fused rings also included in the study. Smaller PAHs (pyrene) exhibit less aggregation, especially when alkane-chain substituents are incorporated to the molecular structure. This indicates that asphaltenes possess large PAHs and, according to the relatively small molecular weights observed, that there is a preponderance of asphaltene molecules with only a single fused ring system. The coal asphaltenes present a significantly smaller propensity toward aggregation than their crude oil counterparts. This finding, coupled with the fact that (1) alkanes inhibit aggregation in LDI and (2) petroleum asphaltenes possess much more alkane carbon, indicates that coal asphaltenes have smaller PAHs on average than petroleum asphaltenes. This is further corroborated by the stronger ultraviolet absorbance of the coal asphaltenes at wavelengths shorter than 400 nm.
Platinum nanomaterials are the focus of great expectations to expand the reach of modern sensor technology. In this work, we have evaluated the ability of different types of platinum nanoparticles to ...enhance the response of analytical techniques based on optical excitation methods, namely nanoparticle-assisted laser desorption/ionization (NALDI), Raman scattering (SERS), and infrared absorption (SEIRA). Physical (laser ablation) and chemical (redox) methods of synthesis have been employed to produce spherical particles with sizes ranging from a few to tens of nanometers, stabilized with different coating agents. The nanoparticles have then been tested as active substrate materials to enhance the sensitivity of NALDI of a model peptide (angiotensin I) and a synthetic polymer (PEG600) and those of SERS and SEIRA of a common chromophore (rhodamine 6G). A fairly similar overall performance is found for nanoparticles homogeneous in size synthesized by chemical reduction as for heterogeneous sets of particles produced either chemically or by laser ablation in solution. These results indicate that size monodispersity is not crucial to attain enhancement effects. Quantitative differences in the relative NALDI, SERS, and SEIRA responses are attributed to the different roughness and porosity of the nanoparticles. The use of a low weight coating agent for nanoparticle stabilization also leads to a better performance compared to bulkier polymeric coatings.
Crown ethers are well known as modulating agents of protein function and interactions. The action of crown ethers is driven by an alteration of the charged moieties of proteins through the capping of ...cationic amino acid side chains. This study evaluates the conformational features involved in the binding of crown ethers to the side chain of arginine. For this purpose, isolated complexes of methyl guanidinium with 12‐crown‐4 and 18‐crown‐6 are characterized with infrared action vibrational spectroscopy and quantum chemical computations. The conformational landscapes of the two complexes comprise an extensive ensemble of conformations close in energy. In the 12‐crown‐4 complex, the crown ether has the plane of its backbone approximately perpendicular to that of the guanidinium moiety and coordinates to two or three of its NHδ+ bonds. In the 18‐crown‐6 complex, the crown ether backbone is partially folded and tilted with respect to guanidinium and fixes its position in order to facilitate up to a four‐fold coordination in the complex. The access of the complexes to multiple conformations leads to broad band structures in the N−H stretching region of their vibrational spectra.
Vibrational action spectroscopy and quantum‐chemical modelling indicates that the macrocyle 12‐crown‐4 (12c4) coordinates to two or three NHδ+ bonds at different positions around methyl guanidinium, whereas 18‐crown‐6 (18c6) fixes its position in order to facilitate up to a tetradentate coordination in the complex.
The side group of the amino acid arginine is typically in its guanidinium protonated form under physiological conditions and participates in a broad range of ligand binding and charge transfer ...processes of proteins. The recognition of phosphate moieties by guanidinium plays a particularly key role in the interactions of proteins with ATP and nucleic acids. Moreover, it has been recently identified as the driving force for the inhibition of kinase phosphorilation activity by guanidinium derivatives devised as potential anticancer agents. We report on a fundamental investigation of the interactions and coordination arrangements formed by guanidinium with phosphoric, phosphate, and pyrophosphate groups. Action vibrational spectroscopy and ab initio quantum chemical computations are employed to characterize the conformations of benchmark positively charged complexes isolated in an ion trap. The multidentate structure of guanidinium and of the phosphate groups gives rise to a rich conformational landscape with a particular relevance of tweezer–like configurations, where phosphate is effectively trapped by two guanidinium cations. The pyrophosphate complex incorporates a Na+ cation, which serves to compare the interactions associated with the localized versus diffuse charge distributions of the alkali cation and guanidinium, respectively, within a common supramolecular framework.
The complexes formed by crown ethers with hydronium and ammonium cations are of key relevance for the understanding of their supramolecular behavior in protic solvents. In this work, the complexes of ...the 15-crown-5 (15c5) and 18-crown-6 (18c6) ethers with H3O+ and NH4 + and their deuterated variants are investigated under isolated conditions. The study employs infrared multiple photon dissociation (IRMPD) vibrational spectroscopy and DFT B3LYP/6-31++G(d,p) calculations for conformational assignment. The 18c6 ether provides two energetically nearby C 3v conformations with commensurate linear O−H···O and N−H···O bonds. The 15c5 ether ring adopts partially folded asymmetric pyramidal geometries, yielding one shorter linear H bond and two longer non−linear H bonds. Remarkably, an appreciable broadening of the IRMPD vibrational bands is observed for the 15c5−H3O+/D3O+ complexes. This can be interpreted as a signature for partial sharing of the proton (or deuteron) between the water and the crown ether along the linear O−H···O intermolecular H bond, which is indeed particularly short for this complex.
We report on a combined experimental and computational study of the chiral recognition of the amino acid serine in protonated form (l/d-SerH+), by the crown ether ...(all-S)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (S-18c6H4). Infrared and vibrational circular dichroism spectroscopies (IR-VCD) are employed to characterize the chiroptical response of the complexes formed by S-18c6H4 with the l-SerH+ and d-SerH+ enantiomers in dried thin films obtained from aqueous solutions. The study focuses on vibrational modes directly related to the intermolecular hydrogen bonds between the crown ether derivative and serine, responsible for crown–serine binding, namely, the CO and CO stretching modes, and on the COH bending mode, which yield intense IR and VCD signals in the range of wavenumbers 900–2000 cm–1. The experimental spectra are analyzed in combination with a computational structural survey and optimization at different levels of density functional theory. The conformational landscape of the complexes is found to be primarily governed by a bowl-like structure of the crown ether host and a tripodal coordination of the protonated R-NH3 + group of serine with the oxygen atoms of the central ether ring. Additionally, one or two of the carboxylic side groups of the crown ether interact with the −COH and −COOH groups of serine. Chiral selectivity is probed by recording the IR and VCD spectra of dried thin films obtained from aqueous solutions with equimolar concentrations of the two serine enantiomers and the macrocycle. The results demonstrate a marked chiral recognition of l-SerH+ relative to d-SerH+ by the S-18c6H4 substrate, which arises from the favorable host–guest coordination through H-bonds at optimum distances and collinear orientations, also involving a limited distortion of the crown ether backbone.
PDF
