Adaptive aggregation of peptide model systems.

Jet-cooled infrared spectra of acetylated glycine, alanine, and dialanine esters and their dimers are reported in the amide A and amide I-III regions. They serve as particularly simple peptide aggregation models and are found to prefer a single backbone conformation in the dimer that is different from the most stable monomer backbone conformation. In the case of alanine, evidence for topology-changing chirality discrimination upon dimer formation is found. The jet spectroscopic results are compared to gas phase spectra and quantum chemical calculations. They provide reliable benchmarks for the evaluation of the latter in the field of peptide interactions.

CO2 laser ionization of acoustically levitated droplets.

For many analytical purposes, direct laser ionization of liquids is desirable. Several studies on supported droplets, free liquid jets, and ballistically dispensed microdroplets have been conducted, yet detailed knowledge of the underlying mechanistics in ion formation is still missing. This contribution introduces a simple combination of IR-MALDI mass spectrometry and an acoustical levitation device for contactless confinement of the liquid sample. The homebuilt ultrasonic levitator supports droplets of several millimeters in diameter. These droplets are vaporized by a carbon dioxide laser in the vicinity of the atmospheric pressure interface of a time of flight mass spectrometer. The evaporation process is studied by high repetition rate shadowgraphy experiments elucidating the ballistic evaporation of the sample and revealing strong confinement of the vapor by the ultrasonic field of the trap. Finally, typical mass spectra for pure glycerol/water matrix and lysine as an analyte are presented with and without the addition of trifluoracetic acid, and the ionization mechanism is briefly discussed. The technique is a promising candidate for a reproducible mass spectrometric detection scheme for the field of microfluidics.

Interaction of levitated ionic liquid droplets with water.

The influence of a humid or dry atmosphere on acoustically levitated ionic liquid droplets was studied by volumetric analysis and vibrational spectroscopy. Imidazolium-based ionic liquids with two types of anions, fluorinated (BF(4) and PF(6)) and alkylsulfate anions, were investigated. The amount of absorbed water was correlated with structural differences of the ionic liquids and analyzed in terms of equilibrium mole fraction as well as absorption rate. The type of anion had the most significant influence on the amount of adsorbed water from the atmosphere. Furthermore, spectral changes in the in situ Raman spectra due to absorbed water were studied for all investigated ionic liquids. For 1-ethyl-3-methylimidazolium ethylsulfate, an exemplary detailed analysis of the intermolecular interactions between cations, anions and water was carried out based on the spectroscopic data. The observed band shifts were explained with a hydrogen bond between the anion and water.

Low temperature infrared spectroscopy study of pyrazinamide: from the isolated monomer to the stable low temperature crystalline phase.

A structural and spectroscopic analysis of the anti-tuberculosis drug pyrazinamide (PZA) was carried out. The PZA molecule was predicted theoretically to possess two conformers differing by internal rotation around the C-C( horizontal lineO) bond, with the E conformer (C(s) symmetry point group; N-C-C horizontal lineO dihedral: 180 degrees ) being ca. 30 kJ mol(-1) more stable than the Z form (C(1) point group; N-C-C horizontal lineO dihedral: ca. +/- 42 degrees ). In consonance with both the large energy difference and low energy barrier between the Z and E conformers, upon isolation in low temperature argon and xenon matrices, only the E form could be observed and characterized spectroscopically. In the argon matrix, this conformer was found to exist in at least three matrix sites, of different stability. In a supersonic jet, besides the monomer (E), the most stable dimer of PZA with two equivalent NH...O horizontal line hydrogen bonds could also be identified. Its spectrum reveals rapid energy flow out of the excited NH stretching mode mediated by one of the heteroatoms in the ring. Finally, the IR spectra of the amorphous solid resulting from fast cooling of the vapor of the compound (initially in the alpha crystalline phase) onto the cold substrate of the cryostat (10 K) and of the crystalline phase resulting from warming the amorphous solid were also recorded and interpreted. The obtained crystalline phase was found to be the thermodynamically most stable delta polymorph of PZA.

Dimer formation in nicotinamide and picolinamide in the gas and condensed phases probed by infrared spectroscopy.

Aggregation of nicotinamide (3-pyridine-carboxamide; NA) and picolinamide (2-pyridine-carboxamide; PA) has been investigated by matrix-isolation, supersonic jet and neat solid state infrared spectroscopy, complemented by DFT(B3LYP)/6-311++G(d,p) calculations. For both compounds, the most stable dimeric structure was shown to be the centrosymmetric dimer where two monomers in their most stable forms establish two NHO[double bond, length as m-dash]C hydrogen bonds. The most stable structures of monomers of NA and PA were characterized in detail experimentally by matrix-isolation spectroscopy and theoretically (at both the DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) levels). For nicotinamide, two conformers were found in the matrices, with ca. 80% of the total population adopting the E form. The monomers and dimers of PA and NA were also investigated by infrared spectroscopy of the studied compounds seeded in supersonic jet expansions. These studies revealed that the constraints on the vibrational dynamics in the PA dimer are different from those in the NA dimer. In the PA dimer, the vibrational energy flow out of the N-H stretching mode was shown to be accelerated substantially by the presence of a secondary intramolecular hydrogen bond. In the glassy state of both compounds, the centrosymmetric dimer seems to be the prevalent structure. In the neat crystalline state (KBr pellet), picolinamide keeps this type of dimeric structure as the constituting unit, whereas nicotinamide molecules assume a different arrangement where one of the NHO[double bond, length as m-dash]C bonds is replaced by an NH...N(ring) bond. The different crystallograpic structures which were formed by the compounds are reflected in the vibrational spectra of the solids. These observations are correlated with the molecular properties of NA and PA, in particular with the greater conformational mobility of NA compared with PA. This is ascribable to the absence in the NA molecule of the intramolecular NH...N((ring)) interaction, which exists in PA.

Elementary peptide motifs in the gas phase: FTIR aggregation study of formamide, acetamide, N-methylformamide, and N-methylacetamide.

Cold, isolated peptide model compounds and their aggregates are generated in pulsed supersonic jet expansions and detected by FTIR spectroscopy in the amide-A region, complemented by amide-I spectra. The most stable, symmetric dimer of formamide is unambiguously assigned in the gas phase for the first time, also by comparison to the analogous acetamide dimer. Efficient quenching of a hot-state Fermi resonance by cooling of the dimers is invoked. As the preferred relative orientation of the C=O and N-H groups in N-methylated formamide and acetamide is trans, these compounds show a fundamentally different dimerization pattern. Their most stable dimers, which would be analogous to those of formamide and acetamide, remain undetected as a consequence of kinetic control in the jet. Accurate benchmark quantities for multidimensional vibrational treatments of these peptide models are derived, and the influence of methyl groups on the N-H stretching dynamics is discussed.