Particle formation by infrared laser ablation of glycerol: implications for ion formation

The quantity and size distribution of micrometer-sized particles ejected from thin films of glycerol were measured using light scattering particle sizing. Thin glycerol films were irradiated at atmospheric pressure with an infrared optical parametric oscillator at wavelengths between 2.95 and 3.1 μm. Particulate material resulting from the ablation was sampled directly into a particle-sizing instrument and particles with diameters greater than 500 nm were detected and sized by light scattering. The fluence threshold for particle formation was between 2000 and 3000 J/m2 for all laser wavelengths. At threshold, fewer than 100 particles/cm3 were detected and this value increased to several thousand particles/cm3 at twice the threshold fluence. The average size of the coarse particles ranged from 900 nm to 1.6 μm at threshold and decreased by 10-20% at twice the threshold fluence. The coarse particle formation observations were compared with ion formation behavior in matrix-assisted laser desorption ionization and interpreted in terms of a photomechanical mechanism for material ablation and ion formation. Copyright © 2006 John Wiley & Sons, Ltd

Halide anion binding to Gly3, Ala3 and Leu3

The structures of Gly3·X-​, Ala3·X-​ and Leu3·X-​ (X = Cl, Br and I) are investigated with computational chem. and IR multiple-​photon dissocn. (IRMPD) spectroscopy. Low-​energy structures calcd. at the B3LYP​/6-​31+G** level of theory (or with the CRENBL basis set and effective core potential implemented for Br and I) for these complexes have similar structural motifs in which the halide anion binds to the peptide via hydrogen bonds at amide, amine, and​/or carboxylic acid H atoms. The IRMPD spectra do not depend significantly on anion identity. Comparisons between measured spectra and those calcd. for low-​energy structures of each of the chloridated complexes indicate that all three complexes have similar binding motifs. These results suggest that the size of the alkyl side chain does not significantly influence how halide anions bind to these peptides. The coordination geometries of Gly3·X-​ and Ala3·X-​ are "inverted" compared to those for the Na+ cationized peptides, where the peptides coordinate to Na+ via lone pair electrons of O and N atoms. The "inversion" in structures between Ala3·Na+ and Ala3·X-​ results in greater steric hindrance for some geometries of the latter. There is a subtle blue shift in the C-​terminal C=O stretch frequency with increasing halide anion size for each peptide, consistent with contributions from Stark and charge transfer effects. In contrast, the N-​H bends red shift with increasing halide anion size, which can only be attributed to the charge transfer effect. This is the first report of IR spectra of peptides complexed with anions, and these results provide insights into anion-​peptide binding interactions