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The angular distribution of neutral poly-alanine molecules produced in a matrix-assisted laser desorption/ionization process

Abstract:

The angular distribution of desorbed neutral and ionized polyalanine molecules, initially embedded in a matrix of 2,5-dihydroxybenzoic acid, has been measured for the first time. The target was irradiated with a pulsed ultraviolet laser beam (355 nm) incident at an angle of 26°with respect to the target normal. The desorbed molecules were collected on silica plates in a hemispherical array and the amount of collected material on each plate was determined using a standard amino acid analysis technique. Alanine polypeptides (14-70 residues) were used as analyte in order to increase the sensitivity of the amino acid analysis of the collector plates. In order to desorb more molecules, the laser fluence used was about three to ten times higher than in normal MALDI applications. In the plane encompassing the laser beam and the surface normal, the distribution is shifted about 13°towards the incident laser direction, whereas a nearly symmetric angular distribution is observed in the perpendicular plane. The relevance of these results for desorption models is discussed in terms of a recent analysis.