Proton Affinities of Eight Matrices Used for Matrix-assisted Laser Desorption/Ionization

Abstract

Since its introduction by Karas and co-workers in 1987, matrix-assisted laser desorption/ionization (MALDI) has rapidly become a vital tool in the study of large molecules.1-10 The mechanism of ionization during MALDI is still poorly understood, and no adequate quantitative model for the complete process exists. The MALDI technique involves both laser ablation and ionization of the matrix/analyte mixture. Spectra are dominated by matrix photoproducts, protonated molecules, and adduct ions. Several groups have developed models for the ablation process. [6][7][8]11 However, full models of MALDI await more detailed knowledge of ionization mechanisms, kinetics and thermodynamics of the species involved.Ablation models are the best developed aspect of MALDI theory. The majority of the energy absorbed by the matrix relaxes by rapid internal conversion from electronic states to vibrationally excited ground states and contributes to rapid heating. Ehring and Sundqvist 12 studied the luminescence of two common MALDI matrices and found that the luminescence quantum yields are very low (0.2) which indicates that most of the energy is converted into matrix heating following photoexcitation. Models proposed to describe the laser ablation of the matrix include quasi-thermal evaporation, layer-by-layer expansion and pressure pulse expansion. In all three models, analyte molecules are assumed to be entrained within the flux of matrix molecules ejected from the surface, but these models do not address the formation of ions.Evidence exists that analyte ionization occurs both in the surface and in the expanding plume. A small proportion of the analyte ionization occurs in the surface layers as proposed by Johnson and co-workers. 11,13,14 At the threshold fluence, the ion yield increases rapidly until a fixed layer depth is ejected, 15 but the ionization is heavily quenched when the ejection yields are large. 16 This model is supported by a recent study by Russell's group which reported two thresholds for analyte ion formation in MALDI.17 At low laser fluences of 2 mJ/cm 2 , vitamin B 12 gives an [M + H] + and a [M -CN + H] + ion in α-cyano-4-hydroxycinnamic acid. These ions are observed from the first laser shot on a spot and only the first shot. At three times higher laser fluences, the [M + H] + ion disappears. Russell and co-workers proposed that a surface component is desorbed and ionized in the first shot at the lower ionization threshold. At the higher laser fluence, a component from the bulk material is desorbed.There also appears to be an important component of ionization originating from analyte molecules entrained in the expanding plume. Mowry and Johnson, 18 studying the relative amounts of ions and neutrals desorbed from various matrices, found that the ratio of neutralsto-ions ejected was of the order of 10 000 or greater at threshold irradiances. The ratio decreases with increasing irradiance of the desorption laser. These authors favor a collisional mechanism for analyte ion formation. Temporal profile analysis o...

A density functional theory (DFT) study on gas-phase proton transfer reactions of derivatized and underivatized peptide ions generated by matrix-assisted laser desorption ionization

“…From the calculations, the CN is the most stable protonation site followed by the ␣-C and the OH (15 kcal/ mol and 39 kcal/mol higher than the value found for CN). The calculated values agree with those found previously using the gas-phase bracketing method in combination with Fourier transform ion cyclotron resonance (FTICR) mass spectrometry [13] and using the kinetic method [12]. Experimental PA values for DHB are in the range 197 Ϭ 204 kcal/mol and for 4HCCA in the range 183 Ϭ 223 kcal/mol.…”

“…Many analytes, especially peptides and proteins, are detected predominantly in protonated form. Reactions between peptides/proteins in the gas phase were reported to have proton affinities (PAs) greater than 215 kcal/mol [10], whereas PAs of the most common matrices are between 200 and 215 kcal/mol [11][12][13], making the proton transfer between protonated matrix and neutral analyte favorable. Strong evidence for proton transfer reactions between protonated matrix and analyte was recently reported using mixtures of conventional matrices and various amino acids varying in gas phase basicity [6].…”

Protonation of Glyn Homologues in Matrix-Assisted Laser Desorption Ionization

“…Recent bracketing experiments have shown markedly lower PA for CHCA (183 ( 2 kcal/mol) than for SA (204 ( 4 kcal/mol). 28 One can argue that proton transfer from the ground state of the protonated CHCA species to Gly n occurs more readily than from protonated SA due to the ∼21 kcal/mol advantage in the PA balance.…”

Sodium Cation Affinities of Commonly Used MALDI Matrices Determined by Guided Ion Beam Tandem Mass Spectrometry

“…Limited theoretical and experimental studies of the gas phase properties of MALDI matrices have been reported in the literature. These studies have focused on determining the gas phase proton affinities [43][44][45][46], basicities/acidities [47][48][49][50], and ionization energies of MALDI matrices [43]. Very few studies have reported cation affinities [51][52][53][54][55] of MALDI matrices.…”