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How crystallography can contribute to MALDI-mass spectrometry: intermolecular interaction of crystalline MALDI-matrix and analyte (part II)

A successful MALDI (Matrix-Assisted Laser Desorption/Ionisation) analysis includes the following steps: sample preparation, excitation of sample and disintegration of condensed phase, ionisation of analyte molecules, and at the end, separation of the ions in the spectrometer according to the mass-to-charge ratio and their detection. In spite of exponential development of the mass spectrometry and its wide use in chemistry and proteomics, the question of matrix-analyte interaction in matrix crystals at molecular level has not been completely understood. The goal of our research is to find out how protein molecule incorporates into MALDI-matrix crystals during experiment and to find out the factors influencing matrix-analyte interactions. The purpose of this research is to detect undesired effects and to select the favourable organic molecules to be used as matrices. The experiments include 2, 6-dihydroxyacetophenone and 2, 4, 6-trihydroxyacetophenone interacting with bovine serum albumin (BSA). First, the conditions of crystallisation of matrices have to be as those during the experiments ; they influence the shape and size of crystals, parameters essential for the matrix-analyte interactions. It is important to know the relation between internal structure of matrix crystals, their morphology and orientations of molecules on their surfaces. A method of choice is a combination of X-ray crystallography and electron microscopy. Crystal structures of 2, 6-dihydroxyacetophenone and 2, 4, 6-trihydroxyacetophenone have already been reported and directions of crystallographic axes of single crystals have been determined using a polarisation microscope and an optical goniometer. Here we report study of crystal morphologies and surfaces of 2, 6-dihydroxyacetophenone and 2, 4, 6-trihydroxyacetophenone, with and without protein molecules incorporated, by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM). Molecules of 2, 6-dihydroxyacetophenone form hydrogen bonded chains running in the [011] direction. The crystal growth is the fastest in the direction specified due to the strongest intermolecular interactions resulting in plate-like crystals. The largest face, (100), indicates the weakest interactions in the [100] direction. A close-up of the (100) face reveals a layered structure. Molecules of 2, 4, 6-trihydroxyacetophenone form 3D hydrogen-bonded network with numerous hydrogen bonds almost parallel to c axis. The crystals are elongated in the [001] direction and stack along the [100] direction forming plate-like aggregates detected as single crystals with a polarisation microscope.

MALDI-matrix; intermolecular interaction; crystal growth; SEM

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