Structural characterization of Substance P using gold nanoparticles in SALDI “in-source decay” mass spectrometry

INTRODUCTION | Laser desorption/ionization mass spectrometry techniques (including MALDI and surface-assisted laser desorption/ionization, SALDI) are routinely used in analytical methods for the study of all kinds of biomolecules. In these techniques, the choice of the assisting material has now become vast, ranging from a large variety of organic matrices to nanoparticles (NPs) and other solid nanosubstrates. The selection of the assisting material appears not straightforward and the capabilities of novel assisting materials for the desorption/ionization for various (bio)molecules are poorly documented. This is typically the case of the SALDI nanosubstrates for which the potential for in-source fragmentation capabilities is seldom studied. METHODS | In this work, gold nanoparticles (AuNPs) were used as assisting materials in SALDI FT-ICR mass spectrometry experiment. The “hardness” in terms of in-source decay (ISD) of the AuNPs was first evaluated based on the survival yield (SY) method, and compared to MALDI matrices. The SY of the AuNPs and the MALDI matrices were determined from the intensities of the parent and fragment benzylpyridinium (BP) thermometer ions: SY = Iparent/(Iparent + Ifragment) PRELIMINARY DATA (RESULTS) | “Hard” assisting materials are characterized by a low SY (i.e. a high fragmentation yield), and therefore by a high energy transfer to the thermometer ions. In this study, the AuNPs were classified as “hard” assisting materials, able to transfer a high amount of energy to the analytes, inducing their fragmentation. Moreover, the SALDI mass spectra were characterized by a clean background, enabling matrix-free interference in the low m/z region. This property is especially useful for easily detecting the ISD fragments in this range, giving directly access to the sequencing of both extremities of any peptide. AuNPs were successfully tested in SALDI in-source decay (ISD) experiments using a dual ion source MALDI/ESI FT-ICR 9.4T SolariX XR mass spectrometer, in the positive ionization mode, for the structural characterization of the substance P peptide. Several types of ISD fragments (a-, b-, c-, x-, y- and z-ion series), originating from two main precursor ions (i.e. either [M + Na]+ or [M – H + Na]+), were detected in the SALDI mass spectra of the substance P, offering a great peptide sequencing coverage. This study encourages to test other SALDI nanosubstrates in structural studies but also to undertake further fundamental investigations to understand not only the desorption and ionization processes but also the fragmentation mechanisms occurring in SALDI-ISD-MS