Abstract :
[en] The wide range of crucial functions fulfilled by biomolecules in biological systems can notably be attributed to the diversity in their structures. It is therefore essential to have effective methods for their structural characterization to support the understanding of their biological activities. In this context, matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) has been extensively used for the analysis of the primary sequence of proteins and peptides. In particular, MALDI “in-source decay” (ISD) experiments can be used to fragment the molecular ions directly in the mass spectrometer ion source. Various matrices have already been studied in MALDI ISD MS experiments. However, the capabilities of surface-assisted laser desorption/ionization (SALDI) ISD MS, which employs nanostructured substrates (e.g. nanoparticles, solid nanosubstrates) instead of organic matrices, are still poorly documented.
Here, we report SALDI ISD FT-ICR MS using citrate-coated gold nanoparticles (AuNPs). The “hardness” of the AuNPs was first evaluated based on the survival yield (SY) method with “thermometer ions”, and compared to commonly used MALDI matrices. The SY values acquired with the AuNPs and MALDI matrices were determined experimentally from the intensities of the parent and fragment benzylpyridinium thermometer ions, as follows :
SY=I_Parent/(I_Parent+I_Fragment )
“Hard” assisting materials exhibit a low SY value (i.e. a high fragmentation yield). In this study, AuNPs turned out to be much harder than the studied MALDI matrices. The “hardness” ranking of the studied assisting materials was correlated with the initial ion velocity obtained by MALDI TOF MS and with previous “hardness” rankings. The SALDI mass spectra were characterized by a clean background in the low m/z range, facilitating the analysis of low molecular weight species and their fragments, due to the lack of interference caused by matrix ions and clusters.
Then, the AuNPs were successfully tested in SALDI ISD MS for the sequencing of peptides, using an ESI/MALDI dual source 9.4T SolariX XR mass spectrometer, in the positive ionization mode. Several types of ISD fragment series (originating from sodium adducts of the molecular ion) were detected, offering a great sequencing coverage. As expected, c-ions produced via a radical-induced fragmentation pathway were detected. However, a- and y-ions were predominant in the mass spectra, suggesting that the fragmentation is mainly assisted by thermal ion activation.
In conclusion, SALDI nanosubstrates offer interesting perspectives for the study of biopolymers, including peptides but also glycans and oligonucleotides, in SALDI ISD MS experiments.
