Probing the limits of liquid droplet laser desorption mass spectrometry in the analysis of oligonucleotides and nucleic acids

Abstract

In the present work we demonstrate the advantages of LILBID mass spectrometry (laser-induced liquid bead ion desorption) in the analysis of nucleic acids and large oligonucleotides. For established methods like matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI), the mass analysis of oligonucleotides or of noncovalent oligonucleotide-protein complexes, in particular of very large ones, still represents a considerable challenge either due to the lack of native solutions or nonspecific adduct formation or due to a reduced salt tolerance or a high charge state of the ions. With LILBID, oligonucleotides, solvated in micro-droplets of aqueous buffer at certain pH and ion strength, are brought into the gas phase by laser ablation. We show that our method is able to detect single- and double-stranded oligonucleotides with high softness, demonstrated by the buffer dependence of the melting of a duplex. The absolute sensitivity is in the attomole range concomitant with a total analyte consumption in the femtomole region. The upper mass limit of oligonucleotides still detected with good signal-to-noise ratio with LILBID is the 1.66 MDa plasmid pUC19. With DNA ladders from short duplexes with sticky ends, we show that LILBID correctly reflects the relative thermodynamic stabilities of the ladders. Moreover, as an example for a specific DNA-protein complex we show that a NF-kappaB p50 homodimer binds sequence specifically to its match DNA. In summary we demonstrate that LILBID, although presently performed only with low mass resolution, due to these advantages, is an alternative mass spectrometric method for the analysis of oligonucleotides in general and of specific noncovalent nucleic acid-protein complexes in particular.