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Proteins fulfill a plethora of biochemical functions within every living organism, and mass spectrometry (MS) has become one of the most powerful and popular modern physical–chemical methods to study the complexities of proteins. In particular, the invention of matrix-assisted laser desorption/ionization (MALDI) [1] and electrospray ionization (ESI) technologies[2, 3] allows one to measure protein molecular weights and sequences, and to probe conformations and post-translational modifications of proteins. In addition, the mass range of species amenable for MS analysis has increased, enabling the transfer of ionized non-covalent species with masses well over one million (e.g., 1.5 MDa 24-Mer flavoprotein vanillyl-alcohol oxidase (VAO) from Penicillium simplicissimum [4]) into the gas phase. These advances moved MS into the range of intact protein oligomers and functional machineries.