Amino Acid Based Deep Eutectic Solvents: Next Generation Charge Reducing Agents for Native Mass Spectrometry

Electrospray ionization coupled with mass spectrometry (ESI-MS) has become a very powerful tool for studying the structure and function of proteins and protein complexes. However, in conventional ESI technique, the noncovalent interactions between biomolecules which are critical to their activity can be disrupted and lead to multiple charge formation. Moreover, peak overlapping, wide charge state distribution and low signal to noise ratio may lead to complex mass spectra. A relatively new approach known as native mass spectrometry (nMS) has become popular for analysis of large protein systems. In nMS non-denaturing solvents and charge reducing agents are used during ESI to obtain better mass spectra. Ammonium acetate, ammonium bicarbonate, triethylamine, trimethylamine oxide, imidazole etc. are traditional charge-reducing agents used in nMS. Deep Eutectic Solvents (DESs) are greener, biodegradable, nonhazardous, cost effective which could be an attractive alternative to the charge reducing agents. In this study, the charge reducing property of amino acid based DESs has been investigated and compared. DES is synthesized simply by mixing two or more components including a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD). Several amino acids (histidine, arginine, proline, serine and lysine) based DESs were synthesized with glycerol. The prepared DESs were confirmed and characterized by infrared spectroscopy, differential scanning calorimetry along with principal component analysis statistical tool. Model proteins and enzyme including lysozyme, cytochrome c and trypsin were mixed with various amino acid based DESs and tested their charge reducing capacity comparing with traditional charge reducing agents. Histidine-glycerol and arginine-glycerol DESs showed better charge reducing capabilities for all three proteins compared to that of traditional charge reducing agents. Moreover, the proline-glycerol DES showed significant charge reducing for trypsin.