'Compatible solutes' are organic compounds involved in osmoregulation in procaryotic and eucaryotic cells [1]. In halophilic/halotolerant and osmophilic/osmotolerant micro-organisms these solutes can represent up to 20% of the dry weight of cells. Using techniques other than mass spectrometry a wide range of compounds had been identified including specific sugars, quaternary ammonium compounds such as betaines, and modified amino acids such as N-acetylated diamino acids, ectoine-type cyclic derivatives and carboxamides of glutamine. They have been shown to have biotechnological potential as stabilisers for biomolecules and whole cells. Efficient screening methods were therefore sought to identify specific organisms' solute 'cocktails' and to select useful strains for production of known and novel compounds.
Liquid Secondary Ion Mass Spectrometry (LSIMS) and desorption chemical ionization (DCI) in conjunction with full high resolution mass spectra and MS/MS have been successfully used on a hybrid instrument to characterise the compatible solutes in crude bacterial extracts. LSIMS (and FAB MS) are very sensitive methods for the detection and identification of quaternary ammonium and zwitterionic solute species, while sensitivity for sugars and polyols is much lower. To minimize the number of ions formed from each solute species in LSIMS, samples were analysed after desalting to improve the yield of [M+H]+ ions relative to [M+Na]+ and [M+K]+ species. Initial experiments were used to confirm the results of studies undertaken using other methods, such as HPLC and NMR. Some minor discrepancies were noted initially, and the MS result was eventually vindicated.
Organic species encountered in this study and identified by high resolution measurements and MS/MS on crude extracts have included glycine-betaine, ectoine, hydroxyectoine, alanine and N-alpha-acetyllysine.
Unusual and novel compounds are occasionally encountered as compatible solutes, and the provision of molecular formulae via high resolution MS and fragmentation pathways via hybrid MS/MS experiments have also been used to aid in the identification of these compounds. DCI high resolution data and MS/MS data were used in the assignment of pyroglutamic acid as a compound in the unidentified strain H53, while high resolution LSIMS and MS/MS data indicated the presence of homobetaine and a C4 aminotriol in Marinococcus albus.