Identification of Isomeric Biomolecules by Infrared Spectroscopy of Solvent-Tagged Ions

The difference in functionality of many isomeric biomolecules requires their analytical identification for life science studies. We present a universal approach for quantitative identification of different small- to medium-sized isomeric biomolecules that can be brought to the gas phase from solution by electrospray ionization (ESI). The method involves infrared (IR) fragment cold ion spectroscopy of analyte molecules that are incompletely desolvated by soft ESI. The use of solvent molecules as natural tags removes a need for adding to solutions any special compounds, which may interfere with liquid chromatography or mass spectrometric measurements. The tested peptides and especially monosaccharides and lipids exhibit highly isomerspecific IR fragment spectra of such noncovalent complexes, which were produced from water, methanol, acetonitrile, and 2butanol solutions. The relative concentrations in solution mixtures of, for instance, two isomeric dipeptides can be quantified with the accuracy of 1.6% and 2.9% for the acquisition time of 25 min and, potentially, 5 s, respectively; for three isomeric phosphooctapeptides, the accuracy becomes 4.1% and 11% for 17 min and, potentially, 10 s measurements, respectively.