A cluster approach to solvation effects in flexible peptides*

Utilizing a method based upon a unique combination of mass spectrometry and laser spectroscopy, our research focuses on capturing and characterizing complexes of interest to provide a molecular-level detailed examination of their structures and interactions. To access microsolvated charged complexes, we have developed new capabilities to perform controlled gas-phase clustering in cryogenic ion trap. This versatile approach enables us to form clusters with >30 solvent molecules surrounding any bare ion obtained by electrospray ionization. The structures of these carefully isolated and prepared complexes are then directly probed in the mass spectrometer using infrared predissociation spectroscopy, yielding well-resolved spectra that contain a wealth of information. Recent studies on the solvation-dependent structures of flexible model peptidic systems, (Gly)_nH⁺ and (Ala)_nH⁺ will be presented. In addition, this talk will highlight two methodologies necessary for deconvoluting the complex vibrational spectra of these species: IR-IR conformer-specific spectroscopy and solvent-specific isotope labeling. This combination of spectroscopic characterizations yields unambiguous spectral assignments and clearly reveals the structural evolution of the flexible peptides in response to increasing solvation. The results also provide insights on the isomerization pathways and energetics involved in these structural changes.