Probing solution- and gas-phase structures of Trp-cage cations by chiral substitution and spectroscopic techniques

Abstract

The relevance of gas-phase protein structure to its solution structure is of the utmost importance in studying biomolecules by mass spectrometry. d-Amino acid substitutions within a minimal protein, Trp-cage, were used to correlate solution-phase properties as measured by circular dichroism with solution/gas-phase conformational features of protein cations probed via charge state distribution (CSD) in electrospray ionization, and gas-phase features revealed by tandem mass spectrometry (MS/MS). The gas-phase features were additionally supported by force-field molecular dynamics simulations. CD data showed that almost any single-residue d-substitution destroys the most prominent CD feature of the “native” all- l isomer, α-helicity. CSD was able to qualitatively assess the degree of compactness of solution-phase molecular structures. CSD results were consistent with the all- l form being the most compact in solution among all studied stereoisomers except for the d-Asn 1 isomer. d-substitutions of the aromatic Y 3, W 6 and Q 5 residues generated the largest deviations in CSD data among single amino acid substitutions, consistent with the critical role of these residues in Trp-cage stability. Electron capture dissociation of the stereoisomer dications gave an indication that some gas-phase structural features of Trp-cage are similar to those in solution. This result is supported by MDS data on five of the studied stereoisomer dications in the gas-phase. The MDS-derived minimum-energy structures possessed more extensive hydrogen bonding than the solution-phase structure of the native form, deviating from the latter by 3–4 Å and were not ‘inside-out’ compared to native structures. MDS data could be correlated with CD data and even with ECD results, which aided in providing a long-range structural constraint for MDS. The overall conclusion is the general resemblance, despite the difference on the detailed level, of the preferred structures in both phases for the mini protein Trp-cage.