Charge, Color, and Conformation: Spectroscopy on Isomer-Selected Peptide Ions

Abstract

Monitoring the chromism induced by intramolecular hydrogen and charge transfers within proteins as well as the isomerization of both protein and cofactor is essential not only to understand photoactive signaling pathways but also to design targeted opto-switchable proteins. We used a dual-ion mobility drift tube coupled to a tunable picosecond laser to explore the optical and structural properties of a peptide chain bound to a chromophore—a prototype system allowing for a proton transfer coupled to conformational change. With the support of molecular dynamics and DFT calculations, we show how proton transfer between the peptide and its cofactor can dramatically modify the optical properties of the system and demonstrate that these changes can be triggered by collisional activation in the gas phase.

Correlation between the shape of the ion mobility signals and the stepwise folding process of polylactide ions

“…Selected ions are then submitted to collisional activation (refer to Experimental Section), and the ATD of the products is measured through a second IM analysis, thus allowing to characterize possible structural modifications (Figs S3 and S6). Such procedure has already been used on the same instrument to study structural transitions of biomolecular ions . However, for the PLA ions under investigation, no change is observed on the recorded ATDs in the accessible collision energy range.…”

“…However, for the PLA ions under investigation, no change is observed on the recorded ATDs in the accessible collision energy range. Although the actual activation energy is difficult to quantify in such an experiment, the present results nevertheless indicate that the interconversion barrier between the observed structural families is too elevated to be overcome under activation conditions that are sufficient to lead to significant unfolding of protein ions . We must here emphasize on the fact that we always managed to avoid ion decompositions upon collisional activation.…”

Ion mobility resolved photo‐fragmentation to discriminate protomers

“…The duration of the selection window is set to typically 500 μs, compatible with the time spread of the ion packet corresponding to a single isomer, due to diffusion. Selected ions can then be activated by collisions with buffer gas atoms before recording the ATD after the second DT . Helium is used as a buffer gas at a pressure of 4 Torr, at the temperature of 300 K, and with typical voltage drops across first and second DTs of 400 V. Experimental collision cross sections (CCSs) are determined from the evolution of arrival times as a function of the inverse voltage across the second DT .…”

“…In the present paper we examine the sensitivity of energy transfer measurements to different protonation patterns in a biomolecular system. The interplay between protonation and the optical and structural properties of small bio‐molecular chromophores has already been investigated, and we recently studied dye‐tagged model peptides including different protonation patterns . This contribution follows the same line by using a well‐controllable model peptide, doubly tagged with a FRET donor‐acceptor dye pair.…”

“…Molecular dynamics simulations were performed with a parametrization already successfully used before for charged gas‐phase peptides . In brief, Amber99/GAFF parameters were used (with parameters for all considered protonation states of terminal chromophore‐tagged cysteine residues re‐evaluated as given in Kulesza et al) employing the GROMACS molecular dynamics simulation software (version 5.1.3) .…”