A novel method for photon unfolding spectroscopy of protein ions in the gas phase

Abstract

In this study, a new experimental method for photon unfolding spectroscopy of protein ions based on a Fourier transform ion cyclotron resonance (FT ICR) mass spectrometer was developed. The method of short-time Fourier transform has been applied here to obtain decay curves of target ions trapped in the cell of the FT ICR mass spectrometer. Based on the decay constants, the collision cross sections (CCSs) of target ions were calculated using the energetic hard-sphere model. By combining a tunable laser to the FT ICR mass spectrometer, the changes of CCSs of the target ions were recorded as a function of the wavelengths; thus, the photon isomerization spectrum was obtained. As one example, the photon isomerization spectrum of [Cyt c + 13H]13+ was recorded as the decay constants relative to the applied wavelengths of the laser in the 410–480 nm range. The spectrum shows a maximum at 426 nm, where an unfolded structure induced by a 4 s irradiation can be deduced. The strong peak at 426 nm was also observed for another ion of [Cyt c + 15H]15+, although some difference at 410 nm between the two spectra was found at the same time. This novel method can be expanded to ultraviolet or infrared region, making the experimental study of wavelength-dependent photon-induced structural variation of a variety of organic or biological molecules possible.

Advancing Orbitrap Measurements of Collision Cross Sections to Multiple Species for Broad Applications

“…This decay constant along with the ion’s path length ( l ) and frequency ( f 0 ) may be used to calculate CCS as shown in eq . N ( t ) = N normalo normale − c t σ = c l f 0 N A similar concept was implemented experimentally on an FT-ICR platform, demonstrating the measurements of CCSs of ubiquitin that matched closely to values determined by IM. This type of CCS method was also successfully used to monitor the unfolding of protein ions following irradiation by a tunable IR laser in an FT-ICR system . Recently, the time-domain decay rate method was demonstrated experimentally on an Orbitrap mass analyzer, facilitated by development of a computational method termed direct decay profile fitting .…”

“…This type of CCS method was also successfully used to monitor the unfolding of protein ions following irradiation by a tunable IR laser in an FT-ICR system. 28 Recently, the timedomain decay rate method was demonstrated experimentally on an Orbitrap mass analyzer, facilitated by development of a computational method termed direct decay profile fitting. 12 In essence, Fourier transformation of the transient data followed by inverse FT allowed the final time-domain data to be fit to an exponential decay model to determine the decay rate.…”

Unimolecular Chiral Stepping Inversion Machine

“…Subsequently, the decay time constants of PT3 and PT5 before and after oxidation were calculated by eq S1. 48 By using the decay time constant, the cross section (Å 2 ) of PT3, PT3 + Fe 3+ , PT5, and PT5 + Fe 3+ were calculated according to eq S2, 48 and the results showed that Å 2 PT3 > Å 2…”

“…First, the mass spectra of PT3 and PT5 before and after oxidation were obtained through Fourier transform ion cyclotron resonance (FT ICR) (Figure S41 and Supporting Information). Subsequently, the decay time constants of PT3 and PT5 before and after oxidation were calculated by eq S1 . By using the decay time constant, the cross section (Å 2 ) of PT3, PT3 + Fe 3+ , PT5, and PT5 + Fe 3+ were calculated according to eq S2, and the results showed that Å 2 PT3 > Å 2 PT3+Fe 3+ and Å 2 PT5 > Å 2 PT5+Fe 3+ (Figure S42, Supporting Information).…”

“…Subsequently, the decay time constants of PT3 and PT5 before and after oxidation were calculated by eq S1 . By using the decay time constant, the cross section (Å 2 ) of PT3, PT3 + Fe 3+ , PT5, and PT5 + Fe 3+ were calculated according to eq S2, and the results showed that Å 2 PT3 > Å 2 PT3+Fe 3+ and Å 2 PT5 > Å 2 PT5+Fe 3+ (Figure S42, Supporting Information). The CCS analysis demonstrated that the cross section of the oxidized form PT3/PT5 is smaller than that of the reduced form PT3/PT5, confirming that the oxidized form of the TTF unit was in the cavity of PT3/PT5 and the reduced form was outside the cavity of PT3/PT5.…”