Infrared Spectroscopy of Mobility-Selected H+-Gly-Pro-Gly-Gly (GPGG)

TLDR: Ion mobility and spectroscopic data combined with density functional theory (DFT) based molecular dynamics simulations confirm the presence of one major conformer per family, which arises from cis/trans isomerization about the proline residue.

About: This article is published in Journal of the American Society for Mass Spectrometry.The article was published on 2015-06-20 and is currently open access. It has received 68 citations till now. The article focuses on the topics: Infrared spectroscopy & Population.

Figures

Figure 4. Infrared H2 predissociation spectra of H +GPGG·H2 for (a) conformer A, and (b) conformer B. The experimental spectrum in (a) was prepared by subtracting contributions from conformer B as described in the text. Geometry optimization and vibrational frequency calculations are performed at the B3LYP/6-311++G(d,p) level of theory and reveal the presence of both (c) trans and (d) cis structures. Calculated spectra are shown below the respective experimental spectra for comparison. The asterisk denotes a peak originating from a minor conformer shown in the Supplementary Material (Supplementary Figure S2)

Figure 1. Schematic structures of the peptideGly-Pro-Gly-Gly highlighting the (a) trans-Pro and (b) cis-Pro forms of the molecule

Figure 5. Cross-section distribution of (a) H+GPGG and (b) a selection where peak A has been isolated. Traces (c)–(e) show that population is transferred from peak A to peak B′ upon collisional activation

Figure 2. Schematic diagram of the IMS-IR-TOF instrument. Ions are generated with electrospray ionization and separated according to their collisional cross-sections as they traverse the 2-m ion mobility drift tube. The ions are then collected in the cold ion trap where they are collisionally cooled and tagged with H2 molecules for subsequent spectroscopic analysis. The inset shows a detailed view of the cold ion trap, which consists of two printed circuit boards as described in the text

Figure 6. Infrared predissociation spectra of H+GPGG⋅H2 acquired while isolating peak A in the ion mobility drift distribution. Trace (a) displays the spectrum acquired for the trans-Pro species. Traces (b) and (c) show how the spectrum evolves as the activation energy is increased. The spectrum for the cis-Pro species is provided in (d) for comparison

Figure 3. Ion mobility cross-section distribution for H+GPGG in (b) with the corresponding H2 predissociation spectrum in (e). By mobility selecting ions having collisional cross-sections of (a) 93.8 Å2 and (c) 96.8 Å2, we obtain H2 predissociation spectra shown in (d) and (f), respectively. This indicates the presence of at least two conformers as described in the text

Frequently asked questions

Q1. What are the contributions in "Infrared spectroscopy of mobility-selected" ?

The authors report the first results from a new instrument capable of acquiring infrared spectra of mobility-selected ions. This suggests that a portion of the trans-proline ion population is kinetically trapped as a higher energy conformer and may retain structural elements from solution. 

Q2. What is the main advantage of the addition of ion mobility?

From a spectroscopic point of view, the addition of ion mobility provides a conformational filter that can simplify the vibrational spectra. 

Q3. How stable is the trans-Pro molecule in D2O?

The GPGG molecule has been studied in solution using NMR spectroscopy, and the trans-Pro species was found to be more stable in D2O by 4.5 kJ mol–1 [33]. 

Q4. What technologies are used to characterize these interactions?

These interactions are characterized using several technologies, including mass spectrometry, ion mobility, and vibrational spectroscopy. 

Q5. How many conformers were obtained from the SA-AIMD runs?

In total, 80 conformers were obtained from the SA-AIMD runs (including the final cooling product and different candidate structures from fast quenches at finite temperature). 

Q6. How do you find out if a molecule is correlated with its gas-phase?

molecules taken from solution undergo several isomerization steps before arriving at their preferred gas-phase conformation [14, 18, 20], but even after these isomerization events, their structure seems to be correlated with its original solution-phase conformation [16]. 

Q7. What is the dominant and more stable class of conformers?

The dominant and more stable class of conformers is that with its proline C=O oriented toward the protonated amine, as shown in Figure 4d. 

Q8. What was the optimum geometry for each conformer?

For each conformer, the geometry was optimized at the DFT B3LYP//6-31G(d,p) level using Gaussian09 v D.01 [60], and the orientationally-averaged geometric cross-section was calculated using the Trajectory Method (TM) with the Mobcal software [61, 62] through consideration of only Lennard-Jones interactions. 

Q9. How many conformers can be populated at 300 K?

Both cis-Pro conformers can be populated at the drift tube temperature of 300 K, and a simple Boltzmann calculation reveals that the minor conformer should make up 10% of the population of the major cis-Pro conformer. 

Q10. How can one separate spectral contributions from multiple conformers?

One can also separate spectral contributions from multiple conformers by performing multi-laser experiments [27–29], as long as each conformer has at least one unique spectroscopic transition. 

Q11. What is the temperature to which the relative populations correspond?

The exact temperature to which the relative populations correspond is not obvious; however, it may reflect that of the drift tube (300 K), but it may also reflect a higher temperature, depending on the rate of cooling in the drift tube and the height of the barriers separating the conformers. 

Q12. What would be the advantages of mass spectrometry and ion mobility?

Massspectrometry and ion mobility would be supplemented by additional structural information, while spectroscopy would be simplified by the presence of mass and conformational filters.