APPLICATION OF CRYOGENIC INFRARED AND ULTRAVIOLET SPECTROSCOPY FOR STRUCTURAL AND DYNAMIC STUDIES OF GAS PHASE IONS
The work presented here employs cryogenic ion spectroscopy for the study of protein structure, kinetics, and dynamics. The main technique used is IR-UV double resonance spectroscopy. Here peptide ions are generated through nano electrospray ionization, guided into a mass spectrometer, mass selected, and then guided into a cryogenically held octupole ion trap. Ions are subsequently cooled to their vibrational ground state through collisions with 5 K helium allowing for high resolution IR and UV spectra to be recorded. The IR spectra are highly sensitive to an ion’s conformation, and the well resolved UV spectra provides a means generate conformer specific IR spectra. With the use quantum mechanical calculations, it is possible to calculate the vibrational spectra of candidate structures for comparison with experimental spectra. Strong correlations between theory and experiment allow for unambiguous structural assignments to be made.
Structural studies are performed on β-turn motifs and well as salt-bridge geometries. Beta-turns are a commonly occurring secondary structure in peptides and proteins. It is possible to artificially encourage the formation of this secondary structural element through the incorporation of the D-proline (DP) stereoisomer followed by a gly or ala residue. Interestingly, the L-proline (LP) stereoisomer is seen to discourage the formation of beta turn structure. Here were probe the inherent conformational preferences of the diastereomeric peptide sequences YALPAA and YADPAA. The findings agree with solution phase studies, the DP sequence is observed to adopt a beta turn however, the LP sequence is found to undergo a sterically driven trans à cis isomerization about the proline amide bond. We find the energetics associated with this unfavorable interaction and show the ability to reverse it by proper substitution of Ala2 for a Gly.
The studies directed towards gas phase salt bridges have been limited to single amino acids or dipeptides. Generally, these species are ionized using a metal ion or adducted with water or excess electrons in order to stabilize a zwitterionic motif. Here we take the first look at a salt bridge motif incorporated into polypeptide in order to understand how the solvation from the secondary structure can aid in stabilizing these motifs in non-polar environments. We find a unique salt bridge motif in the YGRAR sequence in which the tyrosine OH acts as a neutral bridge to form a network between the C-terminal arginine and the ion pair formed between the central arginine and C-terminal carboxylate group. This binding motif has not been discussed in literature and appears as an important structural element in non-polar environments as all salt bridge character is lost upon substituting Tyr for Phe. We are the process of mining the PDB for these types of interactions.
To better understand how cryo-cooling impacts the resulting population distribution at 10 K we measured the distribution among the two major conformation of the YGPAA ion. This was carried out using population transfer spectroscopy. In this method conformational isomerization is induced vis single conformer infrared excitation. The change in population can be related to the final population distribution at 10 K. With this number, we were able to develop a cooling model to simulate the change in the distribution as a function of cooling. The cooling rates, were experimental established, and the isomerization rates and starting population were theoretically derived through RRKM and thermodynamic calculations. With these parameters and cooling model, we found that the room temperature population distribution is largely preserved. When isomerization events involve breaking a hydrogen bond, they become too slow to complete with the cooling time scale of the experiment, effectively freezing in the room temperature structures. These are important physical parameters to characterize when performing structural studies at 10 K.
Finally, we demonstrate a 2-Color IRMPD technique that is able to generate linear spectra at varied temperatures. This is in sharp contrast to traditional IRMPD which results in non-linear and skewed spectra. The importance of generating linear spectra when making structural assignments is highlight by comparing the performance between both techniques. Furthermore, with this technique we show the ability to record the spectra of ion prepared with high internal energies. This provides spectroscopic snapshots of the unfolding events leading to dissociation. Overall, the versatility of this technique to record ground state spectra comparable to IR-UV DR, to record linear spectra at room temperature, and to probe dynamics proves this technique to be useful in the field of ion spectroscopy.