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Free fatty acids and lipid classes containing fatty acid esters are major components of lipidome. In the absence of a chemical derivatization step, FA anions do not yield all of the structural information that may be of interest under commonly used collision‐induced dissociation (CID) conditions. A line of work that avoids condensed‐phase derivatization takes advantage of gas‐phase ion/ion chemistry to charge invert FA anions to an ion type that provides the structural information of interest using conventional CID. This work was motivated by the potential for significant improvement in overall efficiency for obtaining FA chain structural information.

A hybrid triple quadrupole/linear ion‐trap tandem mass spectrometer that has been modified to enable the execution of ion/ion reaction experiments was used to evaluate the use of 4,4′,4″‐tri‐tert‐butyl‐2,2′:6′,2″‐terpyridine (ttb‐Terpy) as the ligand in divalent magnesium complexes for charge inversion of FA anions.

Mg(ttb‐Terpy)₂²⁺complexes provide significantly improved efficiency in producing structurally informative products from FA ions relative to Mg(Terpy)₂²⁺complexes, as demonstrated for straight‐chain FAs, branched‐chain FAs, unsaturated FAs, and cyclopropane‐containing FAs. It was discovered that most of the structurally informative fragmentation from [FA‐H + Mg(ttb‐Terpy)]⁺results from the loss of a methyl radical from the ligand followed by radical‐directed dissociation (RDD), which stands in contrast to the charge‐remote fragmentation (CRF) believed to be operative with the [FA‐H + Mg(Terpy)]⁺ions.

This work demonstrates that a large fraction of product ions from the CID of ions of the form [FA‐H + Mg(ttb‐Terpy)]⁺are derived from RDD of the FA backbone, with a very minor fraction arising from structurally uninformative dissociation channels. This ligand provides an alternative to previously used ligands for the structural characterization of FAs via CRF.

 

Correction for ‘Single-conformation spectroscopy of cold, protonated D PG-containing peptides: switching β-turn types and formation of a sequential type II/II′ double β-turn’ by John T. Lawler et al. , Phys. Chem. Chem. Phys. , 2022, 24 , 2095–2109, https://doi.org/10.1039/D1CP04852J. 

Gas-phase ion-ion reactions between tris-1,10-phenantholine metal dications, [(phen)3M]2+ (where M = Ni and Mg), and the tetraphenylborate anion yield the ion-pairs {[(phen)3M]2+[BPh4]-}+. The ion-pairs undergo transmetalation upon loss of a phen ligand to give the organometallic complexes [(phen)2M(Ph)]+. DFT calculations, used to determine the energy barriers for the transmetalation reactions and the hydrolysis reactions, are entirely consistent with the experimental results. 

d -Proline ( D Pro, D P) is widely utilized to form β-hairpin loops in engineered peptides that would otherwise be unstructured, most often as part of a D PG sub-unit that forms a β-turn. To observe whether D PG facilitated this effect in short protonated peptides, conformation specific IR–UV double resonance photofragment spectra of the cold (∼10 K) protonated D P and L P diastereomers of the pentapeptide YAPGA was carried out in the hydride stretch (2800–3700 cm −1 ) and amide I/II (1400–1800 cm −1 ) regions. A model localized Hamiltonian was developed to better describe the 1600–1800 cm −1 region commonly associated with the amide I vibrations. The CO stretch fundamentals experience extensive mixing with the N–H bending fundamentals of the NH 3 + group in these protonated peptides. The model Hamiltonian accounts for experiment in quantitative detail. In the D P diastereomer, all the population is funneled into a single conformer which presented as a type II β-turn with A and D P in the i + 1 and i + 2 positions, respectively. This structure was not the anticipated type II′ β-turn across D PG that we had hypothesized based on solution-phase propensities. Analysis of the conformational energy landscape shows that both steric and charge-induced effects play a role in the preferred formation of the type II β-turn. In contrast, the L P isomer forms three conformations with very different structures, none of which were type II/II′ β-turns, confirming that L PG is not a β-turn former. Finally, single-conformation spectroscopy was also carried out on the extended peptide [YAA D PGAAA + H] + to determine whether moving the protonated N-terminus further from D PG would lead to β-hairpin formation. Despite funneling its entire population into a single peptide backbone structure, the assigned structure is not a β-hairpin, but a concatenated type II/type II′ double β-turn that displaces the peptide backbone laterally by about 7.5 Å, but leaves the backbone oriented in its original direction.