Infrared Spectroscopy as a Probe of Electronic Energy Transfer

Site-Specific Photochemistry along a Protonated Peptide Scaffold

We present a detailed study of the time-dependent photophysics and photochemistry of a known conformation of the two protonated pentapeptides Leu-enkephalin (Tyrosine-Glycine-Glycine-Phenylalanine-Leucine, YGGFL) and its chromophore-swapped analogue FGGYL, carried out under cryo-cooled conditions in the gas phase. Using ultraviolet-infrared (UV-IR) double resonance, we record excited state IR spectra as a function of time delay between UV and IR pulses. We identify unique Tyr OH stretch transitions due to the S1 state and the vibrationally excited triplet state(s) formed by intersystem crossing, Tn(v). Photofragment mass spectra are recorded out of the S1 origin and following UV-IR double resonance. Several competing site-specific fragmentation pathways are discovered involving peptide backbone cleavage, Tyr side chain loss, and N-terminal NH3 loss mediated by electron transfer. In YGGFL, IR excitation in the S1 state promotes electron transfer (ET) from the aromatic ring to the N-terminal R-NH3+ group leading to loss of neutral NH3. This product channel is missing in FGGYL due to the larger distance for ET from Y(4) to NH3+. Selective loss of the Tyr side chain occurs out of an excited state process following UV excitation and is further enhanced by IR excitation in S1 and Tn(v) states of both YGGFL and FGGYL. Finally, IR excitation in the S1 or Tn(v) states fragments the peptide backbone exclusively at amide(4), producing the b4 cation. We postulate that this selective fragmentation results from intersystem crossing to produce vibrationally excited triplets with enough energy to launch the proton along a proton conduit present in the known starting structure.

Laser Photodissocation, Action Spectroscopy and Mass Spectrometry Unite to Detect and Separate Isomers

The separation and detection of isomers remains a challenge for many areas of mass spectrometry. This article highlights laser photodissociation and ion mobility strategies that have been deployed to tackle...

Characterization of Insulin Dimers by Top-Down Mass Spectrometry

High-molecular weight products (HMWP) are an important critical quality attribute in research and development of insulin biopharmaceuticals. We here demonstrate on two case studies of covalent insulin dimers, induced by Fe²⁺ incubation or ultraviolet (UV) light stress, that de novo characterization in top-down mass spectrometry (MS) workflows can identify cross-link types and sites. On the MS² level, electron-transfer/higher-energy collision dissociation (EThcD) efficiently cleaved the interchain disulfide bonds in the dimers to reveal cross-link connectivities between chains. The combined utilization of EThcD and 213 nm ultraviolet photodissociation (UVPD) facilitated identification of the chemical composition of the cross-links. Identification of cross-link sites between chains at residue level was achievable for both dimers with MS³ analysis of MS² fragments cleaved at the cross-link or additionally the interchain disulfide bonds. UVPD provided identification of cross-link sites in the Fe²⁺-induced dimer without MS³, while cross-link site identification with MS² was not possible for the UV light-induced dimer. Thus, using varied multistage approaches, it was discovered that in the UV light-induced dimer, Tyr14 of the A-chain participated in an -O-S- cross-link in which the sulfur was derived either from Cys7 or Cys19 of the B-chain. In the Fe²⁺-induced dimer, Phe1 from both B-chains were cross-linked through a -CH₂-. The UV chromophoric side chain of Phe1 was indicated in the cross-link, explaining why UVPD-MS² was effective in fragmenting the cross-link and nearby backbone bonds. Our results demonstrated that higher-energy collisional dissociation (HCD), EThcD, and UVPD combined with MS³ were powerful tools for direct de novo characterization of cross-linked insulin dimers.

Direct Ultraviolet Laser-Induced Reduction of Disulfide Bonds in Insulin and Vasopressin

Ultraviolet (UV) light has been shown to induce reduction of disulfide bonds in proteins in solution. The photoreduction is proposed to be a result of electron donation from excited Tyr or Trp residues. In this work, a powerful UV femtosecond laser was used to generate photoreduced products, while the hypothesis of Tyr/Trp mediation was studied with spectroscopy and mass spectrometry. With limited irradiation times of 3 min or less at 280 nm, the laser-induced reduction in arginine vasopressin and human insulin led to significant yields of ∼3% stable reduced product. The photogenerated thiols required acidic pH for stabilization, while neutral pH primarily caused scrambling and trisulfide formation. Interestingly, there was no direct evidence that Tyr/Trp mediation was a required criterion for the photoreduction of disulfide bonds. Intermolecular electron transfer remained a possibility for insulin but was ruled out for vasopressin. We propose that an additional mechanism should be increasingly considered in UV light-induced reduction of disulfide bonds in solution, in which a single UV photon is directly absorbed by the disulfide bond.

Methionine and Selenomethionine as Energy Transfer Acceptors for Biomolecular Structure Elucidation in the Gas Phase

Mass spectrometry affords rapid and sensitive analysis of peptides and proteins. Coupling spectroscopy with mass spectrometry allows for the development of new methods to enhance biomolecular structure determination. Herein, we demonstrate two new energy acceptors that can be utilized for action-excitation energy transfer experiments. In the first system, C-S bonds in methionine act as energy acceptors from native chromophores, including tyrosine, tryptophan, and phenylalanine. Comparison among chromophores reveals that tyrosine transfers energy most efficiently at 266 nm, but phenylalanine and tryptophan also transfer energy with comparable efficiencies. Overall, the C-S bond dissociation yields following energy transfer are low for methionine, which led to an investigation of selenomethionine, a common analog that is found in many naturally occurring proteins. Sulfur and selenium are chemically similar, but C-Se bonds are weaker than C-S bonds and have lower lying σ* anti-bonding orbitals. Excitation of peptides containing tyrosine and tryptophan results in efficient energy transfer to selenomethionine and abundant C-Se bond dissociation. A series of helical peptides were examined where the positions of the donor or acceptor were systematically scanned to explore the influence of distance and helix orientation on energy transfer. The distance was found to be the primary factor affecting energy transfer efficiency, suggesting that selenomethionine may be a useful acceptor for probing protein structure in the gas phase.

Structure-Based Theory of Fluctuation-Induced Energy Transfer in a Molecular Dyad

We present a microscopic theory for the description of fluctuation-induced excitation energy transfer in chromophore dimers to explain experimental data on a perylene biscarboximide dyad with orthogonal transition dipole moments. Our non-Condon extension of Förster theory takes into account the fluctuations of excitonic couplings linear and quadratic in the normal coordinates, treated microscopically by quantum chemical/electrostatic calculations. The modulation of the optical transition energies of the chromophores is inferred from optical spectra of the isolated chromophores. The application of the theory to the considered dyad reveals a two to three order of magnitude increase in the rate constant by non-Condon effects. These effects are found to be dominated by fluctuations linear in the normal coordinates and provide a structure-based qualitative interpretation of the experimental time constant for energy transfer as well as its dependence on temperature.