Energy flow in peptides after UV photoexcitation of backbone linkages

Abstract

We report on the dissociation channels after UV photoexcitation of peptide cations tagged with 18-crown-6 ether (CE). The model peptides chosen for study were singly protonated (Ala)-Pro (n = 1, 2, 3) and Pro-Pro (Ala = alanine, Pro = proline) that all contain at least one tertiary amide group with high absorption cross section at 210 nm (5.90 eV). Statistical dissociation was identified from the loss of CE, a process occuring remotely from the initial site of excitation, and therefore requiring flow of energy to the ammonium group where the CE is bound. However, homolytic breakage of the peptide backbone at the site of excitation is competitive, resulting in so-called a radical cations. Density functional theory calculations of dissociation energies were done on the simplest system [Ala-Pro + H](CE) and found to be 1.87 eV for CE loss and 3.29 eV for the formation of a(CE) and x. These numbers were used to calculate statistical branching ratios for the dissociation processes based on detailed balance. After the absorption of two 210 nm photons (according to power-dependence measurements), the branching ratio between the two channels is calculated to be less than 10, far below the observed ratio of 0.65. Hence both statistical and non-statistical dissociation contribute to dissociation of these photoexcited peptides.

Tandem‐trapped ion mobility spectrometry/mass spectrometry coupled with ultraviolet photodissociation

“…It should be noted that the absorbance band of the peptide bond is rather broad, and thus, for example, 205 nm is often chosen as a more convenient absorption wavelength for concentration measurements in solution 54 . Similarly, in gas‐phase work, it has been shown that the absorption cross section at 210 nm is very appreciable 19 . UVPD experiments of proteins at 213 nm thus combine the high‐absorption cross section of the protein backbone, while minimizing absorption by atmospheric gases, and are thus conducive to elevated pressure conditions compatible with ion mobility spectrometry.…”

“…UVPD, in particular, has proven to be a very versatile tool to study the primary, tertiary, and quaternary structures of proteins and protein complexes 9,10 . UVPD begins when the ion absorbs UV light, producing an electronically excited state of the ion 18–20 . The electronically excited ion then may undergo a variety of relaxation processes, or it may dissociate.…”

“…9,10 UVPD begins when the ion absorbs UV light, producing an electronically excited state of the ion. [18][19][20] The electronically excited ion then may undergo a variety of relaxation processes, or it may dissociate. The photodissociation mechanism of small peptides in the gas phase has been extensively studied, [21][22][23] and UVPD has been comprehensively exploited analytically to identify sequences of peptides, 8 glycopeptides, 24 or carbohydrates, 25 including glycosaminoglycans.…”

Ultraviolet Photodissociation of Protonated Peptides and Proteins Can Proceed with H/D Scrambling

“…This observation supports the notion that two distinct fragmentation mechanisms, statistical vs. nonstatistical fragmentation, are operative in the UVPD process. [31][32][33][34] Statistical fragmentation occurs when the electronic excitation energy is converted into vibrational excitation (internal conversion) followed by intramolecular vibrational energy redistribution (IVR) and subsequent dissociation of the peptide backbone into b/y ions according to the mobile proton model. 35,36 Importantly, H/D scrambling is an inevitable prelude to fragmentation of protonated peptides by statistical fragmentation processes.…”

Unravelling the Keto-Enol Tautomer Dependent Photochemistry and Degradation Pathways of the Protonated UVA Filter Avobenzone

“…However, in the region between 3.7 and 4.7 eV (Vis to near-UVA), photofragmentation is clearly non-statistical with production of the m/z 135 photofragment being photochemically enhanced. 42,[72][73][74][75][76] 4. Discussion…”