Switching radical stability by pH-induced orbital conversion

Gryn’ova, Ganna; Marshall, David L.; Blanksby, Stephen J.; Coote, Michelle L.

doi:10.1038/nchem.1625

Cited by 172 publications

(238 citation statements)

References 45 publications

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“…The TEMPO-Bz moiety does not undergo efficient bond homolysis at 266 nm, despite the stability of the putative distonic nitroxyl radical anion products (1a) and (2a). 65 In the third model compound ( …”

Section: Photodissociation Of Model Compoundsmentioning

confidence: 99%

Photodissociation of TEMPO-modified peptides: new approaches to radical-directed dissociation of biomolecules

Marshall

Hansen

Trevitt

et al. 2014

Phys. Chem. Chem. Phys.

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Photodissociation of TEMPO-modified peptides: new approaches to radical-directed dissociation of biomolecules AbstractRadical-directed dissociation of gas phase ions is emerging as a powerful and complementary alternative to traditional tandem mass spectrometric techniques for biomolecular structural analysis. Previous studies have identified that coupling of 2-[(2,2,6,6-tetramethylpiperidin-1-oxyl)methyl]benzoic acid (TEMPO-Bz) to the N-terminus of a peptide introduces a labile oxygen-carbon bond that can be selectively activated upon collisional activation to produce a radical ion. Here we demonstrate that structurally-defined peptide radical ions can also be generated upon UV laser photodissociation of the same TEMPO-Bz derivatives in a linear ion-trap mass spectrometer. When subjected to further mass spectrometric analyses, the radical ions formed by a single laser pulse undergo identical dissociations as those formed by collisional activation of the same precursor ion, and can thus be used to derive molecular structure. Mapping the initial radical formation process as a function of photon energy by photodissociation action spectroscopy reveals that photoproduct formation is selective but occurs only in modest yield across the wavelength range (300-220 nm), with the photoproduct yield maximised between 235 and 225 nm. Based on the analysis of a set of model compounds, structural modifications to the TEMPO-Bz derivative are suggested to optimise radical photoproduct yield. Future development of such probes offers the advantage of increased sensitivity and selectivity for radicaldirected dissociation. AbstractRadical-directed dissociation of gas phase ions is emerging as a powerful and complementary alternative to traditional tandem mass spectrometric techniques for biomolecular structural analysis. Previous studies have identified that coupling of 2-[(2,2,6,6-tetramethylpiperidin-1-oxyl)methyl]benzoic acid (TEMPO-Bz) to the N-terminus of a peptide introduces a labile oxygen-carbon bond that can be selectively activated upon collisional activation to produce a radical ion. Here we demonstrate that structurally-defined peptide radical ions can also be generated upon UV laser photodissociation of the same TEMPO-Bz derivatives in a linear ion-trap mass spectrometer. When subjected to further mass spectrometric analyses, the radical ions formed by a single laser pulse undergo identical dissociations as those formed by collisional activation of the same precursor ion, and can thus be used to derive molecular structure. Mapping the initial radical formation process as a function of photon energy by photodissociation action spectroscopy reveals that photoproduct formation is selective but occurs only in modest yield across the wavelength range (300 -220 nm), with the photoproduct yield maximised between 235 and 225 nm. Based on the analysis of a set of model compounds, structural modifications to the TEMPO-Bz derivative are suggested to optimise radical photoproduct yield. Future development of such probes offers t...

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Section: Photodissociation Of Model Compoundsmentioning

confidence: 99%

Photodissociation of TEMPO-modified peptides: new approaches to radical-directed dissociation of biomolecules

Marshall

Hansen

Trevitt

et al. 2014

Phys. Chem. Chem. Phys.

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“…. [65,68] However, the effect is consistent across the range of scaffolds studied, and thus the observed structural trends are also representative of neutral alkoxyamines.…”

Section: Dissociation Thresholds Of Alkoxyamine Ionsmentioning

confidence: 52%

“…This is due to a combination of two factors: (i) conventional polar effect of the negative charge Coulombic stabilisation of nitroxyl radicals by remote negative charges arising from their enhanced polarisability. [65,68] Based on these arguments, we predict that inverting the charge polarity would strengthen the O-C bond due to these polar effects. Relocating the charge site onto the opposite side of the O-C bond (as in TEMPO-mediated peptide sequencing) would minimise the latter stabilisation contribution due to the lower alkyl radical polarisability relative to a nitroxyl radical.…”

Section: Dissociation Thresholds Of Alkoxyamine Ionsmentioning

confidence: 95%

“…[64] Calculations were performed at a high level of theory, recently demonstrated to predict gas-and solution-phase bond dissociation energies and associated equilibrium constants to within chemical accuracy. [15,65] Geometries of all species…”

Section: Computational Proceduresmentioning

confidence: 99%

“…This methodology has been shown to accurately predict the gasphase energetics of nitroxyl and other free radical reactions. [65,68] Entropies and thermal corrections at 25, 80, and 110 °C were calculated using standard formulae for the statistical thermodynamics of an ideal gas under the harmonic oscillator approximation in conjunction with the optimised geometries and scaled frequencies.…”

Section: Computational Proceduresmentioning

confidence: 99%

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Experimental evidence for competitive N O and O C bond homolysis in gas-phase alkoxyamines

Marshall

Gryn’ova

Coote

et al. 2015

International Journal of Mass Spectrometry

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The extensive use of alkoxyamines in controlled radical polymerisation and polymer stabilisation is based on rapid cycling between the alkoxyamine (R1R2NO-R3) and a stable nitroxyl radical (R1R2NO•) via homolysis of the labile O-C bond. Competing homolysis of the alkoxyamine N-O bond has been predicted to occur for some substituents leading to production of aminyl and alkoxyl radicals. This intrinsic competition between the O-C and N-O bond homolysis processes has to this point been difficult to probe experimentally. Herein we examine the effect of local molecular structure on the competition between N-O and O-C bond cleavage in the gas phase by variable energy tandem mass spectrometry in a triple quadrupole mass spectrometer. A suite of cyclic alkoxyamines with remote carboxylic acid moieties (HOOC-R1R2NO-R3) were synthesised and subjected to negative ion electrospray ionisation to yield [M − H]− anions where the charge is remote from the alkoxyamine moiety. Collision-induced dissociation of these anions yield product ions resulting, almost exclusively, from homolysis of O-C and/or N-O bonds. The relative efficacy of N-O and O-C bond homolysis was examined for alkoxyamines incorporating different R3 substituents by varying the potential difference applied to the collision cell, and comparing dissociation thresholds of each product ion channel. For most R3 substituents, product ions from homolysis of the O-C bond are observed and product ions resulting from cleavage of the N-O bond are minor or absent. A limited number of examples were encountered however, where N-O homolysis is a competitive dissociation pathway because the O-C bond is stabilised by adjacent heteroatom(s) (e.g. R3 = CH2F). The dissociation threshold energies were compared for different alkoxyamine substituents (R3) and the relative ordering of these experimentally determined energies is shown to correlate with the bond dissociation free energies, calculated by ab initio methods. Understanding the structure-dependent relationship between these rival processes will assist in the design and selection of alkoxyamine motifs that selectively promote the desirable O-C homolysis pathway. homolysis is a competitive dissociation pathway because the O-C bond is stabilised by adjacent heteroatom(s) (e.g., R 3 = CH 2 F). The dissociation threshold energies were compared for different alkoxyamine substituents (R 3 ) and the relative ordering of these experimentally determined energies is shown to correlate with the bond dissociation free energies, calculated by ab initio methods.Understanding the structure-dependent relationship between these rival processes will assist in the design and selection of alkoxyamine motifs that selectively promote the desirable O-C homolysis pathway.

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Gas‐Phase Ion Chemistry of Organic Peroxides

Kato

Blanksby

2014

Patai's Chemistry of Functional Groups

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Over the last 30 years, mass spectrometry and allied technologies have provided significant new insights into the intrinsic, gas phase properties of organic peroxides. In this chapter, the formation and interrogation of neutral peroxides, and their ionized analogues in the gas phase are discussed for several key classes of organic peroxides. Methods for the formation of intact, gas phase cations and anions from labile, often nonvolatile, peroxides using conventional and contemporary soft ionization techniques are described. Studies of the unimolecular dissociation of these ionized peroxides are shown to yield new understanding of energetics and bonding as well as providing diagnostic markers for the elucidation of molecular structure. The bimolecular chemistries of peroxides in the gas phase are considered in two categories: (i) reactions of ions with neutral peroxides; and (ii) the reactions of deprotonated organic hydroperoxides with neutral substrates. Advanced topics cover analytical applications of chemical ionization to the detection of peroxides and peroxyl radicals along with the synthesis of distonic peroxyl radical ions. Studies of the gas phase spectroscopy of organic peroxides that exploit the ionized forms of the peroxides are reviewed including negative‐ion photoelectron spectroscopy, infrared multiple photon dissociation and action spectroscopy.

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Switching radical stability by pH-induced orbital conversion

Cited by 172 publications

References 45 publications

Photodissociation of TEMPO-modified peptides: new approaches to radical-directed dissociation of biomolecules

Photodissociation of TEMPO-modified peptides: new approaches to radical-directed dissociation of biomolecules

Experimental evidence for competitive N O and O C bond homolysis in gas-phase alkoxyamines

Gas‐Phase Ion Chemistry of Organic Peroxides

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