The Long-Lived Radical Cations of Simple Carbon Esters Isomerize to the Lowest-Energy Structure

Abstract. The gas-phase reactivity of two aromatic carbon-centered σ,σ-biradicals (meta-benzyne analogs) and a related monoradical towards small oligonucleotides of differing lengths was investigated in a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer coupled with laser-induced acoustic desorption (LIAD). The mono-and biradicals were positively charged to allow for manipulation in the mass spectrometer. The oligonucleotides were evaporated into the gas phase as intact neutral molecules by using LIAD. One of the biradicals was found to be unreactive. The reactive biradical reacts with dinucleoside phosphates and trinucleoside diphosphates mainly by addition to a nucleobase moiety followed by cleavage of the glycosidic bond, leading to a nucleobase radical (e.g., base-H) abstraction. In some instances, after the initial cleavage, the unquenched radical site of the biradical abstracts a hydrogen atom from the neutral fragment, which results in a net nucleobase abstraction. In sharp contrast, the related monoradical mainly undergoes facile hydrogen atom abstraction from the sugar moiety. As the size of the oligonucleotides increases, the rate of hydrogen atom abstraction from the sugar moiety by the monoradical was found to increase due to the presence of more hydrogen atom donor sites, and it is the only reaction observed for tetranucleoside triphosphates. Hence, the monoradical only attacks sugar moieties in these substrates. The biradical also shows significant attack at the sugar moiety for tetranucleoside triphosphates. This drastic change in reactivity indicates that the size of the oligonucleotides plays a key role in the outcome of these reactions. This finding is attributed to more compact conformations in the gas phase for the tetranucleoside triphosphates than for the smaller oligonucleotides, which result from stronger stabilizing interactions between the nucleobases.

Section: Methodsmentioning

confidence: 99%

Gas-phase Reactivity of meta-Benzyne Analogs Toward Small Oligonucleotides of Differing Lengths

Widjaja

Max

Jin

et al. 2017

“…CS 3 •ϩ ions were the reactive intermediates in this process [28,29]. The reactivity of acetonitrile N-sulfide ions towards pyridine is found quite different in this last case, see Table 1.…”

Section: Ion-molecule Reactions Between Ch 3 -C §N ϩ -S • Radical Catmentioning

confidence: 94%

“…Whereas the very efficient S •ϩ transfer reaction probably involves the intermediacy of several ion-molecule complexes, the thiazolopyridinium ion formation is likely to be initiated by an electrophilic attack of the R-C §N ϩ -S Positively-charged distonic ions have been the focus of intense gas-phase and computational research since the discovery in 1984 that these ions are surprisingly stable, and sometimes even more stable than their conventional counterparts [1,2]. It was also demonstrated that many conventional radical cations can isomerize into their distonic isomeric structures, spontaneously [3] or via the intermediacy of ion-molecule reactions [4]. Numerous other distonic ions have been purposely produced through complex MS n procedures to address several fundamental chemical questions [5].…”

mentioning

confidence: 99%

Experimental and theoretical study of the gas-phase interaction between ionized nitrile sulfides and pyridine

Gerbaux

Haverbeke

Flammang

2003

The gas-phase reactivity of ionized nitrile sulfides, R-C §N ϩ -S • , towards neutral pyridine was studied both experimentally (six sector hybrid mass spectrometer) and theoretically (density functional theory and Møller-Plesset ab initio calculations). An ionized sulfur atom transfer and a cycloaddition process respectively yielding ionized pyridine N-thioxide and a thiazolopyridinium cation were observed. Whereas the very efficient S •ϩ transfer reaction probably involves the intermediacy of several ion-molecule complexes, the thiazolopyridinium ion formation is likely to be initiated by an electrophilic attack of the R-C §N ϩ -S Positively-charged distonic ions have been the focus of intense gas-phase and computational research since the discovery in 1984 that these ions are surprisingly stable, and sometimes even more stable than their conventional counterparts [1,2]. It was also demonstrated that many conventional radical cations can isomerize into their distonic isomeric structures, spontaneously [3] or via the intermediacy of ion-molecule reactions [4]. Numerous other distonic ions have been purposely produced through complex MS n procedures to address several fundamental chemical questions [5].Most of the associative ion-molecule reactions observed when the reactive ion is a distonic species involve a homolytic bond cleavage in the neutral reagent, leading to the abstraction of an atom or a radical by the ion. Many of these reactions are likely to be catalyzed by the charge site. Indeed, initial bond formation at the charge site stabilizes the collision complex and this bonding may also favorably position the neutral molecule for the subsequent radical reaction [6]. Transfer of charged odd-electron groups are also very common for distonic cations. For instance, ionized carbonyl ylide, • CH 2 -O-CH 2 ϩ ions, generated by ionization-induced ring opening of oxirane [7], readily transfer an ionized methylene, CH 2•ϩ , to several nucleophilic molecules [8]. A recent study by Eberlin revealed that distonic acylium ions display, using appropriate neutral reagents, a rich dual reactivity; they react selectively as either free radicals with inert charge sites, or (and more pronouncedly) as acylium ions with inert radical sites. Such radical cations thus display, in a single molecule, dual free radical and ion reactivity [9]. In 1997, Eberlin also found out a third kind of reactions between distonic ions and neutral molecules, namely a 1,3-cycloaddition between • CH 2 -O-CH 2 ϩ ions and carbonyl compounds [10]. This was the first cycloaddition reaction involving an ionized 1,3-dipole observed in the gas phase. More recently, we shown that the interaction between ionized nitrile oxides and several neutral nitriles allows the preparation of substituted azirinyl cations. This reaction was proposed to occur via cyclic intermediates such as ionized nitrosoazirines or ionized 1,2,4-oxadiazoles [11]. The bimolecular associative reactivity of the R-C §N ϩ -X • ions (X¢O, S and Se) has been previously extensively studied a...

“…The uncertainty in the pressure readings of the ion gauge used in this study was corrected by measuring the rate of the highly exothermic electron-transfer reaction between argon radical cation and the neutral reagent (TMB). It can be assumed that this reaction proceeds at the collision rate (k collision ), which can be calculated [39][40][41][42]. Any reaction occurring at collision rate has 100% efficiency.…”

Section: Methodsmentioning

confidence: 99%

“…The theoretical collision rate constants (k coll ) were obtained by using a parameterized trajectory theory [39][40][41][42]. The efficiency of each reaction (the fraction of collisions that leads to reaction) is given by k exp /k coll .…”

Section: Methodsmentioning

confidence: 99%

Differentiation of Protonated Aromatic Regioisomers Related to Lignin by Reactions with Trimethylborate in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

Somuramasami

Duan

Amundson

et al. 2011

Several lignin model compounds were examined to test whether gas-phase ion-molecule reactions of trimethylborate (TMB) in a FTICR can be used to differentiate the ortho-, meta-, and para-isomers of protonated aromatic compounds, such as those formed during degradation of lignin. All three regioisomers could be differentiated for methoxyphenols and hydroxyphenols. However, only the differentiation of the ortho-isomer from the meta- and para-isomers was possible for hydroxyacetophenones and hydroxybenzoic acids. Consideration of the previously reported proton affinities at all basic sites in the isomeric hydroxyphenols, and the calculated proton affinities at all basic sites in the three methoxyphenol isomers, revealed that the proton affinities of the analytes relative to that of TMB play an important role in determining whether and how they react with TMB. The loss of two methanol molecules (instead of one) from the adducts formed with TMB either during ion-molecule reactions, or during sustained-off resonance irradiated collision-activated dissociation of the ion-molecule reaction products, revealed the presence of two functionalities in almost all the isomers. This finding supports earlier results suggesting that TMB can be used to count the functionalities in unknown oxygen-containing analytes.