Mass spectrometry radical cation

Among the major analytical tools for detecting and studying radical cations are mass spectrometry, electron spin resonance spectroscopy, nuclear magnetic resonance spectrometry, and particular variations of these methods. 

Electron impact ionization (commonly used for the production of radical cations in mass spectrometry). 

Mass Spectrometry Ethers like alcohols lose an alkyl radical from their molecular ion to give an oxygen stabilized cation Thus m/z 73 and m/z 87 are both more abun dant than the molecular ion m the mass spectrum of sec butyl ethyl ether... 

Although there has been some controversy concerning the processes involved in field ionization mass spectrometry, the general principles appear to be understood. Firstly, the ionization process itself produces little excess of vibrational and rotational energy in the ions, and, consequently, fragmentation is limited or nonexistent. This ionization process is one of the mild or soft methods available for producing excellent molecular mass information. The initially formed ions are either simple radical cations or radical anions (M ). 

Some of the target molecules gain so much excess internal energy in a short space of time that they lose an electron and become ions. These are the molecular cation-radicals found in mass spectrometry by the direct absorption of radiation. However, these initial ions may react with accompanying neutral molecules, as in chemical ionization, to produce protonated molecules. 

Mass Spectrometry Aldehydes and ketones typically give a prominent molecular- ion peak in their mass spectra. Aldehydes also exhibit an M— 1 peak. A major fragmentation pathway for both aldehydes and ketones leads to formation of acyl cations (acyliurn ions) by cleavage of an alkyl group from the carbonyl. The most intense peak in the mass spectrum of diethyl ketone, for exanple, is m/z 57, conesponding to loss of ethyl radical from the molecular- ion. 

The substituted radical cations [Me2S.. SMe2], [Et2S. .SEt2] and [Et2S.. SMe2] (Fig. 4) have been studied by lilies, McKee and co-workers using a combined experimental/theoretical approach [127-129]. Mass spectrometry experiments on the gas-phase association reactions... 

A different strategy has been applied in our work, that emphasizes the importance of DNA stability on hole transfer within double-stranded DNA. This work is based on determination of the overall yield of oxidized nucleosides that arise from the conversion of initially generated purine and pyrimidine radical cations within DNA exposed to two-photon UVC laser pulses. On the one hand, this work benefits from the excellent current knowledge of chemical reactions involving the radical cations of DNA bases, and on the other hand, from major analytical improvements that include recent availability of the powerful technique of high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (CLHP-ESI-MS/MS) [16-18]. 

It may be noted that competitive deprotonation of 29 at C-l gives rise to 2-deoxyribonolactone (27) with the concomitant release of free 5-methylcy-tosine as minor processes. Interestingly, competitive hydration of 5-MedCyd radical cations (29) occurs exclusively at C-6 as inferred from labeling experiments with 1802 (36) [61]. Thus, mass spectrometry analysis of the four cis and trans diastereomers of 5-MedCyd glycols 36 showed that incorporation of 1802 takes place exclusively at C-5 of 6-hydroxy-5,6-dihydro-2 -deoxycy-tyd-5-yl radicals (34). 

Electron ionization (earlier called electron impact) (see Chapter 2, Section 2.1.6) occupies a special position among ionization techniques. Historically it was the first method of ionization in mass spectrometry. Moreover it remains the most popular in mass spectrometry of organic compounds (not bioorganic). The main advantages of electron ionization are reliability and versatility. Besides that the existing computer libraries of mass spectra (Wiley/NIST, 2008) consist of electron ionization spectra. The fragmentation mles were also developed for the initial formation of a radical-cation as a result of electron ionization. 

Fragmentation of the nitroethylene radical cation has been studied by deuterium labelling and tandem mass spectrometry and compared to the fragmentation of the corresponding nitrosoethylene61. 

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