Strategies for generating peptide radical cations via ion/ion reactions
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Abstract:
Several approaches for the generation of peptide radical cations using ion/ion reactions coupled with either collision induced dissociation (CID) or ultraviolet photo dissociation (UVPD) are described here. Ion/ion reactions are used to generate electrostatic or covalent complexes comprised of a peptide and a radical reagent. The radical site of the reagent can be generated multiple ways. Reagents containing a carbon–iodine (C―I) bond are subjected to UVPD with 266‐nm photons, which selectively cleaves the C―I bond homolytically. Alternatively, reagents containing azo functionalities are collisionally activated to yield radical sites on either side of the azo group. Both of these methods generate an initial radical site on the reagent, which then abstracts a hydrogen from the peptide while the peptide and reagent are held together by either electrostatic interactions or a covalent linkage. These methods are demonstrated via ion/ion reactions between the model peptide RARARAA (doubly protonated) and various distonic anionic radical reagents. The radical site abstracts a hydrogen atom from the peptide, while the charge site abstracts a proton. The net result is the conversion of a doubly protonated peptide to a peptide radical cation. The peptide radical cations have been fragmented via CID and the resulting product ion mass spectra are compared to the control CID spectrum of the singly protonated, even‐electron species. This work is then extended to bradykinin, a more broadly studied peptide, for comparison with other radical peptide generation methods. The work presented here provides novel methods for generating peptide radical cations in the gas phase through ion/ion reaction complexes that do not require modification of the peptide in solution or generation of non‐covalent complexes in the electrospray process. Copyright © 2015 John Wiley & Sons, Ltd.Keywords:
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Abstract In an attempt to probe the extent of internal excitation of protonated ions formed by positive fast‐ion bombardment, the protonated molecules of four commonly used liquid matrices are formed by means of the above technique and their respective mass‐selected collision‐induced dissociation (CID) spectra are investigated within the collision energy range, 5‐200 eV (in the laboratory frame of references). These measurements are also repeated for the same four precursor ions formed by means of positive‐ion chemical ionization in two different reagent gases, NH 3 and CH 4 . The observed dissociation channels and the relative peak heights associated with them imply that the internal excitation of protonated molecules formed by fast‐ion bombardment is much higher than that associated with the protonated ions formed in positive‐ion ammonia CI(NH 3 ‐CI + ). On the other hand, close resemblance between the CID spectra of ions formed by the first technique, by CH 4 ‐CI + and by electron ionization is clearly evident.
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The incorporation of the peptide reagent EEDQ (1) into an insoluble polymeric form is described. The utility of this reagent in peptide syntheses is investigated and its application in automated processes considered.
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Abstract In Freon solvents at temperatures ranging from ‐120 ° to ‐145 °C, the title reagent (I) cleanly converts (II)‐(IV) to the corresponding cation radical salts in yields of 97, 89 and 40%, resp. (the salt prepared from (IV) cannot be isolated).
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The reaction of phenoxyl radicals with acids is investigated. 2,4,6-Tri-tert-butylphenoxyl radical (13), a persistent radical, deteriorates in MeOH/PhH in the presence of an acid yielding 4-methoxycyclohexa-2,5-dienone 18a and the parent phenol (14). The reaction is facilitated by a strong acid. Treatment of 2,6-di-tert-butyl-4-methylphenoxyl radical (2), a short-lived radical, generated by dissociation of its dimer, with an acid in MeOH provides 4-methoxycyclohexa-2,5-dienone 4 and the products from disproportionation of 2 including the parent phenol (3). A strong acid in a high concentration favors the formation of 4 while the yield of 3 is always kept high. Oxidation of the parent phenol (33) with PbO(2) to generate transient 2,6-di-tert-butylphenoxyl radical (35) in AcOH/H(2)O containing an added acid provides eventually p-benzoquinone 39 and 4,4'-diphenoquinone 42, the product from dimerization of 35. A strong acid in a high concentration favors the formation of 39. These results suggest that a phenoxyl radical is protonated by an acid and electron transfer takes place from another phenoxyl radical to the protonated phenoxyl radical, thus generating the phenoxyl cation, which can add an oxygen nucleophile, and the phenol (eq 5). The electron transfer is a fast reaction.
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The kinetics and mechanism of the free radical oxidation of the azo dye, Acid Yellow 9, by sulfate radical anions and hydroxyl radicals have been studied using pulse radiolysis and product analysis. Sulfate radicals react via one electron oxidation, generating the dye radical cation, which has an absorption maximum centered on 470 nm. In basic solutions, the radical cation mainly undergoes deprotonation to yield the aminyl radical which has a strong absorption with a maximum centered on 370 nm, and the pKa for the radical cation is 5.5. Formation of a sulfated product, 1, is indicative of a coupling reaction between the radical cation and sulfate radical anions. Studies also indicate that the hydroxyl radicals react with the dye by both electron transfer as well as by adduct formation.
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T. G. Pearson, J. Chem. Soc., 1934, 1718 DOI: 10.1039/JR9340001718
Primary (astronomy)
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