Amino ions, decomposition

STIRS Prediction of Substructures Not Present. CONGEN can utilize a BADLIST of features known not to be present in the molecule. Negative information is more difficult to determine from an unknown mass spectrum because other parts of the molecule may suppress a fragmentation reaction characteristic of a specific function. However, this is less true of substructures such as amino which strongly influence ion decomposition pathways. Thus it might be useful to carry out an evaluation of the ability of STIRS to identify the absence of specific substructures in the same way used to find the 589 substructures whose presence is best found by STIRS (8). 

Photolysis of 3-phenyl-2,l-benzisoxazole in 48% HBr produced reduction and substitution products via a proposed triplet state nitrenium ion intermediate (71HCA2111). Photolytic decomposition of 5-bromo-3-phenyl-2,l-benzisoxazole in 48% HBr gave 2-amino-5-bromoacetophenone and 2-amino-3,5-dibromoacetophenone (Scheme 18). A nitrenium ion intermediate was also proposed for the photolytic decomposition of 3-phenyl-2,l-benzisoxazole in concentrated HCl (Scheme 19) (7IHCA2111). 

Amino acids have high melting or decomposition points and are best examined for purity by paper or thin layer chromatography. The spots are developed with ninhydrin. Customary methods for the purification of small quantities of amino acids obtained from natural sources (i.e. l-5g) are ion-exchange chromatography (see Chapter 1). For general treatment of amino acids see Greenstein and Winitz [The Amino Acids, Vols 1-3, J.Wiley Sons, New York 1961] and individual amino acids in Chapters 4 and 6. 

The ethylenediamine derivative [31] possesses higher promoting activities than other diamines. This phenomenon may be ascribed to the copromoting effect of the two amino groups on the decomposition of persulfate through a CCT (contact charge transfer complex) formation. So we proposed the initiation mechanism via CCT as the intimate ion pair and deprotonation via CTS (cyclic transition state) as follows ... 

More recently Hand et al. (ref. 9) have studied the decomposition reaction of N-chloro-a-amino acid anions in neutral aqueous solution, where the main reaction products are carbon dioxide, chloride ion and imines (which hydrolyze rapidly to amine and carbonyl products). They found that the reaction rate constant of decarboxylation was independent of pH, so they ruled out a proton assisted decarboxylation mechanism, and the one proposed consists of a concerted decarboxylation. For N-bromoamino acids decomposition in the pH interval 9-11 a similar concerted mechanism was proposed by Antelo et al. (ref. 10), where the formation of a nitrenium ion (ref. 11) can be ruled out because it is not consistent with the experimental results. Antelo et al. have also established that when the decomposition reaction takes place at pH < 9, the disproportionation reaction of the N-Br-amino acid becomes important, and the decomposition goes through the N,N-dibromoamino acid. This reaction is also important for N-chloroamino compounds but at more acidic pH values, because the disproportionation reaction... 

Fig. 20 MIKE/CID spectra of the substituted m/z228 ions from the enantiomers of menthol ((7/ ,25,5R)-(— )-14 (A) and (75,27 ,55)-(+)-14 (B)) formed under CE(5)-2-amino-l-butanol (As) conditions (the m/z2 0 and mlz90 daugther ions are also produced during unimolecular decomposition of the m/z228 ions) (reprinted from ref. 472, with permission from Elsevier).

There are six main decomposition pathways for protonated DKPs and elimination of one of the amino acid side chains or produces an ion which is the origin of five other fragmentation pathways. Owing to the symmetric structure of the DKP ring with regard to R and R, only half of the fragment ions are shown in Figure 3. ... 

LCA toward amino acids and nucleic bases has also been measured. Wesdemiotis and Cerda measured the alkali metal ion affinities of nucleobases in the gas phase from the dissociation of metal ion-bound heterodimers [nucleobase + B]M+, in which B represents a reference base of known affinity and M is an alkali metal. By assessing the dimer decomposition for two different internal energies, entropy is deconvoluted from enthalpy and LCA values are obtained. For guanine, cytosine, adenine, thymine and uracil, the corresponding Li+-nucleobase bond energies are as follows 57.2, 55.5, 54.1,... 

Note the key role of the 2-cyanopyridine ion in promoting the course of the subsequent fragmentation. Proximity effects in 2-substituted pyridines are often instrumental in dictating the nature of decomposition on electron impact, for example in 2-dimethylaminopyridine (68TL3689,73CS(3)139,74JOC285) and in other amino, chloro, and -one derivatives (68JHC647). 

This mechanism is of importance in radical induced amino acid damage catalyzed by copper ions. The study of the decomposition of transients with a metal-carbon -bond containing two potential leaving groups (both an amine and a carboxylate group) at the p position of the carbon centered radical is of special interest. It was reported that the intermediate formed with the amino acid 2-methylalanine with cupric ions decomposes via p-carboxyl elimination whereas the intermediate formed with cuprous ions decomposes via p-amine elimination (102). 

An aqueous aliquot of the amino acid eluate from the ion-exchange column, or standard, is put in a microreaction vial such as the Reacti-Vial (Pierce Chemical), and evaporated just to dryness using dry nitrogen and a 100°C sand bath. To assure removal of water, add 100 yl of CH2Cl2 to vial and the mixture dried. The process is then repeated. It is important not to heat the tube too long when dry, as some sample decomposition may occur. 

In connection with a study of the electron impact-induced fragmentations of 1,2,3-thiadiazoles, the mass spectrum of 5-phenylthiatriazole has been scrutinized.13 Jensen et al.1 have undertaken a detailed investigation including 5-aryl-, 5-amino-, and 5-alkylthiothiatriazoles. The electron impact-induced decompositions resemble the pyrolytic loss of N2S (Section III, A). In all cases the M—N2S ion together with its fragmentation is responsible for the major part of the total ion current. A detailed discussion of the spectra is outside the scope of this review. 

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