Carboxylic acids mass spectra

B. Method for determining the number of —COOH groups in a molecule The TMS derivative of an acid can be converted to the methyl ester using anhydrous methanolic HQ. 1 2 3 Obtain a mass spectrum of the TMS derivative of the acid, and then evaporate the TMS reaction mixture with clean, dry nitrogen. Add 250 pd of anhydrous methanolic HC1 (Pierce cat. no. 33050) and heat at 60° for 20 min. Many TMS derivatives of acids are converted to methyl esters at room temperature after 20 min. If the sample is rerun as the methyl ester, the number of carboxyl groups can be determined by the mass differences before and after making the methyl ester from the TMS derivative. 

Examination of the mass spectrum of n-decanoic acid (Figure 4.1) shows prominent ions at m/z 60 and 73. A m/z 60 ion (Section III) (see also Appendix 10) suggests the mass spectrum may represent an aliphatic carboxylic acid. This ion in combination with m/z 73... 

Figure 5.12 GALDI mass spectrum of shellac (from methanol solution). Peaks at m/z—570 are related to esters of aliphatic hydroxy acids with sesquiterpenoid carboxylic acids (see text and Table 5.4). Signals marked with crosses are contaminants in the spectrometer that accumulated over time (m/z 413, 469, and 507) peaks marked ( ) are contaminating graphite clusters from the matrix (m/z 264, 276, 288).

In addition to the natural waxes, a number of synthetic waxes were analysed with GALDI-MS carboxylic acids (Hoechst wax-L, -S), esters (Hoechst wax-FL, -H, -O), and glycol esters (Hoechst wax-KP, -KPS). In most cases, significant mass spectra were obtained. The mass spectrum of polyethylene glycol (Carbowax 6000) showed a series of signals the spacing of m/z 44 between the signals corresponds to one oxyethylene unit. 

There are at least three possibile ways in which the inhibitor can bind to the active site (1) formation of a sulfide bond to a cysteine residue, with elimination of hydrogen bromide [Eq. (10)], (2) formation of a thiol ester bond with a cysteine residue at the active site [Eq. (11)], and (3) formation of a salt between the carboxyl group of the inhibitor and some basic side chain of the enzyme [Eq. (12)]. To distinguish between these three possibilities, the mass numbers of the enzyme and enzyme-inhibitor complex were measured with matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI). The mass number of the native AMDase was observed as 24766, which is in good agreement with the calculated value, 24734. An aqueous solution of a-bromo-phenylacetic acid was added to the enzyme solution, and the mass spectrum of the complex was measured after 10 minutes. The peak is observed at mass number 24967. If the inhibitor and the enzyme bind to form a sulfide with elimination of HBr, the mass number should be 24868, which is smaller by about one... 

The spectra of 2,3 -bipyridine, ° 2,2 6, 2"-terpyridine/ nem-ertelline (11)/° some substituted 2,3 -bipyridines/ ° and 4,4 -bipyridines ° have been briefly discussed, and the fragmentation patterns of hydroxy-and alkoxy-2,2 -bipyridines, ° 2,2 -bipyridine-5-carboxylic acid, 2,2 -bipyridine-5-sulfonic acid, and 6-halogeno-2,2 -bipyridines ° have been covered in detail. The doubly charged ion mass spectrum of 2,2 -bipyridine has recently been reported. °... 

Carboxylic acids. Monocarboxylic acids normally show the molecular ion in the spectrum. Cleavage of bonds adjacent to the carbonyl group (a-cleavage) results in formation of fragments of mass M — 17 (OH) and M — 45 (C02H). Characteristic peaks arise from the McLafferty rearrangement. 

Another new alkaloid (50) was also isolated from P. heterophylla. Mild acid hydrolysis of (50) gave a desacetyl compound which showed a prominent peak due to the loss of —CH2C02CH3 in its mass spectrum, thereby indicating that the other carboxyl group is esterified to the base. Vigorous acid hydrolysis of (50) gave 1-hydroxymethylpyrrolizidine of undefined stereochemistry together with a neutral compound which yielded (-)-2-isopropylmalic acid on basic hydrolysis. 

In the mass spectrum of phenazine an intense molecular ion at w/z 180 with significant contributions from the doubly charged molecular ion is observed fragmentations by loss of CN, HCN and C Hj (M-27) are characteristic. The mass spectrum of phenazine and isomeric hydroxy- and methoxyphenazine-1-carboxylic acids have been investigated in detail. Phenazine A -oxide shows an M O ion together with an intense M—CjHj ion and M-CO ion (w/z 168). ... 

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