Direct identification of intramolecular disulfide links in peptides using negative ion electrospray mass spectra of underivatised peptides. A joint experimental and theoretical study.

[M--H]- parent anions of underivatised peptides containing an intramolecular disulfide bridge undergo characteristic loss of the elements of H2S2, a process diagnostic of the presence of the disulfide moeity. This facile process is initiated from a side-chain enolate anion. Theoretical calculations (at the HF/6-31G(d)//AM1 level of theory) indicate that the process is exothermic with a small barrier.

Cyclization of N,N-dialkylditiocarbamate and alkylxanthate derivatives of polyhalogenated pyridines in gas and liquid phases.

The intramolecular cyclization of 31 polyhalogen substituted pyridines containing N,N-dialkyldithiocarbamate or alkylxanthate groups has been compared in reaction in solution with sodium N,N-dialkyldithiocarbamates or potassium carbetoxydithiolate and in the gas phase under electron ionization (EI). The scheme of fragmentation of the studied compounds has been proposed. An influence of the nature of leaving groups (Cl, F, CF(3), CN, COOEt), of the presence of electron withdrawing groups (Cl, F, CN, CCl(3), CF(3), COOEt) in ortho-,meta or para-positions to the leaving halogen, of the position of a dithio group toward pyridine nitrogen atom and of the role of oxygen and nitrogen in corresponding alkylxanthates and N,N-dialkyldithiocarbamates on the cyclization process has been investigated.

Cyclization of ortho-cyclopropylphenyl benzamides in gas and liquid phases.

The electron ionization (EI) and collision-induced dissociation (CID) spectra of substituted N-(ortho-cyclopropylphenyl)benzamides 1-7 and N-[ortho-(1-methylcyclopropyl)phenyl]benzamides 8-12 were recorded. In addition to routine bond cleavages, the molecular ions (M+) of 1-12 undergo cyclization into the corresponding 3-aryl-1-alkyl-1-ethyl-1H-benzoxazines and isomeric 5-ethyl-2-oxodibenzoazepines. The presence of a methyl group in the cyclopropyl ring (compounds 8-12) makes the formation of 5-ethyl-2-oxodibenzoazepine less favorable.