Evidence of structure-selective fragmentations in the tandem mass spectra of protonated hydroxyalkylamino-1,4-naphthoquinones formed by electrospray ionisation.

The collision-induced dissociation of protonated hydroxyalkylamino-1,4-naphthoquinones depends strongly on the structure of the substituent [NHCH(2)(CH(2))(n)OH, n = 1-5; or NHCH(2)CH(CH(3))OH] on the quinone ring. Protonated naphthoquinones with an unbranched hydroxypropylamino side chain (n = 3) undergo facile and characteristic CH(2)O loss, whereas isomeric [M + H](+) ions with a branched hydroxypropylamino side chain do not. When n = 1, CH(2)O elimination occurs less readily, accompanied by CH(3)N loss, thus allowing this shorter side chain to be identified. Higher homologous species (n = 3-5) do not expel CH(2)O, but instead eliminate C(n + 1)H(2n)O, C(n + 1)H(2n + 2)O and (for n = 5) C(2n + 1)H(2n + 1)O.