A tandem mass spectrometric investigation of N‐(3‐ferrocenyl‐2‐naphthoyl) dipeptide ethyl esters and N‐(6‐ferrocenyl‐2‐naphthoyl) dipeptide ethyl esters

Abstract

N-(3-Ferrocenyl-2-naphthoyl) dipeptide ethyl esters 1-4 and N-(6-ferrocenyl-2-naphthoyl) dipeptide ethyl esters 5-8 were prepared by coupling either 3-ferrocenylnaphthalene-2-carboxylic acid or 6-ferrocenylnaphthalene-2-carboxylic acid to the dipeptide ethyl esters GlyGly(OEt) (1, 5), AlaGly(OEt) (2, 6), GlyPhe(OEt) (3, 7) and GlyLeu(OEt) (4, 8), using the standard N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride, 1-hydroxybenzotriazole protocol. Electrospray ionization mass spectrometry (ESI-MS) and laser desorption ionization mass spectrometry (LDI-MS) were employed in conjunction with tandem mass spectrometry in the analysis of N-(3-ferrocenyl-2-naphthoyl) dipeptide ethyl esters 1-4 and N-(6-ferrocenyl-2-naphthoyl) dipeptide ethyl esters 5-8. Radical cations, [M](+•) and [M + H](+) species were both observed in the mass spectra. Intense sodium [M + Na](+) and potassium [M + K](+) adducts were also present. An important diagnostic ion at m/z [M-65](+) was observed in both the MS and MS/MS spectra of the N-(3-ferrocenyl-2-naphthoyl) dipeptide derivatives. Sequence-specific ions were generally not observed in the MS/MS spectra of the N-(3-ferrocenyl-2-naphthoyl) series due to formation of the diagnostic [M-65](+) ion. Sequence-specific ions were observed in the MS/MS spectra of the N-(6-ferrocenyl-2-naphthoyl) dipeptide esters with charge retention on the derivatized N-terminal of the dipeptide. Both series of compounds could be successfully analyzed by MALDI without the use of a matrix (LDI).