6-Amidopyrene as a label-assisted laser desorption/ionization (LA-LDI) enhancing tag: development of photoaffinity pyrene derivative

Pyrene-conjugated compounds are detected by label-assisted laser desorption/ionization mass spectrometry (LA-LDI MS) without matrixes. We found that 6-amidopyrene derivatives were highly detectable by the LDI MS instrument equipped with a 355 nm laser. In a certain case of a 6-amidopyrene derivative, a molecular ion peak [M](+•) and a characteristic fragment ion peak [M-42](+•) were detected in an amount of only 10 fmol. The latter peak, corresponding to the 6-aminopyrene fragment, might be generated in situ by the removal of ketene (CH2=C=O) from the parent molecule. A photoaffinity amidopyrene derivative of an antitumor macrolide aplyronine A (ApA-PaP) was synthesized, which showed potent cytotoxicity and actin-depolymerizing activity. In an LDI MS analysis of the MeOH- and water-adducts of ApA-PaP, oxime N-O bonds as well as amidopyrene N-acetyl moieties were preferentially cleaved, and their internal structures were confirmed by MS/MS analysis. Amidopyrene moiety might enhance fragmentation and stabilize the cleaved fragments by intramolecular or intermolecular weak interactions including hydrogen bonding. Our chemical probe methods might contribute to a detailed analysis of binding modes between various ligands and target biomacromolecules that include multiple and weak interactions.