Semisynthesis and spectral characterization of 5-methylpyranopelargonidin and 4-methylfuropelargonidin and their separation and detection in strawberry fruit wine

Abstract

Condensation of anthocyanins and their aglycons with small organic molecules yields more stable natural dyes, e.g. pyranoanthocyanins arising spontaneously in various food products. Reaction of pelargonidin with acetone produces two isomeric anthocyanidin dyes − 5-methylpyranopelargonidin and 4-methylfuropelargonidin. A robust semipreparative liquid chromatographic method was developed to isolate both derivatives from a simple aged solution of pelargonidin in methanol: acetone: 37% aqueous hydrochloric acid (1:1:0.008, v/v/v). Detailed interpretation of mass and nuclear magnetic resonance spectra allowed to assign structures of both dyes in isolated fractions. A fast UHPLC–MS method was optimized for the control of their production in the reaction mixture. Use of reversed stationary phase and acidic mobile phases in gradient mode allowed separation of both isomers in less than one minute. Fragmentation of both dyes after collision activated dissociation in collision cell was studied comprehensively and the observed processes were compared with data from quantum calculations (computational chemistry utilizing DFT methods). When comparing both isomers, retro-Diels-Alder fragmentation appears preferred in furo-derivative, while small losses (i.e. methane and water molecules) were more pronounced in pyrano-derivative. Both studied isomeric dyes were found in laboratory prepared strawberry fruit wine and their content was compared with major present anthocyanins and their derivatives.