Structure elucidation of 4-carboxy-3-deoxyanthocyanidins formed from thermal degradation of hydroxyphenyl-pyranoanthocyanins

Abstract

Hydroxyphenyl-pyranoanthocyanins are vibrantly colored, anthocyanin-derived polyphenols found in some wines and aged juices. Upon heating (90 °C), solutions of 10-catechyl-pyranoanthocyanins (a type of hydroxyphenyl-pyranoanthocyanins) formed a unique, color producing degradation compound. This compound was produced in the greatest quantities at pH 1 and pH 3 where its formation contributed to retention of color intensity despite extensive heating conditions (up to 34 hours). We elucidated the structure of this colored compound and investigated if other hydroxyphenyl-pyranoanthocyanins produced similar colored thermal degradation products. Analysis of the isolated 10-catechyl-pyranoanthocyanin degradation compound by uHPLC-PDA, high resolution MS/MS (QTOF), FTIR, 1D and 2D NMR (¹H, COSY, HMBC, HSQC), spectrophotometry, and color analysis revealed that it was a 4-carboxy-3-deoxyanthocyanidin with a catechol substitution, named 4-carboxy-3-deoxycyanidin. It produced an exact mass of 315.0502 consistent with a chemical formula of C₁₆H₁₁O₇⁺. NMR allowed for structural assignments, and key FTIR discriminating bands supported the presence of aromatic rings and the carboxylic acid moiety. The color of 4-carboxy-3-deoxycyanidin transitioned from yellow (λ_vis-max = 479 nm) at pH 1 to magenta (λ_vis-max = 545 nm) at pH 9.2, analogous with a flavylium system. Similarly, unique, colored degradation compounds formed from three other hydroxyphenyl-pyranoanthocyanins (10-p-hydroxyphenyl-, 10-guaiacyl-, and 10-syringyl-pyranoanthocyanins) each with a m/z 398 units lower than their parent pyranoanthocyanin. These findings revealed the formation of a unique class of pyranoanthocyanin derived flavylium compounds: 4-carboxy-3-deoxyanthocyanidins.