Structure elucidation and antioxidant activity of the radical-induced oxidation products obtained from procyanidins B1 to B4.

A- and B-type procyanidins (PCs) are widely known for their health-promoting properties, such as antioxidant activity. The limited availability of reference substances represents a major challenge, resulting in a low number of systematic studies on their health benefits. In our study, the optimised 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-induced oxidation of the B-types B1 and B2 was carried out yielding the corresponding A-types A1 (1) and A2 (2), which have an additional ether bridge between C2--C7, whereas oxidation of B3 and B4 produced a six-membered spirocyclic ring system including a spiro-carbon atom at C8t (3-5). The compounds (1-5) were fully characterised by combining circular dichroism (CD) and low-temperature NMR spectroscopy at 253.0 K in acetone- including one-dimensional (1D; H and C NMR) and two-dimensional (2D; H-H COSY, H-H TOCSY, H-C HSQC, H-C HMBC, H-H ROESY) NMR experiments. The collision cross section (CCS) values from the trapped ion mobility spectrometry (TIMS) and time of flight mass spectrometry analysis (TOF-MS) allowed a rapid differentiation of the dimeric A-types, C4→C8-linked B-types, and spiro-linked PCs in combination with their retention times. A comparative analysis of the antioxidant properties of the A-, B-types, and spiro-linked PCs was conducted for the first time. The spiro-linked PCs (3-5) were found to have an equal or even higher antioxidant activity compared to the A-types A1 (1) and A2 (2) and the comparative analysis of the different procyanidin types revealed new insights into their antioxidant properties.