Radical-induced oxidation of dimeric B-type procyanidins

A balanced diet is essential for human health, especially the intake of fruits and vegetables that contain phytochemicals such as A‑ and B‑type procyanidins (PCs). These PCs are known for their antioxidant, anti‑inflammatory, and other health‑promoting properties. One approach to increase the yield of A‑type PC is the radical‑induced oxidation, for example, of B‑types B1 and B2, to produce the corresponding A‑types A1 and A2. In this dissertation, a Design of Experiments (DoE) approach was applied to optimise the reaction conditions of the radical-induced oxidation of B‑type PCs, achieving formation rates of 47.4 ± 1.14% for A1 from B1 and 27.5 ± 0.76% for A2 from B2. Furthermore, the systematic oxidation of all C4→C8 linked dimeric PCs (B1–B4) showed that the 4R‑configured dimers B1 and B2 primarily formed A‑type PCs with 2,2‑diphenyl‑1-picrylhydrazyl (DPPH) radicals, while the 4S‑configured dimers B3 and B4 produced spiro‑linked PCs as main oxidation products. The oxidation of B3 and B4 each resulted in two main oxidation products (product 1 and product 2). The optimal formation of product 1 required a short reaction time at room temperature (10.0 min and 25.0 °C), with optimal formation rates of 48.6 ± 4.01% from B3 and 45.0 ± 5.14% from B4. In contrast, the formation of product 2 required higher temperatures and longer reaction times (314 min and 75.0 °C for B3; 360 min and 53.7 °C for B4) to achieve optimal formation rates of 32.0 ± 1.14% from B3 and 60.2 ± 3.68% from B4. Countercurrent chromatography (CCC) proved to be a powerful tool for separating a complex oxidised mixture of B2 and B4. The two‑dimensional coupling of high‑speed CCC (HSCCC) with LC‑ESI‑MS (HSCCC x LC‑ESI‑MS) represented an effective method for analysing the elution profile of the formed oxidation metabolites while simultaneously detecting co‑elution effects of isomeric compounds. To elucidate the structures of the main oxidation products from B1 to B4, one‑ and two‑dimensional NMR experiments were carried out at low temperatures (253 K) in acetone‑d6. Additionally, circular dichroism spectroscopy and ion mobility mass spectrometry were applied. The antioxidant activity of the A‑types A1 and A2, the B‑types B1–B4, and the three isolated spiro‑linked oxidation products from B3 and B4 were investigated regarding their radical scavenging and reducing properties. The spiro‑linked oxidation products from B3 and B4 exhibited similar or even higher antioxidant activity than the A‑types, depending on the assay. Regarding their health‑promoting properties, both A‑type PCs and the spiro‑linked compounds are of interest for further investigation.