Structural Composition and Protein Precipitation Capacity of Condensed Tannins from Purple Prairie Clover (Dalea purpurea Vent.)

Condensed tannins (CT) from purple prairie clover (PPC, Dalea purpurea Vent.) have shown anti-Escherichia coli O157:H7 activity in vitro and in vivo, but the composition and structure of these CTs are unknown. The objectives of this study were to determine the concentration and composition of CT in different tissues of PPC at differing maturities and to determine their protein-precipitating capacity. Condensed tannins were extracted from leaf, stem and flower tissues of PPC harvested at vegetative (VEG), early flowering (EF) and late flowering (LF) stages. The structural compositions of CT were elucidated by in situ thiolytic degradation and liquid-chromatography-mass spectrometry (HPLC-MS). The general composition of the CTs in PPC obtained from thiolytic degradation were corroborated by ¹H-¹³C HSQC NMR spectroscopy. Protein–precipitating capacity of isolated CT was determined by their ability to precipitate bovine serum albumin (BSA) and ribulose 1,5-disphosphate carboxylase (Rubisco). Flowers contained the highest CT concentration followed by leaf and stem tissues, respectively. As the plant matured, CT concentration decreased in stem, but remained relatively constant in leaf and flower tissues. Regardless of plant tissue or maturity, PPC CT consisted mostly of epicatechin (EC; 70-80%) and epigallocatechin (EGC; 20-30%), with minor amounts of catechin (C) and gallocatechin (GC). Leaf CT consisted of more EC (P<0.01) but less (P<0.01) EGC units than CT from stem and flowers at both EF and LF stages. The procyanidin (PC)/prodelphinidin (PD) ratio (PC/PD) was greater (P < 0.05) for leaf CT than for stem and flower CT. The mean degree of polymerization (mDP) was highest for stem CT and lowest for leaf CT. The PC/PD ratio of stem CT decreased and the mDP increased as the plant matured. Leaf CT had higher BSA-precipitating capacity than stem and flower CT (P<0.001), with EF leaf CT exhibiting the highest capacity (P<0.001). Leaf and flower CT had a greater ability to precipitate Rubisco protein than stem CT (P<0.05). These results demonstrated that PPC CT were predominantly of the procyanidin-type and the concentration and chemical structure varied with different plant tissues and growth periods. Condensed tannins in PPC at the early flower stage exhibited the greatest protein-precipitation capacity.