Structures, NMR Spectroscopic Features, and Cytotoxic Properties of Oligomeric Hellinoyl (m-GO- m-GOG)-Type Ellagitannins from the Galls of Tamarix aphylla

Ellagitannin oligomers are large molecules habitually showing complex NMR spectra that are sometimes misinterpreted and lead to incorrect structures. Understanding the NMR spectroscopic features of a group of ellagitannins would overcome these inadequacies. In this study, investigation of the galls of Tamarix aphylla led to the isolation of three new ellagitannin oligomers, phyllagallins T1 (1), T2 (2), and Q1 (3), a known monomer nilotinin M4 (4), four known dimers, nilotinins D7 (5) and D8 (6), hirtellin B (7), and tamarixinin A (8), and a simple phenolic, dehydrotrigallic acid (9). 1D and 2D NMR, HRESI-TOFMS, and ECD experiments show that compounds 1-8 are hellinoyl-type ellagitannins. The NMR spectroscopic features of this type of ellagitannins and the reasons for the abnormal upfield shifts of glucose anomeric proton and hellinoyl moiety proton signals are established considering the experimental results as well as quantum chemical calculation on a simple hellinoyl-type monomer, phyllagallin M2. Based on these results, the NMR assignments reported previously by a different research group for bracteatinin T₁ and hirtellin T₃ are revised. A cytotoxicity study against human oral squamous cell carcinoma cell lines (Ca9-22, HSC-2, and HSC-4) and human mesenchymal normal oral cells (HGF, HPC, and HPLF) showed cytotoxic effects with tumor-specificity higher than 5.2, 3.0, 1.6, and 2.0 for compounds 5, 2, 9, and 3, respectively.