α-Glucosidase Inhibitory Activities and the Interaction Mechanism of Novel Spiro-Flavoalkaloids from YingDe Green Tea

Abstract

Flavoalkaloids are a unique class of compounds in tea, most of which have an <i>N</i>-ethyl-2-pyrrolidinone moiety substituted at the A ring of a catechin skeleton. 1-Ethyl-5-hydroxy-pyrrolidone, a decomposed product of theanine, was supposed to be the key intermediate to form tea flavoalkaloids. However, we have also detected another possible theanine intermediate, 1-ethyl-5-oxopyrrolidine-2-carboxylic acid, and speculated if there are related conjugated catechins. Herein, four novel spiro-flavoalkaloids with a spiro-γ-lactone structural moiety were isolated from Yingde green tea (<i>Camellia sinensis</i> var. <i>assamica</i>) in our continuing exploration of new chemical constituents from tea. The structures of the new compounds, spiro-flavoalkaloids A-D (<b>1</b>-<b>4</b>), were further elucidated by extensive nuclear magnetic resonance (NMR) spectroscopy together with the calculated ¹³C NMR, IR, UV-vis, high-resolution mass, optical rotation, experimental, and calculated circular dichroism spectra. We also provided an alternative pathway to produce these novel spiro-flavoalkaloids. Additionally, their α-glucosidase inhibitory activities were determined with IC₅₀ values of 3.34 (<b>1</b>), 5.47 (<b>2</b>), 22.50 (<b>3</b>), and 15.38 (<b>4</b>) μM. Docking results revealed that compounds <b>1</b> and <b>2</b> mainly interacted with residues ASP-215, ARG-442, ASP-352, GLU-411, HIS-280, ARG-315, and ASN-415 of α-glucosidase through hydrogen bonds. The fluorescence intensity of α-glucosidase could be quenched by compounds <b>1</b> and <b>2</b> in a static style.