Naturaly occurring nanomolecules, tannins - Their structures and functions -

Abstract

Large-size secondary metabolites with dimeric-oligomeric and/or dendrine-type structure from higher plants are regarded as natural nanomolecules which are constructed in a bottom-up pathway from small molecules under biological condition. Among them are polyphenols classified as tannins which have large molecular weights ranging from 500 to 4000, and strong affinity to proteins, alkaloids and heavy metals to form complex molecules (precipitates). They constitute two major groups: one is condensed tannins (proantocyanidins), which are composed of flavan-3-ol units linked through C-C bond, and the other is hydrolyzable tannins, which are principally multiple esters of D-glucose with gallic acid and its oxidative metabolites. This paper reviews the diverse structural characteristics of both tannins, especially ellagitannin oligomers up to pentamers, and examples of their biological activities, formation of insoluble and soluble complex between tannins and proteins are also described.