Higher plant triterpenoids bound in resistant macromolecules in extant and Pliocene fossil Liquidambar fruits

Abstract

Ester-bound compounds (compounds released by saponification) from the resistant macromolecules (RMMs) were analyzed in the extant and Pliocene fossil Liquidambar fruit from the Tokai Group distributed in Gifu Prefecture, central Japan. We compared the distributions of the ester-bound triterpenoids from the fossil with those of free and ester-bound triterpenoids of the extant fruits. The series of triterpenoid acids such as oleanolic and ursolic acids were identified in free compounds but hardly detected in ester-bound compounds from the extant Liquidambar samples. Most of free triterpenoids identified in the extant Liquidambar were also present in the fossil Liquidambar sample as the ester-bound compounds. These results indicate the occurrence of post-depositional incorporation of the free triterpenoids into the macromolecules such as the RMMs via ester bonds. Thus, it is suggested that the ester-bound triterpenoids in RMMs serve as excellent archives of the indigenous assemblage of triterpenoids. The class distributions of the oleanoids and ursanoids in the ester-bound compounds from the fossil sample are considerably different from those in the free polar compounds of the extant samples. Selective preservation of C-28 carboxyl group compared to C-3 oxygenated functions indicates that the triterpenoid acids were incorporated into RMM via ester-bound formed between C-28 carboxyl and hydroxyl groups of the RMM. These results indicate that the ester-bound triterpenoid acids obtained from the fossil sample are selectively preserved by incorporation into the RMM of the fruits.