Novel Polycyclic Sulfides Derived from Regular Polyprenoids in Sediments: Characterization, Distribution, and Geochemical Significance

Abstract

A novel series of di- to pentacyclic polyprenoid sulfides ( 6, 7, 15, 16) which are structurally related to the recently identified highly cyclised regular polyprenoid aromatic ( 1, 2, 3) and saturated hydrocarbons (5) and to the widely distributed tricyclopolyprenanes (e.g., 10) have been characterized in a wide range of sediments. Their distributions, extending from C 13 to C 30, are clearly dominated by the C 15, C 20, C 25 and C 30 homologues, each being represented by several isomers. Two C 30 isomers ( 6a and 6b), which are generally predominant, were isolated from a sample from Sémecourt (Paris basin, France) and their structure established by NMR studies. Interestingly, these two isomers are the sole sulfides present in several samples. The structure of two C 20 isomers ( 7a, 7b) was confirmed by synthesis of standards. These new sulfides clearly derive from precursors resulting from the incomplete cyclization of regular polyprenoids (e.g., 4). The presence of functionalities both on the side chain and, probably in the last ring, allowed intramolecular sulfur incorporation by a process similar to that leading to the formation of low molecular weight sulfides in sediments. Detection of these new polyprenoid sulfides in recent sediments indicates that the sulfurization step occurs at an early stage of diagenesis. The hydrocarbon skeletons of the lower di-, tri- and tetracyclic sulfides may derive from widespread terpenoids present in various organisms. The predominant pentacyclic homologues, however, have a rare and specific carbon skeleton which can not be related to any known biological precursor. The fact that these lower homologues always co-occur with the pentacyclic sulfides points to a common and specific origin for the whole series. Their origin seems, however, distinct from that of the related monoaromatic and saturated regular polycyclic polyprenoid hydrocarbons ( 1-3, 5) which are absent from the samples where sulfides occur. The carbon isotopic composition of two C 30 pentacyclic homologues ( 6a, 6b) could be determined in two samples and suggests that their polyprenoid precursors originate either from primary producers or from heterotrophic organisms living in the oxic part of the water column.