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Highly Branched Isoprenoids
HBIs: an introduction

The abbreviation ‘HBI’ is used by organic geochemists to denote Highly Branched Isoprenoid compounds. These are most commonly hydrocarbons comprising 20, 25 or 30 carbon atoms; some other hydrocarbons and derivatives are sometimes included. The compounds occur in the geosphere mostly as alkanes, as alkenes with one to at least six degrees of unsaturation, or as thiophenes. The latter are thought to be derived from diagenetic reaction of sulphur species with the alkenes. Over fifty different HBI compounds have been identified; some tentatively, many rigourously. Many of the parent alkanes can be considered to be prenylated forms of the common acyclic isoprenoid alkane, farnesane (2,6,10-trimethyldodecane) which is well known due to its common occurrence in petroleum, but unlike farnesane, the HBI hydrocarbons have a further distinctive branch point at C-7.

The widespread occurrence, variable distributions, biological sources and possible geochemical applications of these compounds have fascinated organic geochemists for over thirty years (cf Gearing et al., 1974; Peters et al., 2005). As a result, in 1990 Steven Rowland and John Robson considered it pertinent to review the state of knowledge of HBI research following the synthesis and identification of C25 and C30  parent HBI alkanes and re-synthesis of the C20 (1a) HBI alkane (first synthesized by Yon et al., 1982) by John Robson during his PhD studies at Plymouth Polytechnic U.K (Robson and Rowland, 1986; Robson 1987).

Since the time of that review, further progress in HBI identification, determination of HBI source organisms, elucidation of biosynthetic mechanisms and applications of the uses of HBIs in geochemistry has been made. Whilst these topics have been briefly reviewed in many of the individual research papers and most recently in a book (Peters et al., 2005) it is the intention of the present review to attempt to collate much of this (and some new) information so that it may act as a useful reference source for researchers interested in HBIs.

Use of the term HBI appears in the literature at least as early as 1987 (Sinninghe Damste et al., applied to HBI thiophenes) and generally succeeded the use of other abbreviations following the elucidation of the structures of several of the compounds (e.g. Yon et al., 1982; Robson and Rowland, 1986; 1988) following many years of uncertainty about the identities. The unknown nature of many of the compounds prior to this usually necessitated the use of either gas chromatographic retention indices (RI) or terms such as br (indicating ‘branched’) and c (indicating ‘cyclic’, sic) or both, to delineate the different compounds (e.g. Barrick and Hedges, xxxx). The latter method was employed by Rowland and Robson, for example in their 1990 review of what were otherwise often termed ‘highly branched hydrocarbons’.

Following the identification of the source organisms of several of the C25 and C30 alkenes as microscopic diatomaceous algae of the genera Haslea and Rhizosolenia (Volkman et al., 1994), including axenic (bacteria-free) cultures of Haslea ostrearia (Wraige et al., xxxx), the names haslenes (haslane for the alkane) and rhizenes (rhizane for the alkane) have been proposed for the C25 and C30 compounds respectively (Rowland et al., 2001a, b). Although to date these names have not been widely adopted, they have the advantage of specificity over the more general term ‘HBI’. Therefore the present review uses terms such as ‘hasl-5-ene’ to indicate a C25 HBI monoene with a double bond at position 5 and so on, whereas for example, for C35 compounds of unknown origin and as yet undetermined double bond positions, less specific terms like ‘C35 HBI diene’ will be used.