25th International Meeting on Organic Geochemistry IMOG 2011

O-19
The borolithochromes: boron-containing organic pigments from
a Jurassic red alga
Klaus Wolkenstein 1 , Jürgen H. Gross 2 , Heinz Falk 3
1 Staatliches Museum für Naturkunde Stuttgart, Rosenstein 1, 70191 Stuttgart, Germany, 2 Institute of
Organic Chemistry, University of Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany,
3 Institute of Organic Chemistry, Johannes Kepler University Linz, Altenbergerstraße 69, 4040 Linz, Austria
(corresponding author:klaus.wolkenstein@smns-bw.de)
The distinct pink colouration of specimens of the fossil
calcareous red alga Solenopora jurassica from the
Jurassic of Great Britain and France (Fig. 1) is well
known since the early 20th century [1]. Due to the
conspicuous colour of the fossils the former
occurrence has even been termed ―Beetroot Stone‖.
Previous reports suggested that the pigments may be
porphyrins [1] or polycyclic quinone pigments
(fringelites) [2]. However, detailed chemical analysis
of the pigments using high-performance liquid
chromatography–diode array detection–electrospray
ionization–mass spectrometry (HPLC-DAD-ESI-MS),
Fourier transform ion cyclotron resonance mass
spectrometry (FT-ICR-MS) and
11 B NMR
spectroscopy has now led to the discovery of a novel
class of complicated boron-containing organic
pigments, the borolithochromes [3].
The highly unusual pigments consist of several series
of homologues and their isomers, the main
compounds with the molecular formulae C50H36O12B,
C51H38O12B and C52H40O12B. Solvolysis and
hydrogen/deuterium exchange experiments monitored
by HPLC-DAD-ESI-MS revealed that the pigments
constitute a unique group of spiroborates (boric acid
esters) with two phenolic moieties as boron ligands
(Fig. 1). Astonishingly, the fossil pigments still show
distinct chiroptical properties, indicating that at least
the subchromophores (ligands) are of biological
origin.
The borolithochromes are exceptional due to the
occurrence of the element boron, which is rarely
found in natural products. The fossil compounds
represent the first boron-containing organic pigments
from a natural source at all. Moreover, the
borolithochromes are the first fossil boron-containing
organic compounds and the first specific pigments
from a fossil red alga. However, no analogy with
present-day red algal pigments or other known
biomolecules from any living organism was found.
The presence of the borolithochromes in Solenopora
specimens of different stratigraphic and geographic
origin suggests a widespread occurrence of the
pigments. Because the classification of fossil
calcareous red algae is partly controversial, due to
morphological similarities with other groups of
organisms such as chaetetid sponges, the specific
borolithochromes or their possible diagenetic products
in the fossil record may be of chemotaxonomic
importance.
Fig. 1. Specimen of Solenopora jurassica with preservation
of fossil boron-containing organic pigments
(borolithochromes) and chemical structure of the main single
isomeric borolithochrome (C50H36O12B).
References
[1] Harland, T.L., Torrens, H.S. (1982) Palaeontology 25,
905-912.
[2] Falk, H., Mayr, E., Richter, A.E. (1994) Mikrochim. Acta
117, 1-5.
[3] Wolkenstein, K., Gross, J.H., Falk, H. (2010) Proc. Natl.
Acad. Sci. USA 107, 19374-19378.
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