Hydrocarbon humps in the marine environment: Synthesis, toxicity, and aqueous solubility of monoaromatic compounds

Article abstract of DOI:10.1002/etc.5620201105

A recent study has shown that some monoaromatic hydrocarbon constituents of the so-called unresolved complex mixtures (UCMs) or gas chromatographic humps, which are widespread in the marine environment, are toxic to the mussel Mytilus edulis. Here we describe the synthesis and toxicological assessment of 6-cyclohexyltetralin, 7-cyclohexyl-1-methyltetralin, and 7-cyclohexyl-1-n-propyltetralin, which contain structural features consistent with some monoaromatic UCM hydrocarbons. The compounds were all toxic to M. edulis when measured in the assay used previously to determine the toxicity of a monoaromatic UCM. The aqueous solubilities of the hydrocarbons in fresh and seawater at different temperatures were determined and found to range from about 10 to 110 μg/L (10-60 μg/L in seawater at 15°C). Further studies of the aromatic UCM composition of a wide range of oils and oil residues are required to determine whether such alkylated compounds as 7-cyclohexyl-1-methyltetralin and 7-cyclohexyl-1-n-propyltetralin or their analogues are widespread in oils. If these aromatic compounds prove to be important in UCMs, toxicity experiments should be conducted with other biological end points and monitoring studies of pollutant hydrocarbons should probably include measurement of aromatic UCM hydrocarbons.

Full text of DOI:10.1002/etc.5620201105

Environmental Toxicology and Chemistry, Vol. 20, No. 11, pp. 2428–2432, 2001
2001 SETAC
Printed in the USA
0730-7268/01 $9.00 .00
᭧
ϩ
HYDROCARBON HUMPS IN THE MARINE ENVIRONMENT: SYNTHESIS, TOXICITY,
AND AQUEOUS SOLUBILITY OF MONOAROMATIC COMPOUNDS
E
MMA SMITH, EMMA WRAIGE, PETER DONKIN, and STEVEN ROWLAND*
Department of Environmental Sciences, Plymouth Environmental Research Centre, University of Plymouth, Drake Circus,
Plymouth PL4 8AA, United Kingdom
(
Received 27 November 2000; Accepted 28 March 2001)
Abstract—A recent study has shown that some monoaromatic hydrocarbon constituents of the so-called unresolved complex mixtures
(UCMs) or gas chromatographic humps, which are widespread in the marine environment, are toxic to the mussel Mytilus edulis.
Here we describe the synthesis and toxicological assessment of 6-cyclohexyltetralin, 7-cyclohexyl-1-methyltetralin, and 7-cyclo-
hexyl-1-n-propyltetralin, which contain structural features consistent with some monoaromatic UCM hydrocarbons. The compounds
were all toxic to M. edulis when measured in the assay used previously to determine the toxicity of a monoaromatic UCM. The
aqueous solubilities of the hydrocarbons in fresh and seawater at different temperatures were determined and found to range from
about 10 to 110
and oil residues are required to determine whether such alkylated compounds as 7-cyclohexyl-1-methyltetralin and 7-cyclohexyl-
1- -propyltetralin or their analogues are widespread in oils. If these aromatic compounds prove to be important in UCMs, toxicity
␮g/L (10–60 ␮g/L in seawater at 15ЊC). Further studies of the aromatic UCM composition of a wide range of oils
n
experiments should be conducted with other biological end points and monitoring studies of pollutant hydrocarbons should probably
include measurement of aromatic UCM hydrocarbons.
Keywords—Hump
Hydrocarbons
Solubility
Alkyltetralins
Unresolved complex mixtures
INTRODUCTION
appears to be the case for pollutant aromatic UCMs in mussels
[4]. So-called retrostructural analysis of UCM composition has
suggested that, among these monoaromatics, alkyl-substituted
alkyltetralins may be important [7,9,10]. Thus, ruthenium te-
troxide oxidation of monoaromatic UCMs from several crude
oils produced carbon dioxide from unsubstituted ipso carbons,
Unresolved complex mixtures of hydrocarbons dominate
the gas chromatograms of the weathered residues of petroleum.
Such residues occur widely in the marine environment [1], yet
only a few studies of the toxicity of unresolved complex mix-
tures (UCMs) appear to have been published [2,3]. Although
nonaromatic UCM hydrocarbons appear to be nontoxic or to
contribute in only a minor way to the toxic hydrocarbon burden
of the mussel Mytilus edulis [2], we were recently able to
demonstrate that some monoaromatic UCM components were
toxic to mussels [4]. Furthermore, coastal mussels from the
United Kingdom with impaired health, as measured by Scope
for Growth, had substantial aromatic UCM burdens [4; Fig.
1]. Clearly, further studies of this pollutant burden are required
and the identities of the toxic components need to be estab-
lished.
Since the pioneering attempts of American Petroleum In-
stitute scientists to unravel the composition of Ponca City,
Oklahoma, USA, crude oil in the 1950s, comparatively few
studies of the major unresolved hydrocarbons of crude oil have
in fact been published [e.g., 1,5–10]. Instead, a detailed knowl-
edge of the chromatographically resolved hydrocarbons of pe-
troleum has been accrued [11]. Most recent studies of the
unresolved hydrocarbon composition have been based on the
principle of oxidative or pyrolytic degradation of UCM hy-
drocarbons followed by deuteration or derivatization of the
resulting UCM fragments and gas chromatography-mass spec-
troscopy [1,7] or ion cyclotron resonance-mass spectroscopy
[10] analysis of the resulting products. Many of these studies
have shown that, of the aromatic UCM compounds, the mon-
oaromatics are quantitatively important [5–7,9,10]. This also
straight chain carboxylic acids from n-alkyl substituents, and
branched and cyclic carboxylic acids from naphthenic rings
and branched chain substituents [5,7,9,10,12]. These data sug-
gest that monoaromatic rings with straight chain, branched,
and cyclic substituents are important in monoaromatic UCMs,
and alkyltetralins are an example of possible UCM compounds.
Since a recent study also indicated that the hydrocarbons of a
monoaromatic UCM isolated from a crude oil were somewhat
toxic to mussels, leading to a
Ͼ40% decrease in filtering rate
over 24-h accumulation of a UCM [4], we became interested
in the toxicity of alkyltetralins. In the present study, we syn-
thesized 6-cyclohexyltetralin, 7-cyclohexyl-1-methyltetralin,
and 7-cyclohexyl-1-n-propyltetralin by Haworth-type reac-
tions of phenylcyclohexane and tested their toxicity to the
mussel Mytilus edulis. The final hydrocarbon products were
isolated pure (Fig. 1) and the structures were confirmed by
spectroscopy. Although these compounds have yet to be iden-
tified as hydrocarbon entities of UCMs, they appear at least
to contain structural features consistent with the limited known
composition of monoaromatic UCM components [5–10,12].
The compounds were toxic to M. edulis, reducing the filtering
rate substantially compared with untreated controls. The aque-
ous solubilities of the hydrocarbons ranged from about 10 to
110
␮g/L, depending on molecular weight, temperature, and
salinity. The concentrations of the bioaccumulated monoaro-
matic hydrocarbon residues giving rise to these toxic effects
were within the range of aromatic UCM concentrations de-
tected in mussels from polluted environments. These data fur-
* To whom correspondence may be addressed
(srowland@plym.ac.uk).
2428

Hydrocarbon humps in the environment
Environ. Toxicol. Chem. 20, 2001
2429
Fig. 1. Gas chromatograms showing a typical unresolved complex mixture (UCM) of monoaromatic (four double bond equivalents) hydrocarbons
isolated from environmental mussels collected from Whitby, United Kingdom [13], and the three synthetic tetralins (I–III). *
ϭ internal standard
d12 tetralin, ⌬ ϭ biogenic tetraenes. Gas chromatography conditions are cited in the text and in Wraige [13].
ther suggest that monoaromatic UCM hydrocarbons may have
been overlooked as a pollutant class.
were prepared by adding different concentrations of the tox-
icant in 0.001% of acetone to filtered seawater (0, 12.5, 25,
50, and 100 ␮g/L toxicant). Solutions were stirred for 2 h prior
MATERIALS AND METHODS
to use. Controls comprising filtered seawater and acetone only
(0.001% v/v) were also prepared. Groups of seven mussels,
shell length 12 to 14 mm, were exposed to 1.4 L toxicant or
control solutions in glass beakers. Gentle water movement was
Synthesis and characterization of hydrocarbons
The 6-cyclohexyltetralin, 7-cyclohexyl-1-methyltetralin,
and 7-cyclohexyl-1-n-propyltetralin (I–III) were synthesized
by a Haworth-type reaction scheme (Fig. 2). Intermediates and
final products were characterized by mass spectrometry (MS),
1H and 13C nuclear magnetic resonance spectrometry (NMR),
and infrared spectroscopy. Purity was measured by capillary
gas chromatography (GC). The data for the intermediates are
given elsewhere [13]. Data for the hydrocarbons are summa-
rized in the Appendix.
௡
maintained using a Teflon stirrer bar (10 mm), and care was
taken to position the animals as far away from the stirrer bar
as possible. The animals were fed with an algal culture (Iso-
chrysis galbana) for the duration of the exposure period in
order to ensure that valves remained open and that animals
were filtering. Mussels were exposed to toxicant for 24 h. Two
vessels were prepared for each toxicant concentration. For the
purposes of determining feeding rate, animals were transferred
from the exposure vessel into individual 250-ml glass beakers,
each containing 200 ml toxicant solution at the same exposure
concentration. The animals were allowed an acclimatization
period of 30 min to open their valves and resume pumping
prior to the addition of algae. Algal culture (volume prede-
termined to give a cell concentration of 12,000–15,000 cells/
ml) was then added to each beaker and the water gently stirred
to ensure an even distribution of algae within the beaker. An
aliquot (20 ml) of medium was then immediately taken from
each beaker and the cell count determined in triplicate per
aliquot using a Coulter Counter (Coulter Electronics, Toronto,
Toxicological tests
A widely used, previously published mussel filtering-rate
assay (15ЊC) was employed [13–16]. Briefly, toxicant solutions
ON, Canada) set to measure particles greater than 3
␮m in
diameter. A further aliquot was taken after 15 min and the
decline in cell concentration over 15 min was calculated.
Solubility measurements
Solubility measurements were made according to the fol-
lowing method, which is similar to a literature method [17].
Anthracene was purchased from Aldrich (Poole, UK) with a
purity of
Ͼ99% (as determined by GC). The model aromatic
UCM hydrocarbons, 6-cyclohexyltetralin, 7 cyclohexyl-1-
methyltetralin, and 7-cyclohexyl-1-propyltetralin, were syn-
thesized in good yield and with purities as described above.
Each generator column consisted of a high-pressure liquid
chromatography stainless steel column 30 cm
ϫ 0.46-mm i.d.
Fig. 2. Reaction scheme for synthesis of monoaromatic hydrocarbons
(polyphosphoric acid [PPA]).
with 2 m stainless steel frits at either end, dry packed with
␮

2430
Environ. Toxicol. Chem. 20, 2001
E. Smith et al.
glass beads 60 to 80 mesh coated with test compound. Test
compounds (35 mg) were dissolved in 50 ml of hexane to
which 7 g of beads were added. The solvent was removed by
rotary evaporation followed by a gentle stream of nitrogen to
give a 0.005% coating. Once packed, the generator column
was attached to a pump by means of Teflon tubing and at-
tachments, supplied by a water reservoir. Temperature control
(15 and 25ЊC) was by means of a water bath in which the
generator column and water reservoir were immersed. A 3-L
beaker containing either MilliQ water (Bedford, MA, USA)
or seawater (salinity 33‰, to which was added mercuric chlo-
ride for sterilization, acted as a water reservoir.
The generator column was flushed with 500 ml water to
allow equilibration of the system before measurement. Water
was pumped through the column at a rate of 1 ml/min. Effluent
from the column was collected directly into a 100-ml sepa-
rating funnel containing 25 ml dichloromethane. Once the sam-
ple had been collected, an internal standard (I for the deter-
mination of II and III and II for the determination of I) was
Fig. 3. Feeding rates and hydrocarbon body burdens of populations
of laboratory mussels, Mytilus edulis. Untreated (controls, 0
g/g accumulation) and treated (12–100 g/L, 25–225
cumulation) with 6-cyclohexyltetralin (I, ) and with 7-cyclohexyl-
1-propyltetralin (III, ) for 24 h. Values are means ( 7)
␮
g/L, 0
␮
␮
ϳ
␮g/g ac-
ࡗ
Ⅵ
n
ϭ
Ϯ 1
standard deviation. (Data for 7-cyclohexyl-1-methyltetralin [II] are
not shown in order to maintain the clarity of the diagram; values were
intermediate to those for I and III).
spiked into the water in 100
␮l acetone and the funnel stop-
pered. Each funnel was shaken for 5 min with care so as not
to form an emulsion. The dichloromethane (DCM) fraction
was separated off and the water sample further extracted with
another 25 ml DCM. The DCM extracts were combined and
passed through an anhydrous sodium sulfate column. Analysis
was performed using gas chromatography/mass spectrometry
(GC-MS) operated in single ion monitoring (SIM) mode.
The effect of salinity on solubility was determined by car-
rying out the above procedure but replacing MilliQ water with
seawater. The effect of temperature was determined by car-
and accumulation of up to 150
effect concentrations [TEC50] 0.2 mmol/kg) of 6-cyclohex-
yltetralin and 7-cyclohexyl-1-methyltetralin and up to 220 g/
g wet weight (average TEC50 0.5 mmol/kg) of 7-cyclohexyl-
1- -propyltetralin occurred in 24 h. These correspond to a
maximum of about 800 g/g dry weight. The effects observed
␮g/g wet weight (average tissue
␮
n
␮
for the synthetic compounds in the laboratory are similar to
those observed for other hydrocarbons with similar physico-
chemical properties [16].
Do these concentrations represent environmentally relevant
values for accumulation of UCMs by mussels? The answer,
we believe is, yes; while the aqueous concentrations of hy-
drocarbons used in our experiments may only be encountered
rarely, such as during oil spills, mussels in the environment
are able to accumulate high body burdens of UCMs by filtra-
rying out the above measurements at 25 and 15ЊC by using a
dip chiller unit along with the water bath to achieve the re-
quired temperature. Temperature and salinity were monitored
throughout the experiment.
Instrumental conditions
Gas chromatography-mass spectrometry was carried out
with a Hewlett-Packard MSD GC-MS fitted with a HP-1 Ultra
fused silica column 12 m
Avondale, PA, USA). Auto splitless injection (250
used. Helium was the carrier gas (40 kPa head pressure). Oven
temperature was programmed from 40 to 300 C at 5 C/min
and held at 300 C for 10 min. Mass spectrometer operating
conditions were ion source temperature 250 C, ionization en-
ϫ
0.2-mm i.d. (Hewlett-Packard,
tion. For example, about 100 to 500 ␮g/g dry weight aromatic
Њ
C) was
UCM hydrocarbons were determined in toxicologically im-
pacted (reduced scope for growth [16]) mussels from the east
coast of the United Kingdom [4], and reductions of about 40%
in filtering rates were observed when laboratory mussels ac-
Њ
Њ
Њ
Њ
cumulated 90
␮g/g wet weight (ϳ350 ␮g/g dry wt) monoar-
ergy 70 eV, mass range 35 to 600 Daltons, and selected ion
monitoring.
omatic UCM hydrocarbons [4]. The aromatic UCM concen-
trations and toxic effects on field mussels [4], aromatic UCM-
treated laboratory mussels [4], and laboratory mussels treated
with synthetic compounds herein therefore all suggest a pre-
viously unrecognized toxic effect of monoaromatic UCM hy-
drocarbons.
Although the mechanisms by which mussels accumulate
organic pollutants have been quite extensively studied, to our
knowledge, these studies have not included detailed investi-
gations of the manner in which mussels accumulate monoar-
omatic UCM hydrocarbons. Presumably, whether the UCM
hydrocarbons are present in dissolved, colloidal, or particle-
sorbed phases may have important consequences for uptake
and toxicity, and this will require further study. However, since
aqueous solubility is often an important factor controlling the
concentrations of hydrocarbons in solution, as an initial in-
vestigation into the physical characteristics of the synthetic
tetralins, we determined the aqueous solubilities in freshwater
RESULTS AND DISCUSSION
The effects of the three tetralins on mussel filtering rates
were determined using the assay employed previously to mea-
sure UCM toxicity and which is very widely accepted as a
measure of sublethal narcotic toxicity [14–16]. A substantial
decrease (over 60% reduction compared with controls) in mus-
sel feeding was observed (Fig. 3). The relationship between
exposure concentration and accumulated body burden after 24
h was linear (e.g. I, R2
cyclohexyltetralin, the reduction in feeding rate was signifi-
cantly different ( test, 0.001) from the controls at all
exposure concentrations 12.5 g/L (about 25 g/g wet wt),
and for 7-cyclohexyl-1-propyltetralin, the reduction in filtering
ϭ ϭ 0.986). For 6-
0.998; III, R2
t
p Յ
Ͼ
␮
␮
rate was significantly different (
controls at all exposure concentrations
t
test,
p
Ͼ
Յ
25
0.001) from the
g/L (about 45
␮
␮
g/g wet wt). Clearly, such hydrocarbons are toxic to mussels,
and seawater at 15 and 25ЊC (Table 1) by a well-known gen-

Hydrocarbon humps in the environment
Environ. Toxicol. Chem. 20, 2001
2431
Table 1. Solubilities of alkyltetralins I–III in fresh and seawater (
ϭ
␮
g/L; mean
10); data for anthracene are shown in parentheses for comparison of the method with literature values
[17], ‰ parts per thousand salinity
Ϯ standard deviation, n
ϭ
Aqueous solubility (
␮g/L)
(mean standard deviation)
Ϯ
25
Њ
C
15
Њ
C
25
Њ
C
15
Њ
C
Hydrocarbon
0‰
0‰
35‰
35‰
Anthracene
6-Cyclohexyltetralin (I)
7-Cyclohexyl-1-methyltetralin (II)
7-Cyclohexyl-1-propyltetralin (III)
45
109
45
Ϯ
2 (44) 22
Ϯ
75
27
13
1 (17–23) 29
Ϯ
95
40
17
1 (32)
Ϯ
6
4
3
Ϯ
Ϯ
Ϯ
3
3
2
Ϯ
Ϯ
Ϯ
5
2
2
56
21
9
Ϯ
Ϯ
Ϯ
3
2
2
Ϯ
Ϯ
23
hydrocarbon ‘humps’ in the marine environment: Unrecognised
toxins? Environ Sci Technol 35:2640–2644.
erator column method [17]. The method was calibrated by
determination of the solubility of anthracene, for which lit-
erature data are available (Table 1). The solubilities were found
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unresolved complex mixture (UCM) in gas chromatograms of
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to range from 10 to 110 ␮g/L. These solubility values extend
the measured effects of soluble hydrocarbons on mussel feed-
ing rate according to previously established quantitative struc-
ture–activity relationships for M. edulis [14–16].
Interestingly, Barron et al. [18] recently examined the wa-
ter-accommodated aromatic hydrocarbon content of three en-
vironmentally weathered oils collected from underground
plumes of spilled oil at a coastal California, USA, oil field by
selected ion monitoring gas chromatography-mass spectros-
copy and reported that, although the fractions were toxic to
the mysid shrimp Mysidopsis bahia, the toxicity was, unex-
pectedly, not correlated with polycyclic aromatic hydrocarbons
concentration. Although the actual toxic components were not
identified, the total ion current chromatograms of the oils [18]
show that the hydrocarbon fractions tested were dominated by
aromatic UCMs. Given the solubility of the present synthetic
compounds, it seems reasonable that a contribution of mon-
oaromatic UCM hydrocarbons to mysid toxicity should be
considered.
CONCLUSIONS
13. Wraige EJ. 1997. Studies of the synthesis, environmental occur-
rence and toxicity of unresolved complex mixtures (UCMs) of
hydrocarbons. PhD thesis. University of Plymouth, Plymouth,
Devon, UK.
14. Widdows J, Donkin P, Brinsley MD, Evans SV, Salkeld PN,
Franklin A, Law RJ, Waldock MJ. 1995. Scope for growth and
contaminant levels in North Sea mussels (Mytilus edulis). Mar
Ecol Prog Series 127:131–148.
The experiments outlined above indicate that 6-cyclohex-
yltetralin, 7-cyclohexyl-1-methyltetralin, and 7-cyclohexyl-1-
n
-propyltetralin are toxic to M. edulis. Further studies of the
aromatic UCM composition of a wide range of oils and oil
residues are required to determine whether such alkylated com-
pounds as 7-cyclohexyl-1-methyltetralin and 7-cyclohexyl-1-
15. Widdows J, Donkin P, Evans SV, Page DS, Salkeld PN. 1995.
Sublethal biological effects and chemical contaminant monitoring
of Sullom Voe (Shetland) using mussels (Mytilus edulis). Proc
R Soc Edinb B 103:99–112.
16. Donkin P, Widdows J, Evans SV, Brinsley MD. 1991. QSARs for
the sublethal responses of marine mussels (Mytilus edulis). Sci
Total Environ 109/110:461–476.
17. Whitehouse BG. 1984. The effects of temperature and salinity on
the aqueous solubility of PAHs. Mar Chem 14:319–332.
18. Barron MG, Podrabsky T, Ogle S, Ricker RW. 1999. Are aromatic
hydrocarbons the primary determinant of petroleum toxicity to
aquatic organisms? Aquatic Toxicol 46:253–268.
n
-propyltetralin or their analogues are widespread in oils, and
these are underway in our laboratory. If these aromatic com-
pounds prove to be important in UCMs, toxicity experiments
should be conducted with other biological end points.
Acknowledgement—We thank the Natural Environment Research
Council and the University of Plymouth for studentships for Emma
J. Wraige and Emma L. Smith, and we thank C.A. Lewis and S.T.
Belt for valuable discussions.
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1. Gough MA, Rowland SJ. 1990. Characterisation of unresolved
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2. Thomas KV, Donkin P, Rowland SJ. 1995. Toxicity enhancement
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drocarbon concentrations in the American oyster, Crassostrea
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APPENDIX
Spectroscopic and chromatographic data for the synthetic
alkyltetralins.
6-Cyclohexyltetralin
I; 87% yield;
Ͼ99% gas chromatography (GC) purity: char-
acterized by mass spectrometry (MS), infrared (IR), and nu-
clear magnetic resonance (NMR). MS: m/z 214 (M , 100%);
ϩ
.
4. Rowland SJ, Donkin P, Smith EL, Wraige EJ. 2001. Aromatic

2432
Environ. Toxicol. Chem. 20, 2001
E. Smith et al.
145, 142, 134, 129, 127, 125, 44, 33, 22. H-NMR (ppm):
171; 145; 158; 129. 13C-NMR (ppm): 145.2, 136.8, 134.5,
129.0, 127.4, 124.0, 44.2, 34.6, 29.5, 26.9, 26.2, 23.3. H-
m
1
1
7.0; br m 2.5, 2.8, 2.9; br m 1.3 to 1.9.
NMR (ppm): quintet 6.9;
d 2.7; br m 2.4; m 1.7; m 1.4.
7-Cyclohexyl-1-propyltetralin
III; 91% yield; 97% GC purity: characterized MS and NMR
1
ϩ.
(13C, DEPT, H). MS:
m/z
256 (M , 12%), 213. 13C-NMR
7-Cyclohexyl-1-methyltetralin
(ppm): 145.2, 141.4, 134.4, 128.9, 127.0, 123.8, 44.3, 39.3,
37.4, 34.6, 34.5, 29.4, 27.4, 27.0, 26.2, 20.7, 19.8, 14.3. H-
1
II; 75% yield;
NMR. MS: 145 (100%), 129 (64%), 228 (M , 61%), 213
(51%), 128 (26%), 131 (22%), 55 (17%). 13C-NMR (ppm):
Ͼ90% GC purity: characterized MS, IR, and
ϩ
m/z
NMR (ppm): quintet 6.9, br m 2.7, br m 2.4,
m 1.3 to 1.7, t
0.9.