A new stepwise synthesis of a family of propylamines derived from diatom silaffins and their activity in silicification

Article abstract of DOI:10.1039/b515967a

A new method for the stepwise synthesis of propylamines containing fragments of N-methyl propylamine as found in diatom bioextracts is presented and their activity in silicic acid condensation is described. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515967a

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COMMUNICATION
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A new stepwise synthesis of a family of propylamines derived from
diatom silaffins and their activity in silicification{
Vadim V. Annenkov,*ab Siddharth V. Patwardhan,b David Belton,b Elena N. Danilovtsevaa and
Carole C. Perry*b
Received (in Cambridge, UK) 10th November 2005, Accepted 9th February 2006
First published as an Advance Article on the web 24th February 2006
DOI: 10.1039/b515967a
polyamine structural moieties isolated from several diatom species.
The objective of this work is to establish a convenient universal
method for propylamine synthesis capable of extending the
N-methylpropylamine chain in a linear fashion. Furthermore,
the activity of these PAs in silica formation in vitro is investigated.
To our knowledge, this is the first investigation on the role of a
series of synthetic methylated propylamines in silicification.
N3 (Scheme 3) is the longest synthetic methylated propylamine
to have been synthesised previously. It can be prepared by the
reaction of methylamine with 3-(N-methylamino)propionitrile in
the presence of hydrogen and a hydrogenation catalyst.6 However,
this reaction gives rise to a mixture of amines that needs to be
separated to yield N3 and does not allow the controlled synthesis
of long-chain polyamines. Other non-methylated polyamines
containing four and six nitrogen atoms have been obtained by
reaction of trimethylene dibromide with trimethylendiamine or
dipropylenetriamine.7,8 The reaction is complicated and a mixture
of polymeric and branched products are synthesised requiring
complicated separations. As the amine chain is increased, the
likelihood of branching also increases as the relative concentration
of end amino-groups decreases. Moreover, in the case of long-
chain amines it will be practically impossible to separate a target
linear product from polymeric and branched admixtures.
A new method for the stepwise synthesis of propylamines
containing fragments of N-methyl propylamine as found in
diatom bioextracts is presented and their activity in silicic acid
condensation is described.
Saturated speciality polyamines play important roles in various
biological processes and have attracted the attention of synthetic
chemists. A widely known naturally occurring polyamine is
spermine (N,N9-bis(3-aminopropyl)-1,4-butanediamine), which
had
been
isolated
from
sperm.1
3,39-Methylimino-
bis(N-methylpropylamine) (N3; Scheme 3) has been studied as a
neurotoxic agent2,3 and as an inhibitor of ribonuclease activity.4 In
the last decade, polyamines isolated from siliceous cell walls of
diatoms have been investigated by biologists, biochemists and
materials scientists.
Diatom biosilica polyamines have been found in the free state
and as a part of complex proteins called silaffins in diatom
biosilica.5 Silaffins and polyamines are proposed to play a
significant role in diatom biosilicification producing diverse
nano-structured silica valves.5 Diatom polyamines include
repeated N-methylpropylamine (PA) fragments terminated with
methylated or non-methylated nitrogen (Scheme 1). The propyla-
mines have been shown to promote silica formation in vitro in a
tetramethoxysilane (TMOS) based system, although the only
experimental information available ascribed to a catalytic effect is
obtained from how much precipitable silica is generated during a
fixed time period.5 The interest in such compounds is apparent
from their potential in the biomimetic design of nano-structured
materials via ‘‘green chemistry’’ approaches.
The proposed approach to target polyamines consists of
repetitions of the following reactions: (1) condensation of NH-
amine with methyl acrylate, (2) substitution of the resulting ester
with methylamine and (3) reduction of the amide with lithium
In this communication, we present the successful synthesis of a
series of linear methylated propylamines that are analogous to the
Scheme 1 Structure of polyamines isolated from diatoms.
aLimnological Institute of Siberian Branch of Russian Academy of
Sciences, 3, Ulan-Batorskaya St., P.O. Box 4199, Irkutsk, 664033,
Russia. E-mail: annenkov@lin.irk.ru.; Fax: 3952 425405;
Tel: 3952 428422
bSchool of Biomedical & Natural Sciences, Nottingham Trent
University, Clifton Lane, Nottingham, UK NG11 8NS.
E-mail: Carole.Perry@ntu.ac.uk.; Fax: 0115 848 6636;
Tel: 0115 848 6695
{ Electronic supplementary information (ESI) available: detailed synthetic
methods, NMR, MS and FTIR characterisation results and SEM of silica
samples obtained in the presence of N7. See DOI: 10.1039/b515967a
Scheme 2 The consecutive stages of the polyamines synthesis.
Chem. Commun., 2006, 1521–1523 | 1521
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Scheme 3 Structure of the obtained polyamines.
aluminium hydride (Scheme 2, reactions 1–3 respectively).{ We
have obtained polyamines with 3, 5 and 7 nitrogen atoms starting
from methylamine as in reaction (1). These compounds can be
considered as derivatives of the cyclic monomer 1-methylazetane
(Scheme 3). The propylamines were characterised by proton
Nuclear Magnetic Resonance Spectroscopy (1H-NMR) in a
variety of solvents and by Fourier Transform Infrared
Spectroscopy (FTIR).{ Potentiometric titration (see below) and
mass spectrometry data{ are in good agreement with the molecular
mass of the polyamines.
It has been demonstrated previously that small (bio)molecules
(e.g. diamines, ethyleneamines, amino acids, etc.) ‘‘regulate’’
silicification.8–15 Propylamines isolated from diatom species have
also shown promising prospects in generating tailored silicas.6 It is
of great interest to us to understand how the synthetic
propylamines interact with silicic acid. The effect of N3, N5 and
N7 on orthosilicic acid condensation was studied using a silicon-
catecholate complex as the silica precursor. To a supersaturated
solution of silicic acid (30 mM), the propylamines were added
maintaining the Si : N ratio to unity and the pH was maintained at
neutral without addition of external buffers.
Fig. 2 Silica prepared in the presence of N3 (top) and N5 collected in 6
and 1 min respectively.
The precipitates were collected from as early as after one minute
to seven days, washed and analysed by Scanning Electron
Microscopy (SEM); representative data for N3 and N5 is presented
in Fig. 2 (see ESI{ for N7 data). The formation of spherical silica
particles (y250–400 nm) was observed in the presence of all the
PAs studied herein. Elemental analysis using Energy Dispersive
Spectroscopy (EDS) revealed that the particles were composed of
silicon and oxygen and hence eliminates the possibility that the
precipitates are propylamine complexes (data not shown).
Nitrogen gas adsorption carried out on selected samples showed
that the silica particles possessed unusually low surface areas
(,10 m2 g21) and negligible pore volume (,0.01 cc g21) with most
In the presence of all PAs under consideration, rapid
precipitation was observed in ,1 minute. N.B. the concentration
of silicic acid used in this study (30 mM) is significantly lower than
that used by others (100 mM)5 and hence rapid precipitation of
silica is unexpected. The loss of silicic acid was monitored by the
colorimetric molybdic blue assay and data are presented in Fig. 1.
It is evident that the precipitation was accompanied by a rapid loss
in silicic acid (from initial 30 mM to ca. 15 mM) when PAs were
introduced into the system, thus suggesting a specific action of
propylamines on silica formation. Although several other small-
and poly-amines have been previously shown to promote silicic
acid condensation and/or aggregation,9,11 a rapid drop in silicic
acid concentration (in ,60 seconds) as reported here has not been
reported previously, even when higher silicic acid concentrations
were used (y100 mM).16
˚
of the pores being below 5 A in radius. In contrast, the earliest
silica samples that could be isolated from the blank system (at half
gel time = 18 h) were found to be porous with high surface area
(y700 m2 g21), high pore radius (y20 A) and high pore volume
˚
(y0.3 cc g21).
To our knowledge, the rapid precipitation of non-porous silica
particles from ‘‘pure’’ orthosilicic acid solutions has not been
reported previously. It is noted that ‘‘dense’’ silica particles have
been previously prepared when tetramethoxysilane (TMOS) was
used as the silica precursor.13,17 However, we have shown
previously that TMOS produces a mixture of monomer and
oligomers upon prehydrolysis (with only 20% free silicic acid)12
which produces non-porous silicas in the presence of additives such
as peptides, synthetic polymers and small molecules (e.g.
ethyleneamines).13 Hence our results are significant as we
demonstrate the synthesis of dense silica from ‘‘pure’’ orthosilicic
acid.
In order to understand how the propylamines might be
interacting with the silica species present in solution, a preliminary
investigation using modelling18 and potentiometric titrations on
the protonated and unprotonated propylamine species under our
experimental pH conditions was undertaken. Potentiometric data
(Fig. 3) shows two inflections on the curves, one of them
corresponds to full neutralisation of the amines and the other is
near the point of protonation for alternate nitrogen atoms. This
means that there are 25–30% amine groups which are unproto-
nated at pH 7. This is also confirmed from theoretical calculations
Fig. 1 Silicic acid condensation with or without added propylamines.
Insert shows the initial stages of condensation.
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and for N3 and N5 amines were boiled for 6–10 hours. Reaction (2) was
also performed in ethanol using a 50% excess of methylamine under ester
groups, at r.t. during 3–5 days. The completions of reactions (1) and (2)
were checked by FTIR spectroscopy by disappearance of NH band
(3300 cm21) and ester CLO band (1740 cm21) respectively.
Fig. 3 Potentiometry titration curves for new propylamines (b
=
neutralization degree).
Yield of the target compounds was near 100%. Reduction of the amides
[reaction (3)] was done by dropwise addition of the amide to LiAlH4
suspension in diethyl ether (2.1 moles of LiAlH4 per amide group) with 50–
70% yield. The non-quantitative yield at this stage is connected with
association of the resulting amine with lithium and aluminium hydroxides.
One would hope that the yield would be enhanced in future optimization of
the procedure. The obtained polyamines are colourless liquids, b.p.: 65 uC,
142 uC and 185 uC at 0.2 mm Hg for N3, N5 and N7 respectively. FTIR
(film, cm21): 3292–3296, 2939–2945, 2788, 2839, 1450–1465, 1373, 1315,
1
1150, 1122, 1068, 733–740. N3: HMR, 5% in CDCl3. 1.63 ppm (5t, 4H,
2 6 CH2 (b)), 2.15 (1t, 3H (NMe)), 2.35 (3t, 4H, 2 6 CH2 (c)), 2.40 (1t, 6H,
NHCH3), 2.58 (3t, 4H, (a)). ESI-MS +ve ion. 174.3 ([M + H]+). N5 (n = 2):
1HMR, 5% in CDCl3. 1.62 ppm (mt, 8H (b)), 2.14 (1t, 9H (NMe)), 2.28–
2.35 (mt, 12H (c)), 2.40 (1t, 6H (NHMe)), 2.58 (mt, 4H (a)). ESI-MS +ve
ion. 316.4 ([M + H]+), 245.4 ([M + H]+2 C3H5NHMe), 174.3 ([M + H]+2
C3H5N(Me)C3H6NHMe).
1
N7 (n = 4): H-NMR, 5% in CDCl3. 1.65 ppm (mt, 12H (b)), 2.20 (1t,
Fig. 4 Estimated speciation of N3 and N5 (only species present at
significant fractions are shown). Models of propylamines are shown on the
right of each curve. Charged nitrogen atoms are marked with asterisks.
15H (NMe)), 2.30–2.40 (mt, 20H (c)), 2.42 (1t, 6H (NHMe)), 2.62 (3t, 4H
(a)).
ESI-MS +ve ion. 458.5 ([M + H]+), 387.5 ([M + H]+ 2 C3H5NHMe),
316.3 ([M + H]+ 2 C3H5NMeC3H6NHMe).
(Fig. 4). For example, Fig. 4 also shows the estimated speciation of
N3 and N5 and respective molecular models. It appears that the
rapid formation of dense silicas in the presence of PA arises from
the fact that PAs are partially protonated at neutral pH with labile
protons, thus enabling PAs to act as Brønsted acids, similar to the
proposed activity for ethylamines.13 Future work is being under-
taken in order to investigate in more details the role(s) of
propylamines of a variety of architectures in silica formation and
the data will be presented in due course. It is also possible to
synthesise polyamines with an even number of nitrogen atoms
from N,N9-dimethyl-1,3-propanediamine which is commercially
available or can be obtained using a 1 : 1 ratio of reagents as in
reaction (1). Sequential realisation of reactions (1)–(3) with various
amines and acrylates opens up the way to a wide range of
polyamines that will be evaluated in due course.
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This work was supported by Presidium of the Russian Academy
of Sciences (project # 10.3), The Royal Society short visit grant
(for VVA), AFOSR and EU.
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Notes and references
{ Synthetic procedures are described briefly below, see ESI{ for details on
the synthetic procedures and characterisation of PAs. Condensation of
NH-amines with methyl acrylate [reaction (1)] was carried out in ethanol
solution (15% of the corresponding amine) at 1 : 2.1 amine : acrylate ratio.
In the case of methylamine the reaction mixtures stayed at r.t. for 6 days
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1521–1523 | 1523