Biocatalytic resolution of enantiomeric mixtures of 1-aminoethanephosphonic acid

Article abstract of DOI:10.3390/molecules16075896

Several fungal strains, namely Bauveria bassiana, Cuninghamella echinulata, Aspergillus fumigatus, Penicillium crustosum and Cladosporium herbarum, were used as biocatalysts to resolve racemic mixtures of 1-aminoethanephosphonic acid using L/D amino acid oxidase activity. The course of reaction was analyzed by 31P-NMR in the presence of cyclodextrin used as chiral discriminating agent. The best result (42% e.e of R-isomer) was obtained with a strain of Cuninghamella echinulata.

Full text of DOI:10.3390/molecules16075896

Molecules 2011, 16, 5896-5904; doi:10.3390/molecules16075896
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Biocatalytic Resolution of Enantiomeric Mixtures of
-Aminoethanephosphonic Acid
1
Małgorzata Brzezińska-Rodak *, Magdalena Klimek-Ochab, Ewa Żymańczyk-Duda and
Paweł Kafarski
Department of Bioorganic Chemistry, Faculty of Chemistry, Wroclaw University of Technology,
Wybrzeże Wyspiańskiego 27, 50-370 Wroclaw, Poland
*
Author to whom correspondence should be addressed;
E-Mail: malgorzata.brzezinska-rodak@pwr.wroc.pl; Tel.:+48-71-320-3302; Fax:+48-71-320-2427.
Received: 29 June 2011; in revised form: 12 July 2011 / Accepted: 12 July 2011 /
Published: 14 July 2011
Abstract: Several fungal strains, namely Bauveria bassiana, Cuninghamella echinulata,
Aspergillus fumigatus, Penicillium crustosum and Cladosporium herbarum, were used as
biocatalysts to resolve racemic mixtures of 1-aminoethanephosphonic acid using L/D amino
3
1
acid oxidase activity. The course of reaction was analyzed by P-NMR in the presence of
cyclodextrin used as chiral discriminating agent. The best result (42% e.e of R-isomer) was
obtained with a strain of Cuninghamella echinulata.
Keywords: aminophosphonic acid; fungi; resolution of racemic mixture
1
. Introduction
Aminophosphonic acids and their analogues are an important class of organophosphorous
compounds, mostly because of their varied biological activity [1]. The application of
aminophosphonates range from agriculture to medicine, as exemplified by the herbicides glyphosate
and phosphiniothricin, the antibacterial alafosfalin, the antihypertensive fosinopril or the
antiosteoporetic bisphosphonates [2-10]. Aminophosphonic acids, being considered as structural
analogues of amino acids, exhibit inhibitory activity against different enzymes, especially towards
proteinases such as HIV protease, thrombin, aminopeptidases, and human collagenase [11,12].

Molecules 2011, 16
5897
Various synthetic protocols have been described for the synthesis of α-aminophosphonates,
including a plethora of “green” procedures [8,13-16] and they are thus now readily available in gram
quantities as racemates. However, the preparation of enantiomerically pure forms of these compounds,
still constitutes a challenging issue and most of the methods for their preparation are cumbersome and
of low efficiency.
The biocatalytic resolution of racemic molecules has attracted the interest of chemists for several
decades [17-20]. Microbial biocatalysts in the form of purified enzymes or whole microbial cells have
been successfully used for synthesis of optically pure organophosphonate derivatives [21-29], showing
the utility of biocatalysis in this branch of chemistry. Surprisingly, bioconversion is scarcely used
to obtain optically pure aminophosphonic compounds [30-35]. There is only one literature note
discussing the application of Candida antarctica lipase for the enantioselective acylation of 1-and
2
-aminoalkanephosphonates [36]. Consequently, the development of new biocatalytic methods for the
obtaining of these acids really is a not fully explored area of research.
Here we report a simple strategy of enantioselective oxidation of 1-aminoethanephosphonic acid
(
a warhead of the antibiotic alafosfalin) using whole cells of fungi imperfecti. Resolution of the
racemic mixture of this acid provided its desired L-and D-isomers with moderate, although satisfactory
enantiomeric excesses.
2
. Results and Discussion
Because of the future applicability, there is still a need for the development of effective methods for
preparation of α-aminophosphonic acids in their enantiomerically pure forms. Many approaches have
been undertaken to reach this goal, with the most successful being resolutions of racemic mixtures of
aminophosphonate esters by application of the derivatives of tartaric acid [37,38]. Several other
protocols for asymmetric synthesis of these compounds have also been described although these
methods are considered as less effective and cumbersome [16].
Biocatalysis is a very attractive alternative for chemical asymmetric synthesis because it is, in the
most cases, simple and economic. The biocatalyst (pure enzymes or/and whole microbial cells) are
more and more frequently used in chemical, especially asymmetric synthesis, leading to structurally
diverse, optically pure compounds.
In previous works we have employed various strains of fungi for the synthesis of P-chiral
hydroxyphosphonates [21] and for the oxidative resolution of racemic mixtures of diethyl 1-hydroxy-
phenylmethanephosphonate [39]. The success of these reactions was an inspiration for the use of
whole cells of microorganisms for the resolution of racemic mixtures of 1-aminoethanephosphonic
acid. This molecule was chosen as a standard substrate, because of its simple structure and mentioned
application and, what is also important, because of the available analytical methods for the
identification and absolute configuration assignment.
The phosphonic analogue of alanine is not a physiological substrate for fungi, therefore in some
cases the incubation of the biocatalysts cells (before bioconversion) under starvation conditions was
required to force them to utilize such a compound as an exogenous source of biogenic phosphorus –a
crucial element for viability of the cells. Thus, our strategy relayed on the premise that some fungi are
known for their capacity to degrade slowly aminophosphonates [40,41]. The pathway of this utilization

Molecules 2011, 16
5898
most likely proceeds via oxidative conversion of aminophosphonate to ketophosphonate (Scheme 1).
The ketophosphonate, in turn, is chemically unstable and undergoes chemical degradation to phosphate
and acetic acid. The question, which remained was if this process is stereoselective.
Scheme 1. Enantioselective degradation of 1-aminoethanephosphonic acid by
Cuninghamella echinulata.
NH2
O
NH2
PO H
+
PO H
PO3H2
3
2
3
2
CH COOH + H PO
4
3
3
Five strains of fungi imperfecti have been tested in this work, namely Cuninghamella echinulata,
Aspergillus fumigatus, Beauveria bassiana, Penicillium crustosum, Cladosporium herbarum. In order
to obtain the product of desired configuration, the biomass was prepared with or without the starvation
period following routine cell cultivation. Results reached with C. echinulata, after the inspection of the
3
1
P-NMR spectra of the crude reaction mixture, showed the presence of phosphate as the only second
phosphorus product, supporting our speculation. Thus, enrichment of 1-aminoethanephosphonic acid
with the R-isomer (42% e.e) after two days of biotransformation with fresh cells and without any
starvation period was obtained. Quite interestingly, this fungus preferably decomposed S-enantiomer,
which corresponds to unnatural D-configuration. The same enantioselectivity was exhibited by
C. echinulata, when it was used under starvation conditions (Table 1). Also a drop in enantiomeric
excess was observed as the time of cultivation of the microorganism in the presence of phosphonic
analogue of alanine was increased.
The inversion of the absolute configuration seems to be quite general phenomenon (Table 1). A
possible explanation of this phenomenon is that, that bacterial amino acid oxidases are either
constitutive or inducible and their activity most likely depends on the time of incubation of the cell and
aminophosphonate. Moreover, the protein heterogeneity (e.g., enantiospecificity) often results from the
coexistence of interconvertible isomeric forms of the monomeric enzyme, which also can be an
explanation of the disscused phenomenon.
Table 1. Biooxidative resolution of 1-aminoethanephosphonic acid.
1
fresh cells: e.e values /configuration
cells after starvation:
Microorganism
e.e values/configuration
Day of
transformation
Cuninghamella
echinulata
1
2
3
4
5
1
2
3
4
5
28%
R
42%
R
25%
R
25%
R
13%
R
26%
R
21%
R
13%
R
12%
R
9%
R
Aspergillus
fumigatus
9%
S
10%
S
11%
S
0
11%
S
25%
S
13%
R
6%
R
12%
R
13%
R
Beauveria
bassiana
13%
R
4%
S
10%
R
6%
R
6%
R
2%
S
8%
R
2%
S
5%
S
6%
R
2
2
2

Molecules 2011, 16
5899
Table 1. Cont.
Penicillium
crustosum
Cladosporium
herbarum
1
14%
R
0
14%
R
0
3%
R
6%
S
2%
R
18%
S
6%
S
6%
R
9%
R
0
8%
R
0
7%
S
4%
R
6%
S
12%
S
0
2
2
3%
R
2
2
average value from three runs with statistic error = 3%, results included in the margin of error.
The enrichment of product of S configuration was observed, when the biotransformation was
carried out with whole cells of Aspergillus fumigatus. The highest enantiomeric excess was obtained
after only one day of biotransformation with starved cells (25% e.e; Table 1). Quite interestingly,
cultivation of the fungi for longer periods resulted in a reversal of enantioselectivity. Fresh cells of
A. fumigatus demonstrated poorer activity, yielding predominantly the S enantiomer. The enantiomeric
excess in these experiments was around 10% (Table 1).
The application of other fungal cells was less effective, but still supports the idea that microbial
cells are able to degrade aminophosphonates enantioselectively allowing tapping this process as a
mean to prepare optically pure phosphonic acid. However, the presented results are of preliminary
nature and intensive search for more efficient microorganisms alongside with optimization of the
conditions of this process are required.
3
1
Figure 1. P-NMR spectra of biotransformation products recorded with cyclodextrin: A, fresh
cells of C.echinulata, second day of biotransformation, predominant enantiomer defined as R
isomer; B, cells of A. fumigatus preincubated under starvation conditions, first day of
biotransformation, predominant enantiomer defined as S isomer.
A
B

Molecules 2011, 16
5900
3. Experimental
3.1. General
All the chemicals and solvents were purchased from commercial sources. NMR spectra were
TM
recorded on a Bruker Avance instrument (600 MHz). The optical rotation was measured using the
Polarymeter polAAr 31.
3
.2. Synthesis of 1-aminoethanephosphonic Acid
The substrate was synthesized according to the standard three-component reaction of benzyl
carbamate (0.05 M), triphenylphosphite (0.05 M) and acetaldehyde (0.07 M) followed by hydrolysis in
concentrated hydrochloric acid [42]. The product was obtained with the yield of 90%. NMR data:
3
1
1
P- NMR: δ (ppm) 16.0; H-NMR δ (ppm) 1.46 (dd, J = 7.1, 16.5, CHCH
3
), 3.6 (m, CHCH
3
).
3.3. Microorganism Cultivation
The strains of Beauveria bassiana (DSM 875), Penicillium crustosum (DSM 62837), Aspergillus
fumigatus (individual collection of the University of Pavia, Italy) and Cladosporium herbarum
DSM 63422) were cultivated with shaking (100 rpm) at room temperature in a standard potato
(
dextrose liquid medium (DSMZ Medium 129, 100 mL) in 250 mL cultivation flasks, until the mid-log
growth phase was reached (for 3-4 days). Cuninghamella echinulata strain (DSM 1905) was cultivated
in the standard malt extract peptone liquid medium (DSMZ Medium 90) under the same conditions as
the other microorganisms. After the cultivation time the biomass was separated by centrifugation
(
4,000 rpm/10 min) what allowed obtaining about 8 g of wet cells from every cultivation flask. After
that, depending on the down stream, cells were used directly or preincubated for the next 24 h under
starvation conditions and then used as biocatalysts.
3.4. Biotransformation: General Procedure
Biotransformation (50 mM of the substrate, 8 g wet cells) was carried out (in three simultaneous
experiments for every run) in 50 mL of phosphate buffer (0.017 M, pH 6.11) for varying periods of
time (1-5 days) with shaking at 250 rpm. Cells were then removed by centrifugation (4,000 rpm,
1
0 min) and the supernatant was evaporated. Composition of the product was analyzed by means of
NMR spectroscopy.
3
.5. Derivatization of Enantiomerically Enriched 1-aminoethanephosphonic Acid
The tosyl derivative was prepared by heating a mixture of product (30 mg) in 0.4 M phosphate
buffer (pH 11, 10 mL) with TsCl (135 mg) in acetonitrile (10 mL) at 50 °C for 30 min [43]. Then the
mixture was extracted twice with ethyl acetate, followed by evaporation of volatile components and
NMR analysis was performed.

Molecules 2011, 16
5901
3
1
TM
P-NMR assignments: NMR spectra were recorded on a Bruker Avance instrument operating at
2
43.1 MHz; measurements were made in deuterium oxide at a temperature of 300K.
Determination of product optical purity: enantiomeric excess assignments: Optical purity of the
3
1
product and the enantiomeric excess were evaluated using a P-NMR technique. In order to achieve
separation of enantiomers of 1-aminoethanephosphonic acid by the use of cyclodextrin, this
phosphonate has to be derivatized with p-toluenesulfonyl chloride. Complexation of the aromatic
3
1
ring of p-toluenesulfonate by cyclodextin results in enantiodifferentiation in the P spectra. The
3
1
underivatized aminophosphonic acid is not complexed by cyclodextrin [44]. P-NMR spectra were
recorded with addition of α-cyclodextrin/tosylated 1-aminoethanephosphonic acid (2:1 w/w) and pH of
the samples was set at 10-11.
Determination of the absolute configuration of the products: Absolute configuration was determined
by measurements of the optical rotation (in 1 M NaOH, c=1) after the tosyl group removed by acidic
hydrolysis with concentrated HCl. Representative results were as follows: in the case of (A) - Figure 1,
[α]578 = −6.7 (42% e.e) for R enantiomer; in the case (B) - Figure 1, [α]578 = +4.4 (25% e.e). These
results are in a very good agreement with the literature data for pure enantiomers, which are as
follows: [α]578= −16 for the R enantiomer and + 17 for the S one [37].
4
. Conclusions
Although of preliminary nature, our experiments have indicated that the use of biodegradative
potential of fungi may serve as a mean of separation of chiral phosphonic acids.
Acknowledgments
This work was financed by the project “Biotransformations for pharmaceutical and cosmetics
industry” No.POIG.01.03.01-00-158/09-05, which is partly-financed by the European Union within the
European Regional Development Fund for the Innovative Economy”.
Conflict of Interest
The authors declare no conflict of interest.
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