Mechanistic approach of the difference in non-enzymatic hydrolysis rate between the L and D enantiomers of no-carrier added 2-[18F] fluoromethyl-phenylalanine

Article abstract of DOI:10.1002/jlcr.1811

No-carrier added (n.c.a.) 2-[¹⁸F]fluoromethyl-l-phenylalanine was found to be very sensitive to hydrolysis in aqueous solutions. This problem was solved partially by the addition of calcium ions (0.04M), increasing the shelf-life to at least 6h. In this paper the defluorination reaction was studied in detail to elucidate its mechanism. Therefore, L and D enantiomers of 2-[¹⁸F]FMP and 4-[¹⁸F]FMP were synthesized, as well as 2-[¹⁸F]fluoromethyl-phenethylamine and 4-[¹⁸F] fluoromethyl-phenethylamine, both decarboxylated 'mimetic' molecules of the amino acid analogues. Radiosynthesis, using a customized Scintomics automatic synthesis hotbox^three module, resulted in a high overall yield and a radiochemical purity of >99%. The defluorination rates of all compounds were studied by HPLC. The L enantiomer of n.c.a 2-[¹⁸F]FMP defluorinated seven times faster than the D enantiomer and 2-[¹⁸F]fluoromethyl- phenethylamine. Both enantiomers of 4-[¹⁸F]FMP and 4-[ ¹⁸F]fluoromethyl-phenethylamine were stable. From these data, the reaction mechanism, involving two distinct intramolecular interactions, was derived. First, the interaction between the amine and the benzylic fluorine weakens the carbon-fluorine bond. Secondly, the formation of a second hydrogen bridge between the carboxyl group and one of the benzylic hydrogen atoms renders the fluorine atom even more susceptible to hydrolysis. The latter interaction induces an additional chiral center. The probability of its formation differs considerably between L and D enantiomers of n.c.a. 2-[¹⁸F]FMP, which explains the difference in hydrolysis rate. Copyright

Full text of DOI:10.1002/jlcr.1811

Research Article
Received 22 January 2010,
Revised 3 May 2010,
Accepted 8 June 2010
Published online 5 August 2010 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.1811
Mechanistic approach of the difference
in non-enzymatic hydrolysis rate between
the L and D enantiomers of no-carrier
added 2-[18F]fluoromethyl-phenylalanine
Ã
Ken Kersemans,a John Mertens,a Frank De Proft,b and Paul Geerlingsb
No-carrier added (n.c.a.) 2-[18F]fluoromethyl-l-phenylalanine was found to be very sensitive to hydrolysis in aqueous
solutions. This problem was solved partially by the addition of calcium ions (0.04 M), increasing the shelf-life to at least 6 h.
In this paper the defluorination reaction was studied in detail to elucidate its mechanism. Therefore, L and D enantiomers of
2-[18F]FMP and 4-[18F]FMP were synthesized, as well as 2-[18F]fluoromethyl-phenethylamine and 4-[18F]fluoromethyl-
phenethylamine, both decarboxylated ‘mimetic’ molecules of the amino acid analogues. Radiosynthesis, using
a customized Scintomics automatic synthesis hotboxthree module, resulted in a high overall yield and a radiochemical
purity of 499%. The defluorination rates of all compounds were studied by HPLC. The L enantiomer of n.c.a 2-[18F]FMP
defluorinated seven times faster than the D enantiomer and 2-[18F]fluoromethyl-phenethylamine. Both enantiomers of
4-[18F]FMP and 4-[18F]fluoromethyl-phenethylamine were stable. From these data, the reaction mechanism, involving two
distinct intramolecular interactions, was derived. First, the interaction between the amine and the benzylic fluorine
weakens the carbon–fluorine bond. Secondly, the formation of a second hydrogen bridge between the carboxyl group and
one of the benzylic hydrogen atoms renders the fluorine atom even more susceptible to hydrolysis. The latter interaction
induces an additional chiral center. The probability of its formation differs considerably between L and D enantiomers of
n.c.a. 2-[18F]FMP, which explains the difference in hydrolysis rate.
Keywords: L and D 2-[18F]fluoromethyl-phenylalanine; non-enzymatic; hydrolysis; different; rates
Introduction
Materials and methods
Our group has previously shown that 2-amino-(S)-3-(2-[18F]fluor- All products were at least of analytical grade. All solvents were
omethyl-phenyl)-propionic acid (2-[18F]fluoromethyl-L-phenyla- of HPLC quality or better. NMR data were obtained using an
lanine; 2-[18F]FMLP) showed a high uptake in cancer cells both AVANCE DRX 250 instrument and MS data were acquired on a
in vitro and in vivo. However, in its no-carrier added (n.c.a.) form Fisons VG II Quattro Mass Spectrometer.
it suffered from considerable non-radiolytic radiodefluorination
Geometry optimizations and subsequent calculations of
in water at room temperature.1,2 No-carrier added 2-amino-(S)-3- atomic charges, using the natural population analysis,4–6 were
(4-[18F]fluoromethyl-phenyl)-propionic acid (4-[18F]fluoromethyl- performed at the B3LYP7,8/6-31G(d)9 level of theory using the
L-phenylalanine; 4-[18F]FMLP) was stable in the same conditions. Gaussian 03 software package.10
This suggested that the fast hydrolysis was due to the ortho-
position of the fluoromethyl group on the aromatic ring and Synthetic procedures
more specifically to an intra-molecular interaction between the
Fluorinated amino acids
negatively charged benzyl fluorine atom and the positively
charged NH13 of the amino acid group.2,3 The problem of
The synthesis of the precursor molecules for the radiosynthesis
of R-2-amino-3-(2-[18F]fluoromethyl-phenyl)-propionic acid and
radiodefluorination in the radiopharmaceutical formulation was
solved for the larger part by the addition of calcium ions (0.04 M)
ensuring a shelf-life of at least 6 h. This stabilizing effect was
assumed to be related to the formation of a complex between
aRadiopharmaceutical Chemistry, BEFY, Vrije Universiteit Brussel, Laarbeeklaan
103, 1090 Brussels, Belgium
the Ca21 ions and the negatively charged fluorine and oxygen
atom of the dissociated carboxylic acid.2
In this paper we study the mechanism of the fast defluorina-
tion of n.c.a. 2-[18F]FMLP. We report for the first time the
bDepartment of General Chemistry, ALGC, Vrije Universiteit Brussel, Pleinlaan 2,
1050 Brussels, Belgium
*Correspondence to: Ken Kersemans, Radiopharmaceutical Chemistry, BEFY,
Vrije Universiteit Brussel, Laarbeeklaan 103, 1090 Brussels, Belgium.
E-mail: kkersema@vub.ac.be
difference in non-enzymatic hydrolysis rate between L and
enantiomers, in casu: 2-[18F]fluoromethyl-phenylalanine.
D
J. Label Compd. Radiopharm 2011, 54 80–85
Copyright r 2010 John Wiley & Sons, Ltd.

K. Kersemans et al.
1
S-2-amino-3-(2-[18F]fluoromethyl-phenyl)-propionic acid as well as a pale yellow oil (84%). H NMR (CDCl3, 250 MHz):d7.20–7.01
as their non-radioactive fluorinated analogues was described in (m, 4 H), 5.50 (s, J[19F–1H] = 47.0 Hz, H), 5.37 (s, J[19F–1H] = 47 Hz,
1
detail in earlier publications.2,3
1 H), 3.16 (t, J = 8 Hz, 2 H), 2.73 (t, J = 8.0 Hz, 2 H), 1.22 (s, 2 H). MS
(EI) m/z 154 (MH1).
Synthesis of 2/4-fluoromethyl-phenethylamine
Radiosynthesis
The amine group of commercially available 2/4-methylphenethy-
lamine (Sigma-Aldrich) was protected with N-Boc by reaction with
Boc2O in the presence of triethylamine in tetrahydrofurane, as
described by Leftheris et al.11 After silica gel column chromato-
graphy (50 g Si-gel (Merck), id 2 ꢀ 25cm, petroleum ether/diethyl
ether: 2/8 (v/v)) (2-o-Tolyl-ethyl)-carbamic acid tert-butyl ester was
obtained as a colourless oil (88% yield). 1H NMR (CDCl3,
500 MHz):d7.16–7.12 (m, 4 H), 2.91 (t, J = 8 Hz, 2 H), 2.51
(t, J = 7.5 Hz, 2 H), 2.34 (s, 3 H) and 1.46 (s, 9 H). MS (EI) m/z 236
(MH1). (4-o-Tolyl-ethyl)-carbamic acid tert-butyl ester was
obtained as a colourless oil (88% yield). 1H NMR (CDCl3,
500 MHz):d7.20–7.10 (m, 4 H), 3.33 (m, 2 H), 2.73 (t, J = 7 Hz,
2 H), 2.35 (s, 3 H) and 1.43 (s, 9 H). MS (EI) m/z 236 (MH1).
For the radical bromination of the methyl side chain, (2 or 4-o-
tolyl-ethyl)-carbamic acid tert-butyl ester (270 mg, 1.49 mmol)
was reacted with N-bromosuccinimide (245 mg, 1.38 mmol),
using azobisisobutyronitrile (28 mg, 0.17 mmol) in dichloro-
methane (50 mL) at 501C for 2 h. The crude product was purified
via silica gel column chromatography (50 g Si-gel (Merck), id
2 ꢀ 25 cm, petroleum ether/diethyl ether: 1/9 (v/v)) to yield [2-(2-
bromomethyl-phenyl)-ethyl]-carbamic acid tert-butyl ester as a
yellow oil in 48% yield. 1H NMR (CDCl3, 500 MHz):d7.21–7.15 (m,
4 H), 4.58 (s, 2 H), 2.96 (t, J = 7.5, 2 H), 2.54 (t, J = 7.0 Hz, 2 H) and
1.42 (s, 9 H). MS (EI) m/z 314 (M1). [2-(4-bromomethyl-phenyl)-
ethyl]-carbamic acid tert-butyl ester was obtained as a yellow oil
in (yield 46%). 1H NMR (CDCl3, 250 MHz):d7.14–7.01 (m, 4 H), 4.58
(s, 2 H), 3.27 (m, 2 H), 2.76 (t, J = 6.5 Hz, 2 H) and 1.40(s, 9 H). MS
(EI) m/z 314 (M1).
In the next step, [2-(2 or 4-bromomethyl-phenyl)-ethyl]-
carbamic acid tert-butyl ester (106 mg, 0.339 mmol) was treated
with an excess of AgF (180 mg, 1.42 mmol) in dry acetonitrile
(5 mL) for 3 h at 651C. The reaction product was purified via silica
gel column chromatography (50 g Si-gel (Merck), id 2 ꢀ 25 cm)
using a gradient of ethyl acetate 1–10% (v/v) in petroleum ether
(40–601C). [2-(2-fluoromethyl-phenyl)-ethyl]-carbamic acid tert-
butyl ester was obtained as a white amorphous solid (yield 54%).
1H NMR (CDCl3, 250 MHz):d7.18–7.16 (m, 4 H), 5.52 (dd,
J1 = 10.0 Hz, J2 = 15.0 Hz, J[19F–1H] = 50.0 Hz, 1 H), 5.42 (1H, dd,
J1 = 10.0 Hz, J2 = 15.0 Hz, J[19F–1H] = 50 Hz), 3.37 (m, 2H), 2.88
(t, J = 7.5 Hz, 2 H) and 1.41 (s, 9 H). MS (EI) m/z 254 (MH1). [2-(4-
fluoromethyl-phenyl)-ethyl]-carbamic acid tert-butyl ester was
obtained as a white amorphous solid (yield 58%). 1H NMR
(CDCl3, 250 MHz):d7.23–7.21 (m, 4 H), 5.52 (dd, J1 = 10.0 Hz,
J2 = 15.0 Hz, J[19F–1H] = 50.0 Hz, 1 H), 5.42 (1H, dd, J1 = 10.0 Hz,
J2 = 15.0 Hz, J[19F–1H] = 50 Hz), 3.38 (m, 2H), 2.86 (t, J = 7.0 Hz, 2 H)
and 1.40 (s, 9 H). MS (EI) m/z 254 (MH1).
All radioactive compounds were synthesized using the synthetic
pathways and the customized modular Scintomics hotboxthree
system (Scintomics, Fu¨rstenfeldbruck, Germany) as described
earlier,3 using the appropriate time for recovery of the N.C.A.
[18F]-labeled fully protected compounds after HPLC separation
(for the new compounds 2-[18F]FMPAM and 4-[18F]FMPAM this
was 8.2 min).
Quality control
Quality control of the radiofluorinated amino acid analogues
was performed as described earlier.2,3
Quality control of the n.c.a. 2-(2/4-[18F]fluoromethyl-phenyl)-
ethylamine was achieved by HPLC analysis, performed on a
Polaris C8 125 ꢀ 4 mm, 5 m column (Varian) using an EtOH/
aqueous solution of 1 mM CH3COONH4 and 1 mM NaF: 5/95 (v/v)
of pH 6.5 as mobile phase with a flow rate of 1 mL/min while
monitoring both radioactivity (NaI(Tl), Harshaw Chemie) and UV
absorption at 254 nm (Shimadzu). The k0 values were 0.6 and 8.3
for [18F]fluoride and 2-(2/4-[18F]fluoromethyl-phenyl)-ethyla-
mine, respectively.
Shelf-life study experiments
Quantities of 2.86 GBq of each radiofluorinated compound were
synthesized as described earlier. Immediately after the radio-
synthesis CaCl2 was added to a final concentration of 40 mM in a
volume of 5 mL. Solutions were stored at room temperature.
Follow-up of the stock solution showed that the defluorination
rate was limited to 0.5% per hour.
From the stock solution 10 mL aliquots were added to a 20 mL
vial containing 9.990 mL of water, resulting in a solution
containing 5.72 MBq of radiofluorinated compound and
0.04 mM CaCl2. At this concentration of CaCl2, the stabilizing
effect is negligible, as was demonstrated by comparison with a
sample without CaCl2. Samples of 10 mL, containing about
5.72 kBq at time zero, were directly taken from this solution and
injected on the HPLC system at regular time points. The HPLC
conditions applied were the same as described in the Quality
Control subsection, but using only the radioactivity detector. For
each series a t = 0 analysis was performed and the results
corrected for the free 18Fꢁ if present. The results were expressed
as a fraction of the activity at t = 0, using the surface under the
peaks of 18Fꢁ and the radiofluorinated compound. Collection of
the radioactive peak of a calibrated 18Fꢁ solution showed that in
presence of 1 mM NaF, the amount of 18Fꢁabsorbed in the HPLC
system was negligible.
The final step was the deprotection of [2-(2-fluoromethyl-
phenyl)-ethyl]-carbamic acid tert-butyl ester (137 mg,
0.541 mmol) in 20 mL dichloromethane/trifluoroactic acid: 1/1
(v/v) (30 min at room temperature). After the final reaction,
solvents were removed by rotatory evaporation and 2-(2-
Results and discussion
Radiosynthesis
fluoromethyl-phenyl)-ethylamine was obtained as a pale yellow No-carrier added 2-[18F]fluoromethyl-L-phenylalanine (2-
oil (82%).1H NMR (CDCl3, 250 MHz):d7.10–6.99 (m, 4 H), 5.49 [18F]FMLP), 2-[18F]fluoromethyl-D-phenylalanine (2-[18F]FMDP),
(s, J[19F–1H] = 50.0 Hz, 1H), 5.39 (s, J[19F–1H] = 50 Hz, 1 H), 3.16 4-[18F]fluoromethyl-L-phenylalanine (4-[18F]FMLP) and 4-
(t, J = 8 Hz, 2 H), 2.73 (t, J = 8.0 Hz, 2 H), 1.20 (s, 2 H). MS (EI) m/z [18F]fluoromethyl-D-phenylalanine (4-[18F]FMDP) were synthe-
154 (MH1). 2-(4-Fluoromethyl-phenyl)-ethylamine was obtained sized as described earlier3 with
a 30% yield and a
J. Label Compd. Radiopharm 2011, 54 80–85
Copyright r 2010 John Wiley & Sons, Ltd.
www.jlcr.org

K. Kersemans et al.
radiopharmaceutical purity of at least 99%. The ‘mimetic molecules’ show that the fluorobenzyl moiety was more activated for
n.c.a. 2-[18F]fluoromethyl-phenethylamine (2-[18F]FMPAM) and nucleophilic hydroxylation when the benzylic carbon atom was
n.c.a. 4-[18F]fluoromethyl-phenethylamine (4-[18F]FMPAM) were present in the ortho position compared with the para position
synthesized with a 40% yield and 99% radiopharmaceutical purity of the aromatic ring. Remarkably, a difference in the order of
using the same automated modular system and similar synthesis reaction kinetics between the ortho- and para-substituted
strategies. All radiopharmaceuticals were stabilized by addition of analogues was observed. The defluorination of 4-[18F]FMLP
CaCl2. The samples used in the stability experiments were diluted followed a pseudo-first order reaction (dotted line showing Ln %
to obtain a final CaCl2 concentration of 40 mM. The defluorination 4-[18F]FMLP a.f.o. time), indicating that the concentration of the
in these solutions was proven to be the same as in pure water.
second reactant remained constant during the reaction. This
reactant could be water ([H2O] = 55 M) or the hydroxyl ions,
since the ratio [OHꢁ]/[4-[18F]FMLP] at pH 7.0 amounts to at least
5 ꢀ 102 and [OHꢁ] can assumed to be constant.
Hydrolysis study
Less evident was the fact that the hydrolysis of 2-[18F]FMLP
showed zero-order kinetics. Therefore, defluorination of
2-[18F]FMLP was studied at room temperature as a function of
pH, ranging from pH 3.5 to pH 8.0. The reaction rate constant
Kobs (% 18Fꢁ/min.) was calculated for each pH value and the
results presented in Figure 2. Within the recorded pH range the
hydrolysis rate constant of 2-[18F]FMLP increases exponentially
Defluorination of n.c.a. 2-[18F]FMLP and n.c.a. 4-[18F]FMLP in
water at neutral pH at 501C or 801C is shown in Figure 1. At
equal activities and concentrations (ꢂ5.8 ꢀ 10ꢁ10 M), the initial
defluorination rate Vt = 0 of 2-[18F]FMLP at 501C was approxi-
mately 300 times faster than that of n.c.a. 4-[18F]FMLP. At room
temperature the defluorination of 4-[18F]FMLP is too slow to
allow kinetic measurements (o0.5% after 3 h). These results
Figure 1. Defluorination of 2-[18F]FMLP and 4-[18F]FMLP. Y-axis left: percentage of original compound a.f.o. time at 50 and 801C. Y-axis right: ln (% 4-[18F]FMLP) a.f.o. time
and represented by the dotted line.
5
4.5
4
3.5
y = 1.49E-02e6.82E-01x
3
R2 = 9.94E-01
2.5
2
1.5
1
0.5
0
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
pH
Figure 2. Defluorination of N.C.A. 2-[18F]FMLP at room temperature at different pH values.
www.jlcr.org
Copyright r 2010 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2011, 54 80–85

K. Kersemans et al.
with the [OHꢁ] concentration, ruling out H2O as the nucleophile. [18F]-2-Fluoromethyl-phenylalanine: 2-[18F]FMDP was hydro-
On the other hand, the fact that at pH 3.5 Kobs is approximately lysed approximately seven times slower than 2-[18F]FMLP. To
25 times lower than at pH 8.0 eliminates acid hydrolysis by the our knowledge it is the first time that a considerable difference
bulk protons as the initial step in the overall hydrolysis reaction. is reported between the rates of non-enzymatic hydrolysis of the
The subsequent nucleophilic hydroxylation is related to the L and D enantiomers of a fluorobenzyl analogue of phenylalanine.
solvation of [18F]fluoride ions and not to the formation of HF
This stereospecificity requires either the interaction with a
(pKa of 3.2). These findings can only be explained by the earlier chiral reaction partner or the presence of two chiral centers14 in
proposed intramolecular interaction followed by –OH substitu- the fluoromethyl-phenylalanine analogue, the chiral alfa carbon
tion. This intramolecular interaction must then occur between being one of them. Because the OHꢁ ion, the second partner in
the negatively charged benzyl fluorine atom (natural charge the hydrolysis reaction, is not chiral, the origin of the apparent
ꢁ0.4) and a proton of the positively charged ammonium group stereospecificity must be linked to an additional intramolecular
(the pKa of the amino group in phenylalanine is 9.5), in which a interaction. If one assumes that besides the NH13 F BzL internal
dꢁ
hydrogen bridge is formed resulting in an intermediate bridge, a second hydrogen bridge type interaction can occur
H2N1–H---F dꢁ–CBzl (Bzl = benzyl). This model can explain the zero- between the Oꢁ of the carboxylic group and a proton of the
order kinetics whereby the role of the intramolecular hydrogen benzylic carbon atom, a second apparent asymmetric carbon
bridge formation in the reaction kinetics can be compared with the atom appears. According to the Gutman rules15 on electron
common known keto–enol tautomerism in the halogenation of donor–acceptor complexes, confirmed by ab initio quantum
ketones, the typical example of a zero-order reaction. Our hypo- chemical calculations on H-bonding, this interaction transfers
thesis is supported by literature through analogy of the role of the the negative charge to the F-atom (the spill-over effect),
positively charged tropane –N atom (R3NH1) in the non-enzymatic increasing its leaving group capacity or nucleofugality.16,17
acid catalyzed hydrolysis of the methyl ester group of cocaine This results in a faster hydrolysis by the OHꢁ ions. Similar
in water, proven experimentally and by quantum chemical induction of an asymmetric environment due to H-bonding
first-principle electronic structure calculations.12,13
In order to further our hypothesis we also studied the
hydrolytic properties of n.c.a 2-[18F]FMPAM as a ‘mimetic’
compound, since it is in fact decarboxylated 2-fluoromethyl-
phenylalanine. This means that both the position of the NH31
`
group vis a vis the fluorobenzyl group and the probability of
AA–NH 13 F dꢁ–CBzL (benzyl) interaction are comparable to these
in 2-[18F]FMP. In water at pH 6.5 n.c.a. 2-[18F]FMPAM showed a
defluorination rate of 8.5 and 24% per hour at room
temperature and 371C, respectively. (Figure 3). N.c.a.
4-[18F]FMPAM was stable under these conditions. This proves
our hypothesis that a protonated amine in close proximity of the
benzylic fluoromethyl group activates defluorination by an
intramolecular interaction. As depicted in Figure 4, the
defluorination rate of 2-[18F]FMPAM at room temperature
was comparable to that of 2-[18F]fluoromethyl-D-phenylalanine
(2-[18F]FMDP). Remarkably, there was
a
large difference
enatiomers of
in hydrolysis rate between the
L
and
D
Figure 3. Interaction of the amino group with the benzylic fluorine in 2-[18F]FMLP.
70
60
50
40
30
20
10
y = 0.95x
18F-2-Fme-D-Phe
18F-2-Fme-L-Phe
18F2MFfenetylamine
y = 0.14x
0
0
10
20
30
40
50
60
70
80
90
100
time (min.)
Figure 4. Defluorination of 2-[18F]FMLP and 2-[18F]FMDP in water (pH 6.5) at room temperature.
J. Label Compd. Radiopharm 2011, 54 80–85
Copyright r 2010 John Wiley & Sons, Ltd.
www.jlcr.org

K. Kersemans et al.
Figure 5. Molecular mechanics result of the optimized geometry of 2-[18F]FMLP(right) and 2-[18F]FMDP (left), showing the potential intramolecular interactions.
(conformational induction of asymmetry) has recently been the formation of an additional hydrogen bridge between the
studied by some of the authors in the case of the influence of carboxyl group and one of the benzylic hydrogen atoms renders
conformation on the local dissimilarity of the N atom of the the fluorine atom even more susceptible to hydrolysis. The latter
amino group in the enantiomers of alanine and serine.18,19 Both interaction induces an additional chiral center and the
types of interaction are dynamic processes, and their probability probability of its formation differs greatly between L and
depends on the conformation of the different atoms and groups enantiomers of no-carrier added 2-[18F]FMP.
D
within the molecular structures. This mechanism also explains
The gained insights are important, not only with regard to the
the stabilizing role of Ca21 ions.2 The Oꢁ atom of the dissociated design of future radiofluorinated compounds but also for the
carboxylic acid and the negatively charged F-atom of possible application of 2-[18F]FMDP as a potential tumour tracer
the fluorobenzyl entity are involved as ligands in a bidentate as it is expected to be more stable in vivo than its L enantiomer.
Ca21 complex, preventing the occurrence of aforementioned
interaction.
Acknowledgements
The lower defluorination rate of 2-[18F]FMDP compared with
2-[18F]FMLP can be explained by the lower probability of
hydrogen bridge-like interaction of the D compared with the
L enantiomer. This is represented as a ‘static’ image in Figure 5
by a molecular mechanics result of the optimized geometry of
the D and L enantiomers, in which a second chiral center was
introduced by replacing one of the hydrogen atoms of the
benzylic carbon atom by a deuterium atom (CS Chem3D pro 7.0
representation20). At room temperature and neutral pH,
the defluorination rate of 2-[18F]FMDP is comparable to that of
2-[18F]FMPAM, lacking the acid group, suggesting that the
probability of hydrogen bridge interaction between Oꢁ of the
acid group and the benzylic proton in the D enantiomer is very
low and that the defluorination in that case is due for the larger
part to the interaction between the NH13 and the negatively
charged benzylic F-atom.
K. Kersemans wishes to thank the Research Unit High Resolution
NMR and the department of Organic Chemistry of the Vrije
Universiteit Brussel for performing the 1H NMR and ESI-MS
¨
analysis. Finally we thank Prof. Dr Bengt Langstrom for the
helpful discussion on the chiral aspects in our hypothesis.
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123I]Iodo-phenylalanine compared with the
L enantiomer
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Conclusion
A considerable difference in reaction rate is reported for the
non-enzymatic hydrolysis of the L and D enantiomers of no-
carrier added 2-[18F]FMP. The study of this unique difference in
reaction kinetics leads to the elucidation of the reaction
mechanism, which is governed by two distinct intramolecular
interactions. First, the interaction between the amine and the
benzylic fluorine weakens the carbon–fluorine bond. Secondly,
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