New chiral NMR shift reagent for α-amino acids and N-protected oligopeptides in aqueous solution

Article abstract of DOI:10.1246/cl.2001.4

A europium(III) complex with a new chiral ligand, N,N-bis[2-{N-methyl((1S)-1-carboxy-3-methyl)butylamino}ethyl]-glycine, is described as a new chiral shift reagent for α-amino acids and N-acyl-oligopeptides in aqueous solutions without signal broadening on high resolution NMR spectroscopy. In addition, the molecular structure of lanthanum(III) complex with this ligand is revealed by X-ray crystallography.

Full text of DOI:10.1246/cl.2001.4

4
Chemistry Letters 2001
New Chiral NMR Shift Reagent for α-Amino Acids and N-Protected Oligopeptides
in Aqueous Solution
Masaaki Watanabe, Takayuki Hasegawa, Hiroyuki Miyake, and Yoshitane Kojima*
Department of Chemistry, Graduate School of Science, Osaka City University,
Sugimoto, Sumiyoshi-ku, Osaka 558-8585
(Received October 5, 2000; CL-000909)
A europium(III) complex with a new chiral ligand, N,N-
bis[2-{N-methyl((1S)-1-carboxy-3-methyl)butylamino}ethyl]-
glycine, is described as a new chiral shift reagent for α-amino
acids and N-acyl-oligopeptides in aqueous solutions without
signal broadening on high resolution NMR spectroscopy. In
addition, the molecular structure of lanthanum(III) complex
with this ligand is revealed by X-ray crystallography.
Chiral shift reagents are used to resolve enantiomer signals
on NMR spectroscopy. Major studies have been performed
with organic solvents, and a few water-soluble chiral shift
reagents have also been studied.1–5 The europium(III) based
shift reagent is difficult to use on high resolution NMR spec-
troscopy, because the strong paramagnetic effect of the Eu(III)
ion brings a serial signal broadening problem.6 In this paper,
we report a new water-soluble europium(III) chiral shift reagent
with N,N-bis[2-{N-methyl((1S)-1-carboxy-3-methyl}butyl-
amino)ethyl]glycine (H3L)7 which shows enantiomer shifts
without signal broadening at neutral pH regions on high resolu-
tion NMR spectroscopy.
this condition,9 inducing signal broadening by strong paramag-
netic interactions. Since the Eu(III) complex reported here has
no charge, the interaction between the shift reagent and α-amino
acid is considered to be weak at a neutral pH because of mono-
dentate coordination (carboxylate oxygen). Therefore the enan-
tiomer signals are observed without signal broadening on high
resolution NMR.
A 1H-NMR spectrum of (R,S)-alanine((R):(S) = 1:2) with 1/3
amount of EuL8 in D2O solution is shown in Figure 1. Although
the signals of the α-proton are scarcely resolved, those of β-pro-
tons are clearly separated into a pair of enantiomers with the sig-
nal of (S)-enantiomer at the lower magnetic field without signal
broadening. The chemical shift difference between the enan-
tiomer signals (∆∆δ) is small but high resolution NMR spec-
troscopy could distinguish them. Table 1 shows the ∆∆δ values
of several amino acids and their derivatives in the presence of
EuL. The ∆∆δ values showed pH dependence. The optimum pH
for NMR measurement is between 7 and 8. The enantiomer sig-
nals were not resolved under lower pH and were broadened
under a higher pH in which paramagnetic Eu(III) complex
strongly interacts with the amino acids. Several one-negative-
charge Eu(III) chiral shift reagents with four carboxylates, such
as (R)-propylenediaminetetraacetic acid,2 (S,S’)-ethylenediamine-
N,N’-disuccinate (EDDS),3 and (R,R’) or (S,S’)-1,2-diamino-
bisethylene-N,N,N',N'-disuccinate4 have been studied in aqueous
solutions. Their NMR measurements needed in order to observe
a pair of enantiomer signals required a pH = 9–11 on low resolu-
tion NMR spectroscopy. The substrate should coordinate to the
Eu(III) center in a chelate form (amine and carboxylate) under
The molecular structure of a lanthanum(III) complex with
the ligand described here was determined by X-ray analysis10 at
297 K (Figure 2), revealing that the complex exists in a trimeric
form. The coordination geometry of each La(III) ion is a distort-
ed tricapped trigonal prism. Each La(III) ion is coordinated by
three amine nitrogens and three carboxylate oxygens from the
ligand, a water oxygen and chelated oxygens from one carboxy-
late moiety of the neighboring ligand, making a nine coordination
structure. One of the chelating oxygens bridges between the
La(III) ions to make a trimer to the neighboring La(III) center.
Disordered crystal water molecules occupy the central cavity of
the trimer. Each La–La nonbonding distance is 4.98 Å. The
bond distances from both the terminal amines to the metal ion are
longer than that from the central amine. Among the terminal
amines, the bond lengths of La–N(1), N(4) and N(7) are longer
than that of La–N(3), N(6) and N(9), respectively. This discrep-
ancy is attributed to the bridging carboxylate arm being stretched
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
5
monomeric one in the presence of the third substrate to show
chiral recognition on a 1H-NMR study.
M.W. thanks the support of the Graduate School of
Science, Osaka City University (Special Fund for the Student).
References and Notes
1
T. J. Wenzel, “NMR Shift Reagent”, CRC Press, Boca Ratton,
FL(1987).
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1987, 670.
2
3
4
5
6
7
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(1991).
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4, 147 (1999).
Treatment of L-leucine methylester hydrochloride (Leu-OMe·HCl)
(20.0 g, 0.110 mol) and glyoxal dimethylacetal 45% t-butyl methyl
ether solution (28.0 g, 0.121 mol) with NaBH3CN (7.90 g, 0.111
mol) in MeOH gave N-(2,2-dimethoxyethyl)-Leu-OMe (12.0 g,
0.0571 mol; 47.1%), followed by N-methylation with excess HCHO
aq (12.9 g, 0.155 mol) and NaBH3CN (3.30 g, 0.0524 mol) in
MeOH. N-(2,2-dimethoxyethyl)-N-methyl-Leu-OMe (8.3 g, 0.0336
mol; 65.2%) was treated with 28% HBr/AcOH to give correspon-
ding aldehyde. The aldehyde was condensed with glycine
methylester hydrochloride (1.45 g, 0.0155 mol) in the presence of
NaBH3CN (0.73 g, 0.0156 mol) in MeOH/H2O to give L trimethyl
ester (1.70 g, 0.00372 mol; 32.3%), which was hydrolyzed by
LiOH·H2O (0.703 g, 0.0167 mol) in MeOH/H2O to give lithium salt
(0.829 g, 0.00170 mol; 45.7%) of title ligand. Anal. Calcd for
C20H36N3Li3O6·3H2O (hygroscopic): C, 49.09; H, 8.65; N, 8.59%.
Found: C, 48.95; H, 8.58; N, 8.81%. [α]D = 4.39 ° (MeOH). IR
(KBr) 1624 cm–1 (νC=Ο). FAB-MS m/z 424 [M+Li]+. 1H-NMR (300
MHz, D2O, TSP): δ 0.93(d, J = 6.6 Hz, 12H), 1.36 (m, 2H), 1.55(m,
2H), 1.73 (m, 2H), 2.42 (s, 6H), 2.62–2.95 (m, 8H), 3.21 (s, 2H),
3.25 (dd, J = 4.0 Hz, 10.3 Hz, 2H).
The stock solution of EuL complex was prepared from EuCl3·6H2 O
and equivalent amount of Li3L in D2O. Samples for measurements
were prepared by mixing a substrate (0.05 mmol, (R):(S) = 1:2)),
stock solution of EuL(1 ml, 0.017 M) and small amount of NaOD or
DCl solution for pH adjustment. Pure EuL complex was obtained
as white precipitate from Li3L and EuCl3·6H2O in small amount of
water. This complex is solved in both water and organic solvents
(CHCl3, CH3OH etc.). Anal. Calcd for C20H36N3EuO6·2.5H2O: C,
39.22; H, 6.92; N, 6.83%. Found: C, 39.28; H, 6.87; N, 6.75 %.
FAB-MS m/z 565, 567 [M]–.
8
away from the chelated metal in order to coordinate with the
neighboring metal. In addition, the bond lengths of La–O(5),
O(12) and O(19) are longer than those of La–O(1), O(8) and
O(15), respectively, because of the steric hindrance of the side
chain. The average bond lengths of La–O(2.461 Å) and
La–N(2.754 Å) are similar to those observed in [La(HTTHA)]2–
(average bond length: La–O, 2.492 Å; La–N, 2.812 Å),11
[La2(EDTA)2(H2O)4]2– (La–O, 2.545 Å; La–N, 2.812 Å)12 and
[La2(HEDTA)2(H2O)4] (La–O, 2.520 Å; La–N, 2.785 Å).13
The ESI-MS showed that the Eu(III) complex exists in a
mixture of monomeric and polymeric structures (m/z 597, 599
[EuL+MeOH]–, 1149 [Eu2L2+OH]–, 1732 [Eu3L3+Cl]– etc.) in
methanol. The addition of alanine to this solution reduced these
complicated equilibria to a simple monomeric ternary complex.
(m/z 653, 655 [EuL+alanine]–). A 1H-NMR spectrum of LaL in
D2O showed broad signals, which were changed to sharp sig-
nals in the presence of an equimolar amount of alanine, sug-
gesting that the polymerization of La(III) complex was inhibit-
ed by a substrate as indicated by ESI-MS. The X-ray analyses
of polyaminocarboxylate–lanthanide(III) complexes sometimes
showed polymeric structures.11–14 Furthermore, it has been
reported concerning the [Tb(EDDS)]– complex that polynuclear
species were formed at a low pH in aqueous solutions by lumi-
nescence study.15 So, it is first interesting to observe that poly-
meric lanthanide(III) complex changes its structure to a
9
H. G. Brittain, J. Am. Chem. Soc., 102, 3693 (1980); A. D. Sherry,
C. A. Stark, J. R. Ascenso, and C. F. G. C. Geraldes, J. Chem. Soc.
Dalton Trans., 1981, 2078; L. Spaulding and H. G. Brittain, Inorg.
Chem., 24, 3692 (1985).
10 Crystal data for [La3(L)3(H2O)3]·9H2O: C60H132N9La3O30 (fw =
1876.46), orthorhombic, space group I222(#23), a = 25.6915(3) Å,
b = 25.7038(3) Å, c = 25.6975(3) Å, V = 16969.8(4) Å3, Dcalcd
=
1.469 g/cm3, Dobs = 1.48 g/cm3, µ(Mo Kα) = 15.55 cm–1, R = 0.033
(Rw = 0.043) on 7917 reflections (I > 2.00 σ(I)). Anal. Calcd for
C60H108N9La3O18·13H2O: C, 38.04; H, 7.13; N, 6.65%. Found: C,
38.06; H, 7.16; N, 6.68%, indicating a position of one crystal water
was not determined by X-ray analysis.
11 R. Ruloff, P. Prokop, J. Sieler, E. Hoyer, and L. Beyer, Z.
Naturforsch., 51b, 963 (1996); R. Y. Wang, J. R. Li, T. Z. Jin, G. X.
Xu, Z. Y. Zhou, and X. G. Zhou, Polyhedron, 16, 1361 (1997).
12 S. Y. Chen, C. H. Wang, D. Li, and X. Wang, Sci. China, Ser B, 32,
918 (1989).
13 C. C. Fuller, D. K. Molzahn, and R. A. Jacobson, Inorg. Chem., 17,
2138 (1978).
14 M. B. Inoue, M. Inoue, and Q. Fernando, Acta Crystallogr., Sect. C,
50, 1037 (1994); S. J. Franklin and K. N. Raymond, Inorg. Chem.,
33, 5794 (1994); S. Aime, A. Barge, F. Benetollo G. Bombieri, M.
Botta, and F. Uggeri, Inorg. Chem., 36, 4287 (1997).
15 L. Spaulding and H. G. Brittain, Inorg. Chim. Acta, 110, 197
(1985).