Catalytic and selective conversion of glycine into serine by the reaction with formaldehyde in a neutral aqueous solution

Article abstract of DOI:10.1246/bcsj.75.1383

Di- or trinuclear Cu(II) complexes efficiently catalyzed the condensation of glycine with formaldehyde to yield serine at pH 7.3, 50°C.

Full text of DOI:10.1246/bcsj.75.1383

c. Jpn.
Bull. Chem. Soc. Jpn., 75, 1383–1384 (2002) 1383
e Chemical
ciety of Ja-
n
Short Articles
e Chemical
ciety of Ja-
n
Catalytic and Selective Conversion of
Glycine into Serine by the Reaction
with Formaldehyde in a Neutral
Aqueous Solution
02
M. Yashiro
83
84
Morio Yashiro
SJA8
09-2673
2701
Department of Chemistry and Biotechnology, Graduate
School of Engineering, The University of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113-8656
Fig. 1. Structures of the ligands used in this study and pro-
posed coordination modes of the Cu(Ⅱ) complexes. w de-
notes an appropriate ligand, typically water.
02
(Received January 10, 2002)
02
.4
Di- or trinuclear Cu(Ⅱ) complexes efficiently catalyzed
the condensation of glycine with formaldehyde to yield serine
at pH 7.3, 50 °C.
Table 1. The Condensation of Glycine with Formaldehyde
Catalyzed by Cu(Ⅱ) Complexes at pH 7.3 (200 mmol dm−3
HEPES buffer) and 50 °C for 2 ha)
Entry
Catalyst
Yield of serine/%
Turnoverc)
1
2
CuCl2
1–CuCl2
2–(CuCl2)2
2–(CuCl2)2
3–(CuCl2)2
4–(CuCl2)3
1
1
47
19
24
33
0.2
0.2
9.4
3.8
4.8
6.6
The catalytic role of a multinuclear metal center is of cur-
rent interest in fields related to bio-inorganic chemistry and
catalytic chemistry. The α-CH2 or α-CHR location of an α-
amino acid is activated upon chelating with a metal ion, caus-
ing a racemization1 and/or a C–C bond formation2–7 under ba-
sic conditions. For such activation of α-amino acids, a multi-
nuclear metal ion center is expected to be superior to a mono-
nuclear metal ion center.
3
4b)
5
6
a) [glycine]0 = 20 mmol dm−3, [HCHO]0 = 3 mol dm−3
[complex] = 1 mmol dm−3. b) [HCHO]0 = 0.3 mol dm−3
c) Based on the copper(Ⅱ) complex used.
,
.
Pioneering works by Akabori et al. showed that the conden-
sation of glycine, coordinated to Cu(Ⅱ), with formaldehyde
proceeded under basic conditions to yield serine selectively.3
The condensation is known to occur similarly when a
dipeptide4 or a tripeptide5 of glycine is used instead of glycine,
and both reactions result in the selective modification of the N-
terminal glycine residue. Because the N-terminal amino acid
residue of peptides and proteins often plays important roles in
determining their properties,8 the chemical method for the con-
version of an N-terminal amino acid residue into another one is
potentially fascinating. For the practical applications, howev-
er, the reaction should be conducted under neutral conditions,
in order to avoid undesirable side reactions of peptides.6,7 This
paper deals with the effect of multinuclear Cu(Ⅱ) complexes
for the activation of glycine to promote the condensation with
formaldehyde under neutral conditions.
(CuCl2)2, 3–(CuCl2)2, or 4–(CuCl2)3. The reaction yielded
serine as a main product, and a small amount of 2-(hydroxym-
ethyl)serine4 was formed. In contrast, the yield of serine was
low when CuCl2 or 1–CuCl2 was used.
Figure 2 shows α-amino acid distributions depending on the
reaction time catalyzed by 2–(CuCl2)2. Up to ca. 2 h, the
amount of serine increased with the decrease of that of glycine,
indicating the selective conversion of glycine into serine. Even
Ligands, 2,9 3, and 4,10 which have two or three bis(2-py-
ridylmethyl)amine (DPA, 1) moieties, were used to generate
di- or trinuclear Cu(Ⅱ) complexes (Fig. 1), because DPA is
known to bind Cu(Ⅱ) with a sufficient stability (log K 9.31).11
The Cu(Ⅱ) complexes of 1, 2, 3, and 4 were prepared by sim-
ply mixing CuCl2•2H2O and the ligand in an appropriate molar
ratio in an aqueous solution just before the reaction.
The results for the reactions of glycine with formaldehyde
by using the Cu(Ⅱ) complexes are summarized in Table 1.
Glycine was converted to serine at pH 7.3 and 50 °C in the
presence of a catalytic amount of the Cu(Ⅱ) complex, 2–
Fig. 2. Effect of reaction time for the condensation of gly-
cine with formaldehyde catalyzed by 2–(CuCl2)2.
ꢀ:glycine, ꢁ:serine, ꢂ:2-(hydroxymethyl)serine.
[glycine]0 = 20 mmol dm−3, [HCHO]0 = 3 mol dm−3, [2–
(CuCl2)2] = 1 mmol dm−3, pH 7.3 (200 mmol dm−3
HEPES buffer), 50 °C.

1384 Bull. Chem. Soc. Jpn., 75, No. 6 (2002)
© 2002 The Chemical Society of Japan
8Hz), 7.93 (d, 4H, J = 8Hz), 7.81 (t, 4H, J = 7Hz), 7.08 (s, 4H),
4.27 (s, 8H), 3.74 (s, 4H).
if one prolonged the reaction period, however, the amount of
serine did not increase remarkably in spite of the decrease of
glycine. Furthermore, the total amount of the starting and re-
sulting α-amino acids decreased, indicating that side reactions
besides the condensation occurred by prolonged treatment.
Therefore, under the conditions employed, the best result for
the selective conversion of glycine into serine was achieved
with a short reaction time (ca. 2 h) by using 2–(CuCl2)2.
When glycylglycine was used instead of glycine, the reac-
tion with formaldehyde at pH 7.3 and 50 °C yielded serylgly-
cine (20% by 2–(CuCl2)2 and 30% by 4–(CuCl2)3), but gly-
cylserine was not formed at all. The selective formation of ser-
ylglycine indicates that the condensation proceeds via a Schiff
base intermediate of glycylglycine and formaldehyde, i.e. N-
methylideneglycylglycine. Such a mechanism has been pro-
posed for the CuSO4-assisted condensation of glycylglycine
with formaldehyde in 0.2 M Na2CO3 at 100 °C, which also re-
sulted in the selective formation of serylglycine.4 It is note-
worthy that the present reaction yielded only trace amounts of
fragmentary amino acids as byproducts.4 Because the present
reaction was conducted under neutral conditions, the hydroly-
sis of the peptide bond proceeded only marginally.
The present results explicitly indicate the effectiveness of
the multinuclear Cu(Ⅱ) complexes in the condensation of gly-
cine and formaldehyde under neutral conditions. The Schiff
base intermediate, formed from glycine and formaldehyde, is
in equilibrium between a major aminomethanol form and a mi-
nor imine form in aqueous solutions, and the condensation
proceeds via the imine intermediate.6b Accordingly, possible
roles of a multinuclear Cu(Ⅱ) center in the present reaction are
(1) to bind N-methylideneglycine more strongly than a mono-
nuclear Cu(Ⅱ) and (2) to shift the equilibrium between N-(hy-
droxymethyl)glycine and N-methylideneglycine to the latter.
Hence, the dissociation of the α-methylene proton of the gly-
cine moiety readily occurs, resulting in the smooth condensa-
tion even under neutral conditions.
Condensation of Glycine with Formaldehyde Catalyzed by
the Cu(ꢀ) Complexes. In a typical experiment, to a 9.0-mL por-
tion of an aqueous solution of CuCl2•2H2O (0.020 mmol),
2•4(HClO4) (0.010 mmol), and glycine (0.20 mmol) in buffer (200
mmol dm−3 HEPES, pH 7.5) was added 1.0 mL of aqueous form-
aldehyde solution (35%). The pH of the solution was adjusted to
7.30, and the resulting solution was heated at 50 °C. After an ap-
propriate reaction period, the reaction was quenched by adding
0.20 mL of concentrated HCl to a 1.0-mL aliquot of the reaction
mixture. The resulting solution was analyzed by HPLC: Column,
TOSOH TSKgel Aminopak; eluent, citrate buffer (67 mM, pH
3.41); flow rate, 0.6 mL min−1; fluorometrically detected (ex 345
nm, em 455 nm) after the post column reaction with o-phthalalde-
hyde at 60 °C. Condensations catalyzed by CuCl2, 1–CuCl2, 3–
(CuCl2)2, or 4–(CuCl2)3 were conducted similarly.
The author thanks Mitsutoshi Nakamura, Dr. Tohru
Takarada, and Prof. Makoto Komiyama for performing prelim-
inary tests, and Prof. Kazuhiko Saigo for critical reading of the
manuscript. This work was partially supported by a Grant-in-
Aid for Scientific Research (no. 12650845) from the Ministry
of Education, Culture, Sports, Science and Technology.
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