Interligand CH ... π interaction in binary cobalt(III) complexes of N-pyridoxy-L-amino acids. Biological significance of the pyridoxal 2-methyl group

Article abstract of DOI:10.1039/a702900d

Reaction of K₃[Co(CO₃)₃] with 2 equivalents of N-pyridoxy-L-amino acid (pyridoxy = 3-hydroxy-5-hydroxymethyl-2-methylpyridin-4-ylmethyl; amino acid = alanine, leucine, phenylglycine, phenylalanine, p-nitrophenylalanine, p-

Full text of DOI:10.1039/a702900d

View Article Online / Journal Homepage / Table of Contents for this issue
Interligand CH ؒ ؒ ؒ ð interaction in binary cobalt(III) complexes of
N-pyridoxy-L-amino acids. Biological significance of the pyridoxal
2-methyl group
Koichiro Jitsukawa,*,a Kouji Iwai,a Hideki Masuda,*,a Hisanobu Ogoshib and Hisahiko Einagaa
a Department of Applied Chemistry, Faculty of Engineering, Nagoya Institute of Technology,
Showa-ku, Nagoya 466, Japan
b Department of Synthetic Chemistry, Faculty of Engineering, Kyoto University, Yoshida,
Sakyo-ku, Kyoto 606, Japan
Reaction of K3[Co(CO3)3] with 2 equivalents of N-pyridoxy--amino acid (pyridoxy = 3-hydroxy-5-
hydroxymethyl-2-methylpyridin-4-ylmethyl; amino acid = alanine, leucine, phenylglycine, phenylalanine,
p-nitrophenylalanine, p-methoxyphenylalanine, or tryptophan) afforded the bis(N-pyridoxy--amino
acidato)cobalt() complex as a single band on anion-exchange chromatography. Each complex has been isolated
as its sodium salt and characterized in aqueous solution by UV/VIS circular dichroism (CD), and 1H NMR
spectra and by X-ray crystallographic analysis in the solid state. The cobalt() atom is co-ordinated to two
trans(N)-meridional N-pyridoxy--amino acid ligands. The 2-methyl group of one pyridoxal moiety lies over the
aromatic ring of another pyridoxal ring with separations of 3.67 and 3.56 Å. The 1H NMR resonance of the
2-methyl group shows an upfield shift when compared with that of the free pyridoxyamino acid. The selective
formation of the trans(N)-meridional-Λ-[R(N), R(N)] diastereoisomer is interpreted in terms of an interligand
CH ؒ ؒ ؒ π interaction. The CD and 1H NMR spectra suggest that addition of 1 equivalent HCl weakens this
interaction. The biological significance of the pyridoxy 2-methyl group in vitamin B6 coenzyme is discussed.
Non-covalent interactions such as hydrogen bonding and elec-
trostatic interactions are crucial for molecular recognition and
catalysis in biological systems.1–3 The side-chain groups of
amino acid residues in proteins are sometimes responsible for
such interactions. The interactions at the active site of a
Sigel17 showed that the stability of ternary complexes contain-
ing an amino acid and 2,2Ј-bipyridine or 1,10-phenanthroline
increases with an increase in the interligand interaction between
aromatic and aliphatic groups. Okawa13 also suggested the
presence of a CH ؒ ؒ ؒ π interaction through a synthetic study on
bi- and tri-valent metal complexes of 1--menthoxy-3-
benzoylacetone, although this work gives no direct evidence of
such an interaction. Recently, in the host–guest complexation
between resorcinol cyclic tetramer and organic monool com-
pounds, the presence of an attractive interaction has been pos-
tulated.18 However, this complexation was strongly assisted by
hydrogen-bonding interactions. Direct characterization of these
weak interactions is very difficult in any case.
Previously, in the complexation of N-pyridoxy--
phenylalanine† with CoIII we suggested that the pyridoxy 2-
methyl group causes an inductive CH ؒ ؒ ؒ π interaction with
both of pyridoxy pyridine and phenylalanyl benzene rings.19
The N-pyridoxy--amino acid is a derivative of vitamin B6
coenzyme, pyridoxal 5-phosphate, which plays an important
role in amino acid metabolism, and the biological significance
of the functional groups of the pyridoxal, i.e. pyridyl 1-N, 3-O
and 4-azomethine in its metabolic intermediate, and 5-
phosphate ester, has long been discussed by many chemists,
biochemists, and biologists.20–25 That of the pyridoxy 2-methyl
group, however, is still now open to detailed investigations. In
the present paper, focusing upon a possible CH ؒ ؒ ؒ π interaction
of this group in the pyridoxal coenzyme system, we describe a
further examination on the CH ؒ ؒ ؒ π interaction by use of other
aliphatic or aromatic amino acids in the place of phenylalanine
by means of electronic absorption (UV/VIS), circular dichro-
ism (CD), and 1H NMR spectra and crystal structure analysis.
The findings obtained are interesting from the view point of
molecular recognition.19
enzyme–substrate complex in
a metalloenzyme can be
regarded as ligand–ligand interactions around the metal atom
and these play a vital role in enzyme catalysis. Examples dem-
onstrating such multibonding interactions are carboxypepti-
dase A (CPA) with the model substrate glycyl--tryosine4,5 and
leucine aminopeptidase (LAP) with bestatin.6,7 In the former
the guanidinium group of arginine-145 of CPA and a hydro-
phobic pocket interact with the carboxylate and the tyrosine
phenol group of the substrate, respectively, and in the latter the
carboxylate group of aspartic acid-273 of LAP and a hydro-
phobic pocket interact with the amino and the leucine isopro-
pyl group of the substrate, respectively, in which they fix each
other in a specific way around the central zinc ion. The inter-
actions present in them are the directional bindings and these
binding energies are rather high (3–10 kcal molϪ1; cal = 4.184
J).2 An appropriate orientation of these interacting groups
in biological systems facilitates the reaction selectivity of
substrates.
We have been studying weak interactions such as intra-
molecular aromatic ring stacking, electrostatic ligand–ligand
interaction and hydrogen bonding, as a simple molecular rec-
ognition model for enzyme–substrate complexes, by spectro-
scopic, thermodynamic and X-ray diffraction methods.8–11 On
the other hand, hydrophobic interactions between aliphatic
groups and between aliphatic and aromatic groups, which are
rather weak in comparison with the above-mentioned inter-
actions, are also known to occur in biological systems.2 These
interactions are considered to contribute to determining their
structural conformation. An aliphatic–aromatic or more specif-
ically a CH ؒ ؒ ؒ π interaction has been recognized by some spec-
troscopists and X-ray crstallographers.12–16 There are some pre-
vious reports on this sort of weak interaction. Fischer and
† Pyridoxy = 3-hydroxy-5-hydroxymethyl-2-methylpyridin-4-ylmethyl;
pyridoxal = 3-hydroxy-5-hydroxymethyl-2-methylpyridine-4-carbalde-
hyde.
J. Chem. Soc., Dalton Trans., 1997, Pages 3691–3698
3691

View Article Online
3.23 (dd, 1 H, J = 15.0 and 4.6, β-H of amino acid), 2.97 (dd, 1
H
C
H, J = 15.0 and 8.8 Hz, β-H of amino acid) and 2.31 (s, 3 H,
pyridoxal 2-CH3). H2pw: δ 7.65 (d, 1 H, J = 7.9, indole 4-H),
7.47 (d, 1 H, J = 8.1, indole 7-H), 7.42 (s, 1 H, pyridoxal 6-H),
4.44 (d, 1 H, J = 12.4, pyridoxal 5-CH2), 7.22 (dd, 1 H, J = 7.9
and 7.2, indole 5-H), 7.15 (s, 1 H, indole 2-H), 7.12 (dd, 1 H,
J = 8.1 and 7.2, indole 6-H), 4.16 (d, 1 H, J = 12.4, pyridoxal 5-
CH2), 3.94 (d, 1 H, J = 12.1, pyridoxal 4-CH2), 3.57 (d, 1 H,
J = 12.1, pyridoxal 4-CH2), 3.55 (dd, 1 H, J = 7.3 and 6.4, α-H
of amino acid), 3.13 (dd, 1 H, J = 14.4 and 6.4, β-H of amino
acid), 3.01 (dd, 1 H, J = 14.4 and 7.3 Hz, β-H of amino acid)
and 2.28 (s, 3 H, pyridoxal 2-CH3).
R
CO2H
NH
CH2
HOCH2
OH
4
3
2
5
6
1
CH3
N
H2pa
H2pf
CH3
CH2
H2pnf
CH2
NO2
Cobalt complexes. All cobalt() complexes were prepared as
follows. To an aqueous solution (25 cm3) of K3[Co(CO3)3] (1.25
mmol) was added N-pyridoxy--amino acid hydrochloride (2.5
mmol) and the mixture stirred for 12 h at room temperature.
The resulting mixture was passed through a column of QAE
Sephadex A-25 resin (ClϪ form, 3.6 × 25 cm), and the adsorbed
compounds separated using aqueous 0.1  NaCl. One brown
band was eluted. Desalting with ethanol gave the sodium bis(N-
pyridoxyamino acidato)cobalt() complex as a brown solid;
40% yield for Na[Co(pa)2]ؒ5.5H2O (Found: C, 40.44; H, 6.02;
N, 8.74. Calc. for C22H28CoN4NaO8ؒ5.5H2O 1: C, 40.19; H,
5.98; N, 8.51%). Yields of the other complexes were as follows:
32% for Na[Co(pf)2] 2, 48% for Na[Co(pl)2] 3, 26% for
Na[Co(ppg)2] 4, 20% for Na[Co(pnf)2] 5, 9.0% for Na[Co(p-
mf)2] 6 and 4.3% for Na[Co(pw)2] 7. 1H NMR spectra data (400
MHz D2O) were as follows. Na[Co(pa)2] 1: δ 7.79 (s, 1 H, pyri-
doxal 6-H), 4.87 (d, 1 H, J = 12.5, pyridoxal 5-CH2), 4.83 (d, 1
H, J = 12.5, pyridoxal 5-CH2), 4.59 (d, 1 H, J = 12.5, pyridoxal
4-CH2), 4.10 (d, 1 H, J = 12.5, pyridoxal 4-CH2), 4.37 (q, 1 H,
J = 7.0, α-H of amino acid), 1.18 (s, 3 H, pyridoxal 2-CH3) and
1.86 (d, 3 H, J = 7.0 Hz, β-CH3). Na[Co(pf)2] 2: δ 7.71 (s, 1 H,
pyridoxal 6-H), 7.50 (d, 2 H, J = 7.3, benzene), 7.36 (dd, 2 H,
J = 7.3 and 7.3, benzene), 7.28 (t, 1 H, J = 7.3, benzene), 4.77 (s,
2 H, pyridoxal 5-CH2), 4.71 (dd, 1 H, J = 5.7 and 3.4, α-H of
amino acid), 4.54 (d, 1 H, J = 12.8, pyridoxal 4-CH2), 4.02 (d, 1
H, J = 12.8, pyridoxal 4-CH2), 3.82 (dd, 1 H, J = 14.8 and 5.7,
β-CH2), 3.62 (dd, 1 H, J = 14.8 and 3.4 Hz, β-CH2) and 0.84 (s,
3 H, pyridoxal 2-CH3). Na[Co(pl)2] 3: δ 7.87 (s, 1 H, pyridoxal
6-H), 4.93 (d, 1 H, J = 12.7, pyridoxal 5-CH2), 4.84 (d, 1 H,
J = 12.7, pyridoxal 5-CH2), 4.67 (d, 1 H, J = 12.7, pyridoxal 4-
CH2), 4.18 (d, 1 H, J = 12.7, pyridoxal 4-CH2), 4.35 (dd, 1 H,
J = 3.2 and 2.4, α-H of amino acid), 2.40–2.05 (m, 3 H, β- and
γ-H of amino acid), 1.31 (s, 3 H, pyridoxal 2-CH3), 1.13 (d, 3 H,
J = 6.4, δ-H of amino acid) and 1.05 (d, 3 H, J = 6.4 Hz, δ-H of
amino acid). Na[Co(ppg)2] 4: δ 7.72 (s, 1 H, pyridoxal 6-H),
7.69–7.62 (m, 5 H, benzene), 5.44 (s, 1 H, α-H of amino acid),
4.52 (d, 1 H, J = 12.2, pyridoxal 5-CH2), 4.42 (d, 1 H, J = 12.2,
pyridoxal 5-CH2), 4.38 (d, 1 H, J = 12.2, pyridoxal 4-CH2), 4.28
(d, 1 H, J = 12.2 Hz, pyridoxal 4-CH2) and 1.17 (s, 3 H, pyri-
doxal 2-CH3). Na[Co(pnf)2] 5: δ 7.71 (s, 1 H, pyridoxal 6-H),
7.27 (d, 2 H, J = 8.1, benzene), 6.76 (d, 2 H, J = 8.1, benzene),
4.75 (s, 2 H, pyridoxal 5-CH2), 4.64 (dd, 1 H, J = 5.5 and 2.9, α-
H of amino acid), 4.53 (d, 1 H, J = 12.8, pyridoxal 4-CH2), 4.02
(d, 1 H, J = 12.8, pyridoxal 4-CH2), 3.71 (dd, 1 H, J = 15.0 and
5.5, β-H of amino acid), 3.51 (dd, 1 H, J = 15.0 and 2.9 Hz, β-H
of amino acid) and 0.87 (s, 3 H, pyridoxal 2-CH3). Na[Co-
(pmf)2] 6: δ 7.70 (s, 1 H, pyridoxal 6-H), 7.42 (d, 2 H, J = 8.6,
benzene), 6.94 (d, 2 H, J = 8.6, benzene), 4.76 (s, 2 H, pyridoxal
5-CH2), 4.67 (dd, 1 H, J = 5.6 and 3.0, α-H of amino acid), 4.52
(d, 1 H, J = 12.8, pyridoxal 4-CH2), 4.02 (d, 1 H, J = 12.8, pyri-
doxal 4-CH2), 3.88 (s, 3 H, methoxy), 3.78 (dd, 1 H, J = 15.0
and 5.6, β-H of amino acid), 3.58 (dd, 1 H, J = 15.0 and 3.0 Hz,
β-H of amino acid) and 0.83 (s, 3 H, pyridoxal 2-CH3).
Na[Co(pw)2] 7: δ 7.84 (d, 1 H, J = 7.9, indole 4-H), 7.63 (s, 1 H,
pyridoxal 6-H), 7.42 (s, 1 H, indole 2-H), 7.41 (d, 1 H, J = 8.1,
indole 7-H), 7.19 (dd, 1 H, J = 7.9 and 7.0, indole 5-H), 7.13
(dd, 1 H, J = 8.1 and 7.0, indole 6-H), 4.55 (d, 1 H, J = 12.8,
H2pmf
H2pw
CH2
CH2
OCH3
CH3
CH
H2pl
CH2
CH3
NH
H2ppg
C6H5
Experimental
Synthesis
N-Pyridoxy amino acids. A mixture of -amino acid (20
mmol), KOH (40 mmol), and pyridoxal hydrochloride (20
mmol) in MeOH (40 cm3) was stirred at room temperature until
it became yellow. The pyridoxylidene--amino acid Schiff-base
solution was then treated with hydrogen gas (3 atm, ca. 3 × 105
Pa) for 3–4 h in the presence of PtO2 (0.2 g). Following
filtration, an appropriate amount of methanolic HCl was added
to afford a quantitative yield of the N-pyridoxy--amino acid
hydrochloride. These were characterized by 1H NMR spec-
troscopy as follows.
H2pa: δ 7.54 (s, 1 H, pyridoxal 6-H), 4.58 (s, 2 H, pyridoxal
5-CH2), 4.18 (d, 1 H, J = 13.2, pyridoxal 4-CH2), 4.07 (d, 1 H,
J = 13.2, pyridoxal 4-CH2), 3.45 (q, 1 H, J = 7.2, α-H of
amino acid), 2.33 (s, 3 H, pyridoxal 2-CH3), 1.40 (d, 3 H, J = 7.2
Hz, β-CH3). H2pf: δ 7.58 (s, 1 H, pyridoxal 6-H), 7.34 (dd, 2 H,
J = 7.6 and 6.4, benzene), 7.31 (t, 1 H, J = 6.4, benzene), 7.21 (d,
2 H, J = 7.6, benzene), 4.49 (s, 2 H, pyridoxal 5-CH2), 4.16 (d, 1
H, J = 14.0, pyridoxal 4-CH2), 3.93 (d, 1 H, J = 14.0, pyridoxal
4-CH2), 3.56 (dd, 1 H, J = 7.9 and 5.4, α-H of amino acid), 3.12
(dd, 1H, J = 14.1 and 5.4, β-H of amino acid), 2.95 (dd, 1 H,
J = 14.1 and 7.9 Hz, β-H of amino acid) and 2.25 (s, 3 H, pyri-
doxal 2-CH3). H2pl: δ 7.49 (s, 1 H, pyridoxal 6-H), 4.59 (d, 1 H,
J = 12.1, pyridoxal 5-CH2), 4.51 (d, 1 H, J = 12.1, pyridoxal 5-
CH2), 3.92 (d, 1 H, J = 12.1, pyridoxal 4-CH2), 3.59 (d, 1 H,
J = 12.1, pyridoxal 4-CH2), 3.22 (dd, 1 H, J = 7.3 and 7.0, α-H
of amino acid), 2.31 (s, 3 H, pyridoxal 2-CH3), 1.60–1.30 (m, 3
H, β- and γ-H of amino acid), 0.89 (d, 3 H, J = 6.4, δ-H of
amino acid) and 0.84 (d, 3 H, J = 6.4 Hz, δ-H of amino acid).
H2ppg: δ 7.44 (s, 1H, pyridoxal 6-H), 7.36–7.41 (m, 5 H, ben-
zene), 4.41 (d, 1 H, J = 12.2, pyridoxal 5-CH2), 4.38 (d, 1 H,
J = 12.2, pyridoxal 5-CH2), 4.24 (s, 1 Hz, α-H of amino acid),
3.88 (d, 1 H, J = 12.2, pyridoxal 4-CH2), 3.66 (d, 1 H, J = 12.2
Hz, pyridoxal 4-CH2) and 2.31 (s, 3H, pyridoxal 2-CH3). H2pnf:
δ 7.55 (s, 1 H, pyridoxal 6-H), 7.00 (d, 2 H, J = 7.5, benzene),
6.74 (d, 2 H, J = 7.5, benzene), 4.56 (s, 2 H, pyridoxal 5-CH2),
4.36 (d, 1 H, J = 14.2, pyridoxal 4-CH2), 4.18 (d, 1 H, J = 14.2,
pyridoxal 4-CH2), 3.78 (dd, 1 H, J = 8.7 and 4.4, α-H of amino
acid),3.19(dd,1H,J = 15.0and4.4,β-Hof aminoacid),2.94(dd,
1 H, J = 15.0 and 8.7 Hz, β-H of amino acid) and 2.36 (s, 3 H,
pyridoxal 2-CH3). H2pmf: δ 7.54 (s, 1 H, pyridoxal 6-H), 7.14
(d, 2 H, J = 8.4, benzene), 6.90 (d, 2 H, J = 8.4, benzene), 4.55
(s, 2 H, pyridoxal 5-CH2), 4.35 (d, 1 H, J = 14.0, pyridoxal 4-
CH2), 4.16 (d, 1 H, J = 14.0, pyridoxal 4-CH2), 3.82 (s, 3 H,
methoxy), 3.78 (dd, 1 H, J = 8.8 and 4.6, α-H of amino acid),
3692
J. Chem. Soc., Dalton Trans., 1997, Pages 3691–3698

View Article Online
pyridoxal 5-CH2), 4.51 (d, 1 H, J = 12.8, pyridoxal 5-CH2), 4.47
(d, 1 H, J = 12.1, pyridoxal 4-CH2), 4.00 (d, 1 H, J = 12.1 Hz,
pyridoxal 4-CH2), 0.73 (s, 3 H, pyridoxal 2-CH3); α- and
β-protons of amino acid moiety were not identified.
A
A
B
B
A
A
Co
Co
C
C
B
B
C
C
Protonation
trans(B)-mer-∆-[R(N),R(N)]
trans(B)-mer-Λ-[R(N),R(N)]
The complex Na[Co(pa)2] 1 was protonated by addition of
HCl, the progress of which was monitored by UV/VIS, CD and
1H NMR spectra. The proton number and protonation con-
stant were estimated by use of a CD spectrophotometer. The
pH values were measured with an Orion Research pH-meter,
Scheme 1 A, B and C denote the co-ordinated atoms of the N-
pyridoxy--amino acid, i.e. carboxylate oxygen, amino nitrogen and
phenol oxygen
model EA-920. Hydrogen-ion concentration, [Hϩ], was calcu-
26
ϩ
lated from aH /γ , using γ = 0.83.
On standing for several days an aqueous solution of complex
2 gave dark brown crystals (2Ј) (Found: C, 56.41; H, 7.97; N,
5.53. Calc. for C34H37CoN4O8ؒ2H2O: C, 56.35; H, 7.73; N,
5.56%).
Measurements
Electronic absorption spectra were recorded on a JASCO
UVIDEC-660 spectrophotometer, and circular dichroism spec-
tra on a JASCO J-500C spectropolarimeter (0.01–1 m solu-
tions in D2O, room temperature). Proton NMR spectra were
recorded on a JEOL JNM-GX 400 MHz spectrometer in
(CD3)2SO and CD3OD (SiMe4 as reference) and in D2O
[sodium
4,4-dimethyl-4-silapentanesulfonate
(dss)
as
reference].
Crystallography
Single crystals of Na[Co(pa)]ؒ5.5H2O 1 and Na[Co(pf)]ؒ6H2O
2 suitable for X-ray analyses were recovered from eluates follow-
ing chromatography and standing for several days. Those of 2Ј
were obtained from a solution of 2 prepared by addition of
1 equivalent HCl. Crystal data and experimental details are
summarized in Table 3. Diffraction data were collected at 293 K
with an Enraf-Nonius CAD4-EXPRESS four-circle automated
diffractometer. The crystals were sealed in a glass capillary tube
to avoid loss of water. Reflection intensities were monitored by
three standard reflections every 2 h, and the decay was within
2% in each case. Reflection data were corrected for Lorentz and
polarization effects. An empirical absorption correction, based
on ψ scans, was applied.
The structures were solved by the heavy-atom method and
refined anisotropically for non-hydrogen atoms by full-matrix
least-squares calculations. Refinements were continued until all
shifts were smaller than one-third of the standard deviations
of the parameters involved. Atomic scattering factors and
anomalous dispersion terms were taken from ref. 27. The posi-
tions of hydrogen atoms except those of the pyridinium moi-
eties of complex 2Ј and water molecules were obtained from the
Fourier-difference maps, and their parameters were isotropically
refined. The R and RЈ values were 0.0541 and 0.0631 for 1,
0.0638 and 0.0765 for 2, and 0.0406 and 0.0449 for 2Ј respect-
ively. The weighting scheme wϪ1 = σ2(Fo) ϩ (0.015Fo)2 was
employed for all crystals. The final Fourier-difference maps did
not show any significant features. The calculations were per-
formed on a Micro VAX-3100 computer using the program
system SDP-MOLEN.28 Selected bond lengths and angles are
listed in Table 4.
Fig. 1 Electronic absorption (upper) and circular dichroism (lower)
spectra of the complex Na[Co(pa)2] 1
below, they are optically active and their crystals are asym-
metric. This indicates that an isomer was formed selectively
under the preparation conditions, although many kinds of iso-
mers, such as trans(N) or cis(N) for the arrangement of donor
atoms, meridional or facial for the configuration of chelate
rings, ∆ or Λ for the helicity of two unsymmetrical tridentate
ligands, R or S for the configuration of asymmetric nitrogen
atoms of the amino acid moiety, etc., are possible for these
complexes (Scheme 1). In the course of complexation of the N-
pyridoxyamino acid with cobalt ion, some regulation may
occur through an intramolecular ligand–ligand interaction.
UV/VIS and CD spectra
The electronic absorption spectrum of the Na[Co(pa)2] 1 com-
plex as a typical example is given in Fig. 1 together with the CD
spectrum. Well separated first absorption bands were observed
near 15 000 and 20 000 cmϪ1, which is a typical spectral pattern
for a complex of trans(X)-[CoX2Y4] type.29 Similar spectra were
observed for all other complexes as summarized in Table 1.
Since the spectral patterns of the well separated first absorption
bands resemble well those of
a
bis(pyridoxylidene--
phenylalaninato)cobaltate complex anion with a meridional
trans(N) configuration, these cobalt() complexes are also
expected to take the same configuration.30
Corresponding to the well separated first absorption bands,
two negative CD bands were observed in the same spectral
region. The spectral patterns for complexes 2–7 are essentially
the same as that for 1. The electronic absorption and CD spec-
tra for 1–7 were the same both in isolated crystals and in their
mother-liquors, indicating that all these complexes have the
same absolute configuration in both the solid and solution
states and that the co-ordination geometries around the central
metal ion are not affected by the difference in amino acid side
chains.
CCDC reference number 186/657.
Results
Syntheses
The N-pyridoxy--amino acids H2L were synthesized from the
corresponding -amino acid and pyridoxal by the reductive
alkylation method. The reaction of H2L with K3[Co(CO3)3] in a
2:1 molar ratio gave the complex anions [CoL2]Ϫ. As shown
J. Chem. Soc., Dalton Trans., 1997, Pages 3691–3698
3693

View Article Online
Table 1 Electronic absorption and CD spectral data for the Na[CoL2]
complexes*
Complex
Absorption (log ε)
CD (∆ε)
1 Na[Co(pa)2]
14.6 (1.93)
19.3 (2.32)
14.5 (Ϫ10.1)
19.4 (Ϫ8.2)
24.5 (3.28, sh)
24.1 (ϩ6.0)
25.4 (ϩ11.0)
30.0 (ϩ18.9)
14.8 (Ϫ8.3)
2 Na[Co(pf)2]
3 Na[Co(pl)2]
14.9 (1.78)
19.8 (2.36, sh)
24.0 (3.09, sh)
19.3 (Ϫ6.4)
26.9 (ϩ17.3, sh)
30.0 (ϩ19.2)
14.9 (Ϫ6.5)
14.9 (1.78)
19.3 (2.16)
19.4 (Ϫ3.4)
24.7 (2.97, sh)
23.8 (ϩ2.1, sh)
22.9 (ϩ7.7)
29.7 (ϩ12.5)
14.8 (Ϫ7.9)
19.4 (Ϫ5.6)
22.0 (ϩ4.1, sh)
24.6 (ϩ6.8, sh)
28.3 (ϩ8.8)
4 Na[Co(ppg)2]
14.7 (1.88)
19.3 (2.26, sh)
23.7 (3.28, sh)
30.0 (ϩ9.0)
14.8 (Ϫ6.9)
19.7 (Ϫ5.9)
23.7 (ϩ5.1, sh)
27.0 (ϩ6.9, sh)
30.0 (ϩ9.0)
14.8 (Ϫ7.1)
19.7 (Ϫ6.0)
23.7 (ϩ5.1, sh)
27.0 (ϩ6.9, sh)
30.0 (ϩ8.8)
14.7 (Ϫ5.7)
19.5 (Ϫ3.9)
5 Na[Co(pnf )2]
6 Na[Co(pmf )2]
7 Na[Co(pw)2]
14.9 (1.78)
19.7 (2.12, sh)
24.0 (3.06, sh)
14.9 (1.78)
19.7 (2.10, sh)
24.0 (3.03, sh)
14.6 (1.69)
19.3 (2.00)
24.7 (2.93, sh)
23.2 (ϩ3.4, sh)
26.8 (ϩ7.7, sh)
29.7 (ϩ7.8)
* Wavenumbers are given in 103 cmϪ1; sh = shoulder.
Table 2 Proton NMR chemical shifts (δ) of the 2-methyl groups for
N-pyridoxy--amino acid ligands and their cobalt() binary complexes
in D2Oa
Fig. 2 (a) Changes in electronic absorption (upper) and circular
Amino acid
Free
Complexed
dichroism (lower) spectra for the complex Na[Co(pa)2] 1 on proton-
ation. (b) Plot of ∆ε vs. pH for the CD peak at 19 600 cmϪ1. The curve
is a theoretical one drawn by using the ideal values
H2pa
H2pf
H2pl
H2ppg
H2pnf
H2pmf
H2pw
2.33
2.25
2.31
2.31
2.36
2.31
2.28
1.18 (ϩ1.15)b
0.84 (ϩ1.41)b
1.31 (ϩ1.00)b
1.17 (ϩ1.14)b
0.87 (ϩ1.49)b
0.83 (ϩ1.48)b
0.73 (ϩ1.55)b
ppm) than those of bis(N-pyridoxy--aliphatic amino acidato)-
cobalt() complexes, such as those of pa and pl (1.00–1.15
ppm). The complex with N-pyridoxy--tryptophan having the
larger aromatic side chain, Na[Co(pw)2], demonstrated the larg-
est upfield shifts. These are explained in terms of an additional
ring-current effect caused by an approach of the aromatic ring
of amino acid to the 2-methyl group. The relatively small upfield
shift of [Co(ppg)2]Ϫ can be understood from the fact that the
phenyl ring of ppg cannot approach the 2-methyl group, as is
obvious from the Corey–Pauling–Koltun (CPK) model.
a Chemical shifts from dss. b The difference between the 2-methyl pro-
tons for N-pyridoxy--amino acid and its cobalt() binary complex;
ϩ denotes an upfield shift.
1H NMR spectra
1
In general, the H NMR spectra of ligands show downfield
1
Previously, the H NMR upfield shift observed for the 2-
shifts upon complex formation with a diamagnetic metal ion.
Those of the pyridoxyamino acids prepared here also showed
0.1–1.2 ppm downfield shifts upon complex formation with
CoIII; those of the α-hydrogen of the amino acid moieties were
1.0–1.2 ppm downfield. However, only the 2-methyl groups
showed significant upfield shifts (Table 2). They were observed
at concentrations less than ≈0.1 m, which can be explained by
a ring-current effect due to an intramolecular proximity
between the 2-methyl group and the pyridoxal ring. This
undoubtedly indicates that a CH ؒ ؒ ؒ π interaction is present
between the 2-methyl group and the pyridoxal ring in these
complexes. The upfield shifts of the 2-methyl protons were
larger for bis(N-pyridoxy--aromatic amino acidato)cobalt()
complexes, such as those of pf, pnf, pmf and pw (1.41–1.55
methyl protons of bis(pyridoxylideneamino acidato)alumin-
ium() complexes was described in terms of the shielding effect
due to the adjacent azomethine group.31,32 Our results, however,
suggest that the upfield shift is caused by the ring current of the
adjacent pyridoxy pyridine ring.
Protonation of the [Co(pa)2]Ϫ complex in solution
The successive addition of HCl to an aqueous solution of com-
plex 1 gave rise to a pH-dependent spectral change. The two
characteristic electronic absorption and CD peaks in the region
of 14 600 and 19 400 cmϪ1 demonstrated shifts to higher energy
with an increase in the hydrogen-ion concentration [Hϩ], as
shown in Fig. 2(a), in which the CD peaks shifted with isosbes-
3694
J. Chem. Soc., Dalton Trans., 1997, Pages 3691–3698

View Article Online
tic points at 16 800 and 21 000 cmϪ1. Furthermore, the strength
of the negative CD peak near 20 000 cmϪ1 decreased with
increasing hydrogen-ion concentration, although such
a
change was not observed in the electronic absorption spectra.
The higher-energy shift in the first two absorption bands and
the decrease in the strength of the negative CD peak may suggest
a change in the co-ordination structure around the metal ion.
The protonation equilibrium of the complex 1 can be expressed
by equation (1) where Kn represents the equilibrium constant
Kn
[Co(pf)2]Ϫ ϩ nHϩ
[CoHn(pf)2](nϪ1)ϩ
(1)
(2)
Ϫ
]
Kn = [CoHn(pf)2(nϪ1)ϩ]/[Hϩ]n[Co(pf)2
and [Co(pf)2Ϫ] and [CoHn(pf)2(nϪ1)ϩ] denote the concentrations
of non-protonated, [Co(pf)2]Ϫ, and protonated cobalt() com-
plexes, [CoHn(pf)2](nϪ1)ϩ, respectively. According to the litera-
ture,33 equation (3) can be derived where Amax, A and Amin
nؒlog[Hϩ] = log[(A Ϫ Amin)/(Amax Ϫ A)] Ϫ log Kn (3)
Fig. 3 An ORTEP35 drawing of the complex anion [Co(pa)2]Ϫ 1 show-
ing the atom labelling scheme. Ellipsoids enclose 50% probability; H
atoms are omitted for clarity
represent the strength (∆ε) of the negative CD peak for
[Co(pf)2]Ϫ, that for the coexisting metal complex at [Hϩ] and
that for the protonated metal complex, respectively. A plot of
∆ε for the peak at 19 600 cmϪ1 against pH is shown in Fig. 2(b),
which gives Amax = Ϫ4.03, A = Ϫ6.15, Amin = Ϫ8.24 and log
Kn = 6.1. The substitution of these values in equation (3) gave
n = 1, and the addition of two Hϩ ions does not satisfy the
above theoretical equation (3). This indicates that one Hϩ is
added to the complex. From a comparison of the protonation
constant with those reported previously for trivalent binary
metal complexes of pyridoxyethane-1,2-diamine (M = InIII,
GaIII or FeIII),34 the protonation is considered to have occurred
at pyridine nitrogen.
1
The H NMR spectrum also showed a very interesting fea-
ture. The 2-methyl- and 6-protons of complex 1 exhibited 0.47
and 0.21 ppm downfield shifts, respectively, on protonation,
although the other protons did not show any significant shifts.
Furthermore, the shift of the 2-methyl protons, which are hard-
ly affected by co-ordination, was larger than that of the 6-
proton. These facts allow us to suggest that the shift of the 6-
proton is caused by the protonation of the pyridine nitrogen
and that the larger shift for the 2-methyl protons is due to the
decrease in the intramolecular interaction between the aromatic
ring and 2-methyl group. Interestingly, the downfield shift of
the 2-methyl protons in complex 2, 0.81 ppm, was much larger
than that for the free pyridoxyamino acid, but the chemical shift
of the 2-methyl protons for the protonated complex 2Ј was
almost the same as that for the protonated complex 1. This
finding may indicate that the phenyl ring of the phenylalanyl
group in complex 2 also interacts with the 2-methyl group.
Crystal structures of complexes 1, 2 and 2Ј
As is obvious from the selected bond lengths and angles for
complexes 1, 2 and 2Ј (Table 3), the co-ordinate bond param-
eters around the cobalt atoms are within the normal values. All
these crystal structures [Figs. 3, 4(a) and 5, respectively] reveal
that each CoIII is six-co-ordinated with two 3-hydroxy oxygen,
two amino nitrogen, and two carboxyl oxygen atoms of two
tridentate pyridoxy--amino acid ligands in meridional trans-
(N) configuration, as was postulated from the spectroscopic
results in the solution state. All the structures are comprised of
only one diastereoisomer; the configurations around the central
metal atom and around the amino nitrogen atom are Λ and R,
respectively, i.e. trans(N)-mer-Λ-[R(N), R(N)]-[CoL2]Ϫ.
As predicted from the 1H NMR upfield shift of the 2-methyl
groups, the crystal structures of the complexes 1 and 2 also
demonstrated an anomalous intramolecular assembly between
the 2-methyl group and adjacent pyridoxy pyridine ring. The
Fig. 4 (a) An ORTEP drawing of the complex anion [Co(pf)2]Ϫ
showing the atom labelling scheme. Details as in Fig. 3. (b) Side view
2
J. Chem. Soc., Dalton Trans., 1997, Pages 3691–3698
3695

View Article Online
Table 3 Crystal data and experimental details* for Na[Co(pa)2]ؒ7H2O 1, Na[Co(pf)2]ؒ6H2O 2 and [CoH(pf)2]ؒ2H2O 2Ј
1
2
2Ј
Formula
M
C22H28CoN4NaO8ؒ7H2O
684.5
C34H36CoN4NaO8ؒ6H2O
818.7
C34H37CoN4O8ؒ2H2O
724.6
Crystal size/mm
Crystal system
Space group
a/Å
b/Å
c/Å
0.6 × 0.3 × 0.2
Monoclinic
P21
9.5478(3)
15.912(2)
10.585(2)
90.770(9)
1607.9(3)
2
0.5 × 0.3 × 0.2
Orthorhombic
P212121
8.719(1)
20.151(2)
21.993(2)
—
0.3 × 0.3 × 0.3
Monoclinic
C2
15.272(1)
13.852(1)
9.200(1)
116.929(4)
1735.4(2)
2
β/Њ
U/Å3
3864.2(6)
4
Z
2θmax/Њ
52.64
1.41
6.11
52.64
1.41
5.20
52.64
1.39
5.51
Dc/g cmϪ3
µ/cmϪ1
Transmission coefficient range
No. reflections observed
0.89–1.11
3555
0.85–1.09
4381
0.93–1.01
1837
No. reflections used [Io > 3σ(Io)]
R
RЈ
3546
0.0541
0.0631
2920
0.0638
0.0765
1429
0.0406
0.0449
* In common: dark brown; ω–2θ scans; λ(Mo-Kα) 0.710 73 Å.
Table 4 Selected bond lengths (Å) and angles (Њ) for complexes 1, 2
and 2Ј
Complex 1
Co᎐O(3A)
Co᎐O(11A)
Co᎐N(8A)
1.890(3)
1.884(3)
1.930(4)
Co᎐O(3B)
Co᎐O(11B)
Co᎐N(8B)
1.884(3)
1.899(3)
1.927(4)
O(3A)᎐Co᎐O(3B)
O(3A)᎐Co᎐O(11B)
O(3A)᎐Co᎐N(8B)
O(3B)᎐Co᎐O(11B)
O(3B)᎐Co᎐N(8B)
O(11A)᎐Co᎐N(8A)
O(11B)᎐Co᎐N(8A)
N(8A)᎐Co᎐N(8B)
91.5(1)
89.3(1)
86.3(1)
178.7(1)
96.5(1)
85.1(1)
92.8(1)
176.5(2)
O(3A)᎐Co᎐O(11A)
O(3A)᎐Co᎐N(8A)
O(3B)᎐Co᎐O(11A)
O(3B)᎐Co᎐N(8A)
O(11A)᎐Co᎐O(11B)
O(11A)᎐Co᎐N(8B)
O(11B)᎐Co᎐N(8B)
178.4(1)
96.1(1)
89.6(1)
86.0(1)
89.6(1)
92.5(1)
84.6(1)
Complex 2
Co᎐O(3A)
Co᎐O(11A)
Co᎐N(8A)
1.874(5)
1.903(5)
1.911(7)
Co᎐O(3B)
Co᎐O(11B)
Co᎐N(8B)
1.868(5)
1.929(5)
1.911(6)
O(3A)᎐Co᎐O(3B)
O(3A)᎐Co᎐O(11B)
O(3A)᎐Co᎐N(8B)
O(3B)᎐Co᎐O(11B)
O(3B)᎐Co᎐N(8B)
O(11A)᎐Co᎐N(8A)
O(11B)᎐Co᎐N(8A)
N(8A)᎐Co᎐N(8B)
93.9(2)
88.9(2)
87.7(2)
176.8(2)
95.7(2)
85.8(3)
93.3(2)
178.4(3)
O(3A)᎐Co᎐O(11A)
O(3A)᎐Co᎐N(8A)
O(3B)᎐Co᎐O(11A)
O(3B)᎐Co᎐N(8A)
O(11A)᎐Co᎐O(11B)
O(11A)᎐Co᎐N(8B)
O(11B)᎐Co᎐N(8B)
175.9(2)
93.6(3)
90.1(2)
85.1(2)
87.1(2)
92.9(2)
85.9(2)
Fig. 5 An ORTEP drawing of the complex [CoH(pf)2] 2Ј showing the
atom labelling scheme. Details as in Fig. 3
separations between the 2-methyl carbon and another pyridine
ring plane were 3.67 and 3.67 Å for complex 1 and 3.52 and 3.62
Å for 2, respectively, the former of which are comparable to the
sum (3.7 Å) of the van der Waals radii of the methyl group (2.0
Å) and aromatic carbon (1.7 Å)36 and the latter of which are
slightly shorter in comparison. Simultaneously, the two pyri-
dine rings also showed intramolecular stacking with mean sep-
arations 3.29 and 3.59 Å and with tilting angles of 43.4 and
24.5Њ, respectively, which may also contribute to the intra-
molecular C᎐H ؒ ؒ ؒ π interaction. Such an abnormal assembly
must cause great strain around the metal ion, as is evident from
CPK model considerations, and there is great distortion around
the metal atom [Fig. 4(b)].
Complex 2Ј*
Co᎐O(3)
Co᎐N(8)
1.890(3)
1.935(2)
Co᎐O(11)
1.903(3)
O(3)᎐Co᎐O(3Ј)
O(3)᎐Co᎐O(11Ј)
O(3)᎐Co᎐N(8Ј)
O(3Ј)᎐Co᎐O(11Ј)
O(3Ј)᎐Co᎐N(8Ј)
91.9(1)
89.8(1)
85.4(2)
177.9(1)
96.2(2)
O(11)᎐Co᎐N(8)
O(11Ј)᎐Co᎐N(8)
O(11)᎐Co᎐O(11Ј)
N(8)᎐Co᎐N(8Ј)
85.2(2)
93.1(2)
88.5(1)
177.7(2)
* Atoms corresponding to the other half of the dimer are denoted by
primes.
Furthermore, in the crystal structure of complex 2 the phenyl
ring plane also approached the 2-methyl group, although at
distances of 3.92 and 4.26 Å. Such an additional interaction
is supported by the fact that the H NMR upfield shift of the
2-methyl protons in 2 in D2O is larger by 0.34 ppm than that
been observed in CD3OD or (CD3)2SO solutions. These facts
indicate that the attractive approach of the phenyl ring to the 2-
methyl group occurs in both solid and solution states.
1
As described above, the UV/VIS, CD and 1H NMR spectral
changes upon protonation of complex 2 suggested some struc-
tural changes in the solution phase. Fortunately, we obtained
in 1 having no phenyl group. A similar upfield shift has also
3696
J. Chem. Soc., Dalton Trans., 1997, Pages 3691–3698

View Article Online
Table 5 Proton NMR chemical shifts (ppm) of complexes 1 and 2 in D2O on protonationa
Na[Co(pa)2] 1
Na[Co(pf)2] 2
2-CH3
6-H
2-CH3
6-H
Complex (pH ≈ 7)
Protonated complex (pH ≈ 2)
Free pyridoxyamino acid
1.18(ϩ1.15)b
1.65(ϩ0.68)
2.33
7.79(Ϫ0.25)
8.00(Ϫ0.46)
7.54
0.84(ϩ1.41)
1.65(ϩ0.60)
2.25
7.79(Ϫ0.21)
7.89(Ϫ0.31)
7.58
a Chemical shifts from dss. b The values in parentheses denote the differences between chemical shifts of the complexes and free pyridoxyamino acids,
and ϩ and Ϫ denote up- and down-field shifts.
Table 6 Upfield 1H NMR chemical shifts (ppm) estimated according to the Johnson and Bovey37 equation
Ring-current effect by pyridine ring*
Ring-current effect by benzene ring*
H(2MA)
H(2MB)
H(2MC)
H(2MA)
H(2MB)
H(2MC)
L/Å
θ/Њ
Z/Å
ρ/Å
2.998
7.35
2.97
0.84
1.60
3.681
17.49
3.51
1.11
0.93
4.481
3.97
4.47
0.31
0.59
4.111
28.39
3.96
1.96
0.43
5.288
24.94
4.80
2.23
0.26
4.505
9.85
4.44
0.77
0.56
Calculated upfield shift (ppm)
Average of upfield shifts (ppm)
1.04
0.42
* Co-ordinates of hydrogen atoms H(2MA), H(2MB) and H(2MC) established for complex 2 were employed to estimate the ring-current effect
by pyridine and benzene rings.
single crystals of monoprotonated complex 2Ј. The structure
has almost the same configuration as those of complexes 1 and
2. Subtle but clear differences, however, were detected in the
bond lengths around the metal ions; the Co᎐O and Co᎐N
bonds for complex 2Ј are respectively shorter and longer on
average in comparison with those for 2 as can be seen from
Table 4. The proton in 2Ј seems to be located on the pyridoxal
nitrogen atoms, which is supported by the fact that the solvated
water molecule O(1W) is associated with the two nitrogen
atoms, N(1) ؒ ؒ ؒ O(1W) 2.66 Å. The distance between the pyri-
doxal mean plane and the adjacent 2-methyl carbon, 3.56 Å,
was almost the same as those for complex 2, although the two
pyridoxal rings approached each other with a distance of 3.32
Å and a tilting angle of 17.3Њ. As expected from the difference
1
in the H NMR chemical shifts of the 2-methyl protons for
complexes 2 and 2Ј (Table 5), the phenyl ring of 2Ј was removed
from the 2-methyl group with a distance of 4.45 Å in com-
parison with that in 2 (3.92 and 4.26 Å), which may be
explained by considering that there exists a repulsive intra-
molecular, interligand interaction between the phenyl and the
pyridoxal groups.
Fig. 6 View of the [CoH(pf)2] complex 2Ј showing the manner of
CH ؒ ؒ ؒ π interactions, with the atomic labelling scheme. Ellipsoids
enclose 50% probability; H atoms except for those of the 2-methyl group
are omitted for clarity
structure of the Na[Co(pf )2] complex, which shows an intra-
molecular approach of the phenyl ring of the amino acid side
chain to the pyridoxy 2-methyl group, it may be suggested that
Discussion
CH ؒ ؒ ؒ ð Interaction in solution and solid states
1
the larger H NMR upfield shifts are due to the ring-current
The 1H NMR spectra of the pyridoxy 2-methyl groups in these
complexes all exhibited upfield shifts in comparison with those
in the free pyridoxyamino acids; the magnitude of the shifts was
in the order, pw > pnf, pmf > pf > pa, ppg > pl, as is obvious
from Table 2. These upfield shifts can be explained by the ring-
current effect due to the intramolecular proximity between the
2-methyl group and the pyridoxal ring, as is suggested from the
crystal structures of the Na[Co(pa)2] and Na[Co(pf)2] com-
plexes. Such large shifts are specific to the bis(N-pyridoxy--
amino acidato)cobalt() complexes with aromatic side chains,
such as those of pf, pnf, pmf and pw. In view of the crystal
effect by the aromatic side chains in addition to that by the
pyridoxal pyridine ring: the order of these upfield shifts corres-
ponds to that of the sizes of the aromatic rings. The larger shift
in the [Co(pw)2]Ϫ complex anion strongly supports this
consideration.
As was previously reported, we succeeded in accurate
determination of the crystal structure of the complex
Na[Co(pf)2] 2.19 It revealed that two of the three C᎐H bond
vectors of the 2-methyl group were directed in the planes of the
pyridoxy pyridine and phenylalanyl benzene rings of the neigh-
bouring ligand (Fig. 6). The hydrogen atoms of the 2-methyl
J. Chem. Soc., Dalton Trans., 1997, Pages 3691–3698
3697

View Article Online
groups in complexes 1 and 2 obtained from the Fourier-
difference maps were also oriented in the same manner as those
in 2Ј. The presence of an interaction in the latter is also sup-
ported by the fact that the 1H NMR upfield shift of the 2-methyl
protons in 2 is larger by 0.34 ppm than that in 1, although the
separations between the 2-methyl carbon and phenyl ring (3.92
and 4.26 Å) are long. It is concluded from these results that
such an anomalous arrangement of aromatic rings with respect
to the 2-methyl group is enforced by the CH ؒ ؒ ؒ π interaction.
In order to explain the magnitude of the upfield shifts in 1H
NMR spectra stated above and the anomalous arrangement of
aromatic rings in the crystal structure, the shift values due to the
ring-current effect were estimated by using the coordinates of
the Na[Co(pf)2] crystal established here according to the equa-
tion of Johnson and Bovey.37 Table 6 shows that the estimated
ring-current shift values by pyridine and benzene rings are 1.04
and 0.42 ppm, respectively. Interestingly, the latter value is
unexpectedly large and the sum of the calculated upfield shifts
(1.46 ppm) is in striking agreement with the experimental value
(1.41 ppm), which may indicate that the conformation demon-
strated in the crystals is maintained also in solution.
The protonation of the pyridoxy pyridine nitrogen resulted
in a reduction in the strength of negative CD peak near 20 000
cmϪ1 and a downfield shift in the 1H NMR peak of the 2-methyl
group (0.81 ppm). The former may be due to the vicinal effect38
and suggests a significant conformational change in the prim-
nary co-ordination sphere around the central metal ion. The
latter may be explained by the decrease in the CH ؒ ؒ ؒ π inter-
action between the 2-methyl protons and the aromatic rings.
These spectral features are related to the asymmetric and aro-
matic amino acid side chain, indicating that the aromatic rings
are distant from the 2-methyl group. As was expected, the
phenyl ring was removed from the 2-methyl group to a separ-
ation of 4.45 Å. The long separation in the protonated complex
2Ј can be attributed to the closer contact between the two pyri-
dine rings in comparison with that in the unprotonated com-
plex 2. Hence, it is concluded that protonation of pyridine
nitrogen reduces the electron density on the aromatic rings and
therefore weakens the interligand CHؒ ؒ ؒ π interaction.
NMR spectra. This work was supported by a Grant-in-Aid for
Specially Promoted Research from the Ministry of Education,
Science and Culture, Japan.
References
1 B. Alberts, D. Bray, J. Lewis, M. Raff, K. Roberts and J. D. Watson,
Molecular Biology of the Cell, Garland Publishing, New York, 3rd
edn., 1994, p. 89.
2 E. Frieden, J. Chem. Educ., 1975, 52, 754.
3 W. P. Jencks, Adv. Enzymol. Relat. Areas Mol. Biol., 1975, 43, 219.
4 S. K. Burley, P. R. David, A. Taylor and W. N. Lipscomb, Proc.
Natl. Acad. Sci. USA, 1990, 87, 6878.
5 S. K. Burley, P. R. David, R. M. Sweet, A. Taylor and W. N.
Lipscomb, J. Mol. Biol., 1992, 224, 113.
6 H. Kim, S. K. Burley and W. N. Lipscomb, J. Mol. Biol., 1993, 230,
722.
7 S. K. Burley, P. R. David, R. M. Sweet and W. N. Lipscomb, Proc.
Natl. Acad. Sci. USA, 1991, 88, 6919.
8 T. Mizutani, T. Ema, T. Tomita, Y.Kuroda and H. Ogoshi, J. Am.
Chem. Soc., 1994, 116, 4240.
9 H. Masuda, A. Odani, T. Yamazaki, T. Yajima and O. Yamauchi,
Inorg. Chem., 1993, 32, 1111.
10 K. Jitsukawa, T. Morioka, H. Masuda, H. Ogoshi and H. Einaga,
Inorg. Chim. Acta, 1994, 216, 249.
11 O. Yamauchi, A. Odani, H. Masuda and H. Sigel, Metal Ions in
Biological Systems, eds. A. Sigel and H. Sigel, Marcel Dekker, New
York, 1996, vol. 32, p. 207.
12 Y. Kodama, K. Nishihata, M. Nishio and Y. Iitaka, J. Chem. Soc.,
Perkin Trans. 2, 1976, 1490.
13 H. Okawa, Coord. Chem. Rev., 1988, 92, 1.
14 M. Levitt and M. F. Perutz, J. Mol. Biol., 1988, 201, 751.
15 M. Nishio and M. Hirota, Tetrahedron, 1989, 45, 7201.
16 M. Nishio, Y. Umezawa, M. Hirota and Y. Takeuchi, Tetrahedron,
1995, 51, 8665.
17 E. Fischer and H. Sigel, J. Am. Chem. Soc., 1980, 102, 2998.
18 K. Kobayashi, Y. Asakawa, Y. Kikuchi, H. Toi and Y. Aoyama,
J. Am. Chem. Soc., 1993, 115, 2648.
19 K. Jitsukawa, K. Iwai, H. Masuda, H. Ogoshi and H. Einaga,
Chem. Lett., 1994, 303.
20 D. M. Smith, N. R. Thomas and D. Gani, Experimentia, 1991, 47,
1104.
21 A. M. Catherine, G. V. Michel and N. J. Johan, J. Mol. Biol., 1992,
225, 495.
22 P. A. Clark, J. N. Jansonius and E. L. Mehler, J. Am. Chem. Soc.,
1993, 115, 1894.
23 P. A. Clark, J. N. Jansonius and E. L. Mehler, J. Am. Chem. Soc.,
1993, 115, 9790.
Conclusion
24 P. Christen and D. E. Metzler (Editors), Transaminases, Wiley, New
York, 1985.
Although weak bondings such as the CH ؒ ؒ ؒ π interaction are
affected by the solvent, they are still present in aqueous solu-
tions, as is seen from the above-mentioned spectral findings.
The N-pyridoxy--amino acids employed here are derivatives of
vitamin B6 coenzyme, pyridoxal-5-phosphate, which plays an
important role in amino acid metabolism in biological systems,
and the significance of the functional groups of the pyridoxal
molecule has long been discussed by many chemists, biochem-
ists and biologists.20–25 However, that of the 2-methyl group is
open to discussion even at present, in spite of the experimental
result that the absence of the 2-methyl group from the pyridoxal
coenzyme decreases vitamin B6 activity.24 The recent crystal
structure analysis of aspartate aminotransferase bound to the
cofactor pyridoxal 5-phosphate has demonstrated that the two
aromatic rings of tyrosine and tryptophan are close to the 2-
methyl group in the enzyme. The CH ؒ ؒ ؒ π interactions
between the 2-methyl group and aromatic rings, described in
this paper, may therefore suggest that the 2-methyl group acts as
the recognition group of the active site in the enzyme through
the CH ؒ ؒ ؒ π interaction with the aromatic rings.
25 K. Dakshinamurti, Ann. N.Y. Acad. Sci., 1990, vol. 585.
26 J. Kielland, J. Am. Chem. Soc., 1973, 59, 1675.
27 J. A. Ibers and W. C. Hamilton (Editors), International Tables for
X-Ray Crystallography, Kynoch Press, Birmingham, 1974, vol. 4.
28 MOLEN, An Interactive Structure Solution Procedure, Enraf-
Nonius, Delft, 1990.
29 J. E. Fergusson, Stereochemistry and Bonding in Inorganic
Chemistry, Prentice-Hall, New York, 1974.
30 K. Jitsukawa, T. Yamamoto, H. Masuda and H. Einaga, unpub-
lished work.
31 E. H. Abbott and A. E. Martell, J. Am. Chem. Soc., 1969, 91, 6866.
32 E. H. Abbott and A. E. Martell, J. Am. Chem. Soc., 1970, 92, 5845.
33 M. Noritake, K. Okamoto, J. Hidaka and H. Einaga, Bull. Chem.
Soc. Jpn., 1990, 63, 353.
34 R. J. Motekaitis, Y. Sun and A. E. Martell, Inorg. Chim. Acta, 1989,
159, 29.
35 C. K. Johnson, ORTEP, Report ORNL-5138, Oak Ridge National
Laboratory, Oak Ridge, TN, 1976.
36 L. Pauling, The Nature of the Chemical Bond, Cornell University
Press, Ithaca, 3rd edn., NY, 1960.
37 C. E. Johnson and F. A. Bovey, J. Chem. Phys., 1958, 29, 1012.
38 T. Yasui, J. Hidaka and Y. Shimura, J. Am. Chem. Soc., 1965, 87,
2762.
Acknowledgements
Thanks are due to the Instrument Center, the Institute for
Molecular Science, for assistance in obtaining the 400 MHz 1H
Received 28th April 1997; Paper 7/02900D
3698
J. Chem. Soc., Dalton Trans., 1997, Pages 3691–3698