ISBN: 978-83-60043-10-3 - eurobic9

Eurobic9, 2-6 September, 2008, Wrocław, Poland
P206. Chiral Interactions in Metal Complexes Containing Amino Acids
and its Derivatives
T. Yajima a , S. Ito a , J. Morita a , M. Yumoto a , Y. Shimazaki b , O. Yamauchi a , T. Shiraiwa a
a Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan.
b College of Science, Ibakaki Univeristy, Mito, Ibaraki 310-8512, Japan
e-mail: t.yajima@ipcku.kansai-u.ac.jp
A number of bio-active compounds have chirality, and their stereoisomers exhibit various activities in biological
systems and play key roles in sustaining life. Especially, optically active amino acids have been used in foods,
medicines, pesticides, cosmetics, and even as chiral reagents for asymmetric syntheses. Enantiomeric
compounds have been obtained by refining natural products, asymmetric sysntheses, optical resolution from
racemates, and so on. Optical resolution of racemic amino acids (AAs) is achieved by various procedures to give
their enantiomers. However, the procedures are limited to resolution of certain AAs, and there is a strong
demand for the method of general applicability. Transition metal ions can bind two or more ligands, so that a
metal complex containing a chiral AA may coordinate the second ligand (AA’) enantioselectively for steric or
other reasons, enabling separation of the racemic mixture of this ligand by differences in solubility or reactivity.
We first studied selective incorporation of an enantiomer of a racemic AA’ into Cu(II) complexes with an active
AA, Cu(AA)(AA’).
The ternary Cu(II) complex containing L-isoleucine (ile) and D-alanine (ala), [Cu(L-ile)(D-ala)], is less soluble
than [Cu(L-ile)(L-ala)], because of the difference in configuration: [Cu(L-ile)(D-ala)] has a cis-configuration,
whereas [Cu(L-ile)(L-ala)] has a trans-configuration.[1] Cis-trans isomerism may have an influence on the
properties of ternary Cu(II) complexes; for complexes containing D/L-aminobutanoic acid (abu) instead of D/Lala,
[Cu(L-ile)(L-abu)] is over 10 times more soluble than [Cu(L-ile)(D-abu)]. Such a wide difference in solubility
may arise from the stability difference between the diastereomers of the ternary complexes, whose stability
constants, logβ, were found to be 15.692(9) for [Cu(L-ile)(D-abu)] and 15.166(25) for [Cu(L-ile)(L-abu)].
On the other hand, two ternary complexes containing L-alloisoleucine (aile) and D/L-ala, [Cu(L-aile)(D/L-ala)],
exhibit similar solubilities and the same cis-configuration.
O
H2 O N
Cu
N O
H2 O
L-ala O O
D-ala
OH2 Cu
N OH
H 2
2
In order to attain efficient separation of racemic AAs, we studied
potentiometrically and spectroscopically the M-L-AA ternary systems, where M
refers to Zn 2+ , Cu 2+ , and Ni 2+ and L to tris(2-pyridylmethyl)amine (TPA) and
(S)-N, N-bis(2-pyridylmethyl)-1-(2-pyridyl)ethylamine ((S)-MeTPA).[2]
Potentiometric titrations of the M-TPA-AA systems revealed that stable ternary
complexes, [M(TPA)(L-AA)], are formed for M = Ni 2+ , while ternary
complexes with M = Zn 2+ or Cu 2+ N
N
N
are not formed or less stable. The stability
constants of [Ni(TPA)(AA)] for AA = chiral amino acids are slightly smaller
R = H
CH3
TPA
(S)-MeTPA
than the value of [Ni(TPA)(gly)], suggesting that the side chain of AA may give rise to steric hindrance with the
pyridine ring of TPA and that it may be possible to resolve efficiently a number of amino acids by a chiral TPAtype
ligand. The circular dichroism spectra of Cu-(S)-MeTPA-L-Phe and Cu-(S)-MeTPA-D-Phe systems
exhibited a large negative and a positive extremum at ~600 nm, respectively, as compared with Cu-TPA-D/L-
Phe, which may indicate that AA has a higher affinity for Cu-(S)-MeTPA than Cu-TPA.
Acknowledgement: We would like to thank Professor Akira Odani, Kanazawa University, for kind advice on
potentiometric titrations.
References:
[1] T. Shiraiwa, H. Fukuoka, M. Yoshida, H. Kurokawa, Bull. Chem. Soc. Jpn., 57, 1675 (1984).
[2] J.W. Canary, Y. Wang, R. Roy, Jr., Inorg. Synth., 32, 70 (1998).
O
Cu
N
H2 N
R
[Cu(L-ile)(L-ala)] [Cu(L-ile)(H2O) 2]
[Cu(L-ile)(D-ala)]
_____________________________________________________________________
325
O
N
H 2
O O