Two different incorporation sites of manganese in single-crystalline monohydrated L-asparagine
studied by electron paramagnetic resonance
K. Krambrock, K. J. Guedes, and L. O. Ladeira
Departamento de Física, ICEx, Universidade Federal de Minas Gerais, Caixa Postal 702, 30123 Belo Horizonte, Minas Gerais, Brazil
M. J. B. Bezerra, T. M. Oliveira, G. A. Bezerra, and B. S. Cavada
Laboratório de Bioquímica Molecular, Departamento de Bioquímica, Universidade Federal do Ceará, Campus do Pici,
60455-900 Fortaleza, Ceará, Brazil
M. C. F. de Oliveira
Departamento de Química Orgânica e Inorgânica, Universidade Federal do Ceará, Campus do Pici, 60455-900 Fortaleza, Ceará, Brazil
M. Z. S. Flores, G. A. Farias, and V. N. Freire*
Departamento de Física, Universidade Federal do Ceará, Caixa Postal 6030, Campus do Pici, 60455-900 Fortaleza, Ceará, Brazil
Received 23 June 2006; revised manuscript received 10 December 2006; published 21 March 2007
Single crystals of monohydrated L-asparagine have been grown from aqueous solutions using MnCl
2
as
doping material. Electron paramagnetic resonance EPR was used to determine the incorporation sites of Mn
2+
ions in the crystal structure. Depending on small pH changes and crystal growth kinetics in the aqueous
solutions, Mn
2+
ions are incorporated in two chemically distinct sites in asparagine crystals. The first shows
isotropic six-line hyperfine-split EPR spectra, whereas the second shows anisotropic multiple line splitting due
to Mn
2+
fine structure S =5/2 and hyperfine interaction I =5/2. Angular dependencies of the Mn
2+
EPR
spectra in three mutually perpendicular crystal planes were measured and analyzed. The results are discussed
in terms of the metal incorporation site symmetry in the crystal structure of monohydrated L-asparagine.
DOI: 10.1103/PhysRevB.75.104205 PACS numbers: 71.20.Rv, 78.20.e, 78.40.Me, 78.55.Kz
I. INTRODUCTION
About half of all proteins contains metal ions, which per-
form a wide variety of specific functions associated with life
processes. In particular, transition metals such as Fe, Cu, and
Mn are involved in many redox processes requiring electron
transfer, and play an important role in the folding and bio-
functionality of proteins, taking part of many enzymes and
being indispensable in several catalytic reactions.
1
For ex-
ample, the interaction between a tetranuclear Mn cluster and
its protein ligand has a central role in photosystem II,
2
while
the manganese ion in the vicinity of the saccharide-binding
site in native Dguia lectin interacts with the asparagine resi-
due Asn 14, contributing to the stabilization of the binding
pocket.
3
The growth of amino acid crystals from aqueous
solutions containing transition-metal ions allows us to study
in the solid state the transition metal—amino acid interac-
tion, helping to provide a solid foundation for the under-
standing of the role of transition metals in proteins.
To understand basic aspects of the role of metal in pro-
teins, amino acid crystals doped with transition metals are
appropriate model systems. Several works were published on
different aspects of these crystals. Windsch and co-workers
4
investigated copperII-doped single crystals of glycine and
triglycine sulfate, showing that each copperII ion is coor-
dinated with two amino acid molecules. Takeda et al.
5
stud-
ied single crystals of copperII-doped L-alanine, demon-
strating the existence of four chemically identical but
magnetically nonequivalent sites through electron paramag-
netic resonance EPR measurements. Winkler et al.
6
pre-
sented a study of low-concentration FeIII doping in crys-
talline L-alanine by means of EPR, Raman scattering, and
photoluminescence, showing that FeIII occupies two in-
equivalent sites of rhombic symmetry in the L-alanine crys-
tal. Other FeIII-related centers with isotropic EPR spectrum
were mentioned; however, they were not analyzed. Calvo
and co-workers
7–9
performed EPR studies of copper ion dop-
ants in several amino acid crystals. For example, Dalosto et
al.
7
performed EPR studies of copper ion dopants and
ZnD , L-histidine,
7
interpreting their experimental results
with a model where the copper atoms hop randomly between
different states, relating this dynamics to the fluctuating dis-
order in He lattices. Zeeman and hyperfine coupling tensors
were determined for L-arginine phosphate monohydrate
single crystals by Santana et al.,
8
who suggested that Cu
impurities have three N ligands in this case. EPR was also
used by Santana et al.
9
to study CuII dopant ions in single
crystals of bisL-asparaginatoZnII, indicating that the
CuII impurities replace ZnII ions in the host crystal. Re-
cently, Pinheiro et al.
10
performed EPR detection and first-
principles calculations of manganese clusters in highly doped
L-alanine crystals, demonstrating that manganese is incorpo-
rated into L-alanine crystals as Mn
2+
ions at magnetically
equivalent single interstitial sites in the unit cell for crystals
grown with MnCl
2
concentrations smaller than 3.0% in the
mother solutions, and at two or more neighboring interstitial
sites in the case of higher MnCl
2
concentrations, which gives
rise to manganese clusters in the doped L-alanine crystals.
First-principles quantum mechanics and force field calcula-
tions suggested four interstitial sites for the manganese at-
oms in the L-alanine unit cell, and a high spin configuration
sextet with S =5/2 for the manganese electronic state.
Asparagine C
4
N
2
O
3
H
8
is one of the 20 natural amino
acids, having an important role in the metabolic control of
PHYSICAL REVIEW B 75, 104205 2007
1098-0121/2007/7510/1042055 ©2007 The American Physical Society 104205-1