www.elsevier.nl/locate/ica
Inorganica Chimica Acta 297 (2000) 404 – 410
Synthesis and characterization of magnesium and zinc complexes
of 1,4,7-triazacyclononane-N -acetate. Potential models for the
active site of RuBisCo
Duncan Odom
a
, Christine J. Gramer
a
, Victor G. Young Jr.
b
, Scott A. Hilderbrand
a
,
Suzanne E. Sherman
a,
*
a
Diision of Natural Sciences, New College of the Uniersity of South Florida, Sarasota, FL 34243, USA
b
X-Ray Crystallographic Laboratory, Department of Chemistry, Uniersity of Minnesota, Minneapolis, MN 55455, USA
Received 30 August 1999; accepted 1 September 1999
Abstract
Reaction of the tetradentate pendant-arm macrocycle 1,4,7-triazacyclononane-N-acetate (L) with Zn(O
3
SCF
3
)
2
or
Mg(O
3
SCF
3
)
2
in alcohol produces the six-coordinate complexes [ZnL(H
2
O)]O
3
SCF
3
·C
2
H
5
OH (1) and [MgL(H
2
O)
2
]O
3
SCF
3
(2).
Compounds 1 and 2 were characterized by single-crystal X-ray crystallography. The complex cation of 1 forms a polymeric chain
with zinc centers bridged in a syn, anti fashion by acetate pendant arms. The complex cation of 2 is mononuclear, with two
adjacent coordination sites on magnesium occupied by water molecules. These labile coordination sites may provide a binding site
for activation of substrates in functional modeling of magnesium enzymes such as RuBisCo.
1
H and
13
C NMR spectroscopy
demonstrates that L remains bound to magnesium in methanol solution. Crystal data: 1, orthorhombic, space group P 2
1
2
1
2
1
,
a =9.0536(7), b =14.3750(11), c =14.4741(11) A , V =1883.7(2) A
3
, Z =4, R =0.041; 2, orthorhombic, space group P 2
1
2
1
2
1
,
a =8.5949(3), b =13.9694(4), c =14.0076(5) A , V =1681.83(10) A
3
, Z =4, R =0.0248. © 2000 Elsevier Science S.A. All rights
reserved.
Keywords: Azamacrocycle complexes; Crystal structures; Magnesium complexes; Zinc complexes; RuBisCo
1. Introduction
Several known enzymes are thought to utilize adja-
cent coordination sites on a single magnesium ion to
bind substrates and aid in catalytic transformations
[1,2]. An important example is ribulose-1,5-bisphos-
phate carboxylase – oxygenase (RuBisCo), a principal
catalyst in photosynthesis and probably the most abun-
dant protein in the world [3–5]. RuBisCo is a magne-
sium-activated enzyme that catalyzes initial steps in two
competing metabolic pathways, CO
2
fixation (carboxy-
lation) and photorespiration (oxygenation). The former
pathway is the first step in the earth’s food chain,
whereas the latter pathway depletes carbon and energy
that could otherwise be used for plant growth [6]. A
large body of evidence, including X-ray crystallo-
graphic information, indicates that the active site mag-
nesium bonds to the carbohydrate substrate, ribulose-
1,5-bisphosphate, and participates in catalysis [7 – 19].
Initially, the carbohydrate coordinates to two cis sites
on magnesium.
Synthetic model chemistry may help to identify fac-
tors that influence the relative rates of carboxylation
and oxygenation reactions catalyzed by RuBisCo. Such
information might improve current efforts [3] to engi-
neer an enzyme with higher turnover and higher specifi-
city for CO
2
. A functional model for RuBisCo might
also impact synthetic strategies for regenerating hydro-
carbon fuels from CO
2
. With these possibilities in mind,
we are interested in the preparation of complexes of
non-redox-active metals as potential functional models.
Such complexes should be soluble and stable in polar
solvents and should contain cis or fac ‘open’ sites
coordinated by labile solvent molecules for subsequent
activation of substrate molecules. * Corresponding author.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII:S0020-1693(99)00387-4