Download The Hydroxyl Group Is a Surprisingly Versatile Metal Ion

EuAsC2S-12/S1-OP19
The Hydroxyl Group Is a Surprisingly Versatile Metal Ion-Binding Site !!
Helmut Sigel, Astrid Sigel
University of Basel, Department of Chemistry, Inorganic Chemistry, Spitalstrasse 51, CH-4056 Basel, Switzerland
<[email protected]>
Knowledge on the binding of biologically relevant divalent metal ions (M2+) to hydroxyl groups
is very scarce and this depite its importance, e.g. for ribozymes [1]. Being interested in weak
interactions [2,3] already for a long time [4], we joined forces with several colleagues and
reviewed the literature [5]. The metal ion-hydroxyl group interaction is always then of relevance
if a suitable primary binding site (PBS) is available next to the hydroxyl group. For example, for
the following series of ligands allowing the formation of 5-membered chelates, containing a
hydroxyl group and as a monodentate PBS a phosph(on)ate, carboxylate, amino, imidazole or
pyridyl group, for a given metal ion the extent of chelate formation increases in the given order;
an order, which also reflects the decrease of the charge transfer from the primary coordinating
atom to the metal ion [5]. As far as possible, the alkaline earth ions, several 3d ions (Mn2+, Co2+,
Ni2+, Cu2+) as well as Zn2+, Cd2+, and Pb2+ were used in the evaluations [5].
The existence of an intramolecular equilibrium between an open (op) and a closed (cl) or
chelated isomer (Eq. 1) is always refleced by an increased complex stability; i.e., an enhanced
complex stability (log ) compared to the stability of the open isomer [4]. The interrelations
between Equilibrium (1) and Equations (2) and (3) are obvious, as is evidenced below for
hydroxyacetate (also known as glycolate), HO–CH2–COO– (HOAc–) [5]:
O
H
HC
O
H
HC
C
O–
HO
H
C
O–
H
HO
2+
M
M(HOAc)+op
[M(HOAc)+cl ]
M
(1)
2+
M(HOAc)+cl
= 10log  – 1
(2)
+
% M(HOAc)cl = 100 ·KI / (1 + KI)
(3)
KI =
[M(HOAc)+op ]
Note, from the Table it is evident that (i) Equilibrium (1) definitely exists, and that (ii) the results
obtained for the complexes with lactate or hydroxy-iso-propionate, HO–CH(CH3)–COO–
(HOiPr–), correspond to those of the M(HOAc)+ species. As one might expect, with
methoxyacetate, CH3–O–CH2–COO– (MeOAc–), the formation degree of the chelate is decreased
[5]. The relevance of the indicated results for biological systems, especially nucleic acids, is
obvious. This is even more true if one notes that a reduced solvent polarity favors the metal ionhydroxyl group interaction significantly. For example, the stability enhancement, log , rises for
Cu(HOAc)+ from 0.79 ± 0.08 in aqueous solution to 1.10 ± 0.06 log units in water containing
50% (v/v) 1,4-dioxane, and, e.g., in the active sites of ribozymes a reduced permittivity
(dielectric constant [1]) exists.
Supported by the Department of Chemistry of the University of Basel, Switzerland
[1] R.K.O. Sigel, A.M. Pyle, Chem. Rev. 2007, 107, 97–113.
[2] R.K.O. Sigel, H. Sigel, Acc. Chem. Res. 2010, 43, 974–984.
[3] N.A. Corfù, A. Sigel, B.P. Operschall, H. Sigel, J. Indian Chem. Soc. 2011, 88, 1093–1115 (Sir Prafulla
Chandra Ray issue)
[4] H. Sigel, L.E. Kapinos, Coord. Chem. Rev. 2000, 200-202, 563–594.
EuAsC2S-12/S1-OP19
[5] F.M. Al-Sogair, B.P. Operschall, A. Sigel, H. Sigel, J. Schnabl, R.K.O. Sigel, Chem. Rev. 2011, 111, 4964–5003.