“ uptake factor ” is defi ned as KML × [M], where KML is the stability constant K 1
and [M] is the concentration of metal ion. Since naturally occurring aqueous
systems have metal ion concentration varying roughly as
K,Na
MCa , Mg
MZn
MCu
MFe
M++
−++
−+
−+
−+101 10 << 10 10 10 −22
32
92
122∼∼^17great selectivity for metal species is necessary to concentrate the necessary
ions at sites where they are needed. Differentiating ligands are those preferred
by the cation in question. A much more detailed discussion takes place in ref-
erence 3. Table 1.8 is adapted from this source.
1.5.2 Kinetics,
As students learned in their introductory chemistry courses, rates of reaction
are divided into several classes, depending on the number of reactants involved
in rate determination. These are: (1) zero order — the reaction rate is indepen-
dent of the concentration of that reactant; (2) fi rst order — the reaction rate is
dependent on the concentration of one reactant; (3) second order — the reac-
tion rate is dependent on the concentration of two reactants; and (4) higher
order — the reaction rate is dependent on more than two reactants. Higher -
order reaction rates are very rare because the possibility of bringing more than
two reactants together productively is very small. Bioinorganic kineticists,
studying the reaction rates of complex enzymatic reactions, often simplify
matters to isolate a reaction of interest and relate it to a proposed mechanism
for the enzyme ’ s catalytic activity. For instance, in a pseudo - zero - order reac-
tion — that is, one that would be fi rst order under normal circumstances — the
concentration of the enzyme may be held constant while a particular sub-
strate ’ s concentration is varied but does not affect the reaction rate. This condi-
tion may apply when the enzyme is saturated with substrate over the range of
TABLE 1.8 KML and KML ¥ [M] for Some Cations and Their Differentiating
Ligands
K + , Na + Ca 2+ , Mg 2+ Zn 2+ , Cu 2+ Differentiating Ligand
K + , Na + O - Macrocycles such as crown
ethers, cryptates and naturally
occurring macrocyclic
antibiotics such as nonactin
and valinomycin
K ML > 10 < 10^2 < 10^6
K ML × [M] > 1.0 < 0.1 < 0.1
Ca 2+ , Mg 2+ Oxygen donors such as di - or
KML 1 .0 < 10^3 < 10^6 tricarboxylates
KML × [M] < 0.1 > 1.0 < 0.1
Zn 2+ , Cu 2+ Nitrogen and sulfur ligands
KML 0.1 < 10^2 > 10^6
KML × [M] < 0.1 < 0.1 > 1.0
Source : Adapted from reference 3.
BIOLOGICAL METAL ION COMPLEXATION 9