8
0
Transition Met Chem (2011) 36:79–85
site. The catalytic activity of the complexes is many orders
of magnitude lower than that of the enzyme, despite the use
of a more activated substrate [16].
medium affects the substrate binding to the metal center
and the hydrolysis rate.
The effectiveness of metal atoms in promoting the
hydrolysis of phosphate species is a function of many
factors, including the nature of the metal atom, the attached
ligands, the phosphate species, and the compositions and
structures of the complexes formed between them. The
availability of a nucleophile, such as a suitably positioned
coordinated hydroxide, is also critical to an effective
hydrolysis process [1, 16, 22]. It has been widely accepted
that metal atoms cooperatively play three common roles in
these metalloenzymes. First, increased acidity of coordi-
nated water ligands promotes the formation of hydroxide,
which is a better nucleophile. In the case of RNA hydro-
lysis, the metal-bound hydroxide acts as a base to generate
Experimental
IR spectra were recorded as KBr pellets on an Excalibur
Biorad 3500 GX FTIR spectrophotometer. Electronic
spectra were recorded on an HP 8452A UV–Vis Diode
Array Spectrophotometer in the 280- to 800-nm range.
Elemental analyses were determined on a Perkin-Elmer
2400 CHN analyzer. Metal contents were measured using a
graphite stove Shimadzu Model 8100 Atomic Absorption
Spectrometer. Cyclic voltammetry experiments were
performed using an EG & G Princeton PARC 263A
Potentiostat (working electrode: glassy carbon, reference
electrode: Ag/AgCl, support electrode: platinum, and sup-
port electrolyte: tetrabutylammonium hexafluorophos-
phate). The molar conductivities of acetonitrile solutions of
the complexes were measured on a Digimed D-20 equip-
-
an alkoxide sugar moiety (2-O ), which acts as a nucleo-
phile for the phosphate groups. Secondly, bonding of a
metal atom to the anionic oxygen of a substrate (dianionic
monoester or monoanionic diester), or bonding at peptide
carbonyl oxygen, increases the electrophilic susceptibility
of the neighboring atom to the nucleophilic attack. Finally,
the metal center can assist in the departure of a leaving
group by interaction with the partial negative charge on the
leaving oxygen, in the transition state [23].
-
1
ment (platinum electrode K = 1 cm ). EPR experiments
were performed on an EPR Bruker ESP 300E spectrometer
(standard concavity: 4102-SP, frequency X band 9.5 GHz)
at 293 and 77 K, using liquid N2.
All chemicals were purchased from Aldrich, Sigma or
0
As the active form of the enzyme is a heterovalent state,
most of the model complexes are mixed-valence com-
III II
Merck and were of analytical grade. N,N -bis(2-hydroxyben-
0
zyl)-N,N -bis(2-methylpyridyl)–1,3-propanediamine-2-ol
(H bbppnol) and its complex [Fe (bbppnol)(l-AcO) ]
plexes Fe M (M = Zn, Cu, Mn, or Ni). However, vari-
3
2
2
III III
ous Fe Fe model complexes have also demonstrated
(PF ) (2) were synthesized and characterized according to
6
phosphatase activity. Maximum activity was found at pH
-
published procedures [25]. The structural formula of com-
4 -1
s
6
.0 [kobs = 5 9 10
dinitrophenyl)phosphate promoted at 50 °C by a binuclear
] for the hydrolysis of bis(2,4-
plex 2 is shown in Fig. 1.
complex [Fe O(phen) (OH) (NO ) [16, 24].
2
Synthesis of [Fe (bbppnol)(l-AcO)(H O) ](ClO ) (1)
4
2
3 2
2
2
2
4 2
In this study, we report the reactivity of three binuclear
0
non-heme iron(III) compounds with the ligand N,N -bis(2-
0
Sodium acetate (0.082 g, 1.0 mmol) and Fe(ClO ) ꢀ6H O
4
2
2
hydroxybenzyl)-N,N -bis(2-methylpyridyl)–1,3-propanedi-
amine-2-ol (H bbppnol, Fig. 1). This ligand has phenolate
(0.3 g, 2.0 mmol) were added to a solution of H bbppnol
3
(0.491 g, 1.0 mmol) in methanol (40 mL). The clear deep
blue solution was heated to 50 °C and stirred for 10 min.
After cooling the solution to room temperature, 20 mL of
nanopure water was added. The microcrystalline precipi-
tate was filtered off and washed with cold water and ether.
3
arms to mimic the tyrosinate residues found in the enzyme.
The complexes were studied for the hydrolysis of bis-(2,4-
dinitrophenyl)phosphate as models for metallohydrolases
activity. Although the PAPs activity is high in acidic
media, we decided to test these model compounds in
neutral to basic media to check whether a more alkaline
-
1
Yield: 66%. C H Cl Fe N O (887.23 g mol ): Ele-
3
1
36
2
2 4 15
mental analysis, found (calcd.) % C 43.0 (42.0), H 4.8
Fig. 1 Structures of the ligand
+
N
N
3
H bbppnol and complex 2 [25]
N
N
N
O
N
N
N
Fe
Fe
OH
O
PF -
6
O
O
O
O
O
OH
HO
H bbppnol
3
2
1
23