ACID–BASE PROPERTIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 66

provide insight into active sites involved in reaction mechanisms of many
heme and nonheme enzymes [100-103]. High-valent iron species as intermedi-
ates in the activation of oxygen by iron enzymes has also been invoked [101,
104]. Examples include aromatic dihydroxylation by arene-containing protein-
bound iron complex [105]. Complexes of high-valent iron have therefore been
synthesized, followed by their structural characterization using spectroscopic
techniques such as Mössbauer spectroscopy, resonance Raman spectroscopy,
and X-ray absorption spectroscopy (XAS) [100, 103, 106-108]. These tech-
niques have identified species of Fe(IV), Fe(V), and Fe(VI) such as 6-coordinate
Fe(IV)-oxo(N 4 Py) and Fe(VI)-nitride complexes and 5-coordinate Fe(V)-
oxo(TAML) (TAML = tetraamidomacrocyclic) species [109-111]. The reactiv-
ity of high-valent iron-oxo and -nitride species is presented in Chapter 6.
2.2.2.2 Copper, Zinc, and Nickel. The complexation of Cu(II) with bioligands
is imperative in biological systems, and hence the formation constants of 1 : 1
binary complexes of Cu(II)-Gly, Cu(II)-Ala, Cu(II)-Val, Cu(II)-Leu, Cu(II)-
Glu, and Cu(II)-Asp and 1 : 2 binary complexes of Cu-Gly 2 and Cu-Glu 2 have
been determined using potentiometric and spectrometric techniques [74]. The
enthalpy changes of complexes were negative, except Cu(II)-Glu and Cu(II)-
Asp complexes, while the positive entropy changes for all of the complexes were
obtained. This suggests that both enthalpy and entropy changes derived the com-
plexation. The complexation of similar amino complexes with Zn(II) and Ni(II)
has also been studied [45]. Knowledge on Cu(II), Zn(II), and Ni(II) complex-
ation with studied amino acids may provide insight on structural differences in
different superoxide dismutases in order to describe oxidation and reduction
reactions of their catalytic metal ions [112]. Cu(II), Zn(II), and Ni(II) complexes
of salicylaldehyde (Sal)-amino acid Schiff bases were also studied as nonenzy-
matic models for pyridoxal-amino acid systems [45]. The stability constants, logβ 1
and logβ 2 , were in the order Sal-Gly > Sal-Ala > Sal-Ser > Sal-Tyr > Sal-Phe,
except Sal-Gly for the Cu(II) complex. This indicates the steric effect and basicity
of the Schiff base controls the Cu(II) complexation process [45]. Results may
help in understanding complicated metal-protein interactions.
Interactions of Cu(II), Zn(II), and Ni(II) with terminally blocked
(CH 3 CONH- and −CONH 2 ) peptides (-TESHHK-, -TASHHK-, -TEAHHK-,
-TESAHK-, and -TESHAK-) have been studied [113]. The selected His-
containing peptides are models of histone H2A and the complexation study
may elucidate toxicity of metals [113]. For example, Ni(II) binding to bioli-
gands may lead to oxidative degradation of biomolecules via reactions with
• OH radicals. The studied peptides interacted strongly with metal ions, particu-
larly Cu(II) and Ni(II) ions, which, on reactions with H 2 O 2 , generate reactive
oxygen species, which can efficiently oxidize biomolecules such as 2′-deoxy-
guanosine [114].
Binary and ternary complexes of Cu(II), Zn(II), and Ni(II) (M) with dicar-
boxylic amino acids (Asp and Glu, B) and adenosine-5′-triphosphate (A) as
ligands have been reported due to the importance of M-A-B interactions in