ACID–BASE PROPERTIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 64

complexes have been studied for the last several decades [76-84]. Examples
of such studies include the interaction of metals with sulfur-containing amino
acids in the 1950s [76-79] and Cu(II) complexes of dipeptides and amino acids
in the 1970s [80, 81]. In a recent study, the coordination dynamics of Zn in
proteins has been reviewed [75]. In enzymes, the imidazole nitrogens from His,
the carboxylate oxygen(s) from the side chain of Asp and Glu, and sulfur from
the sulfhydryl groups of Cys were the donors of proteins to coordinate Zn
(Fig. 2.5a). The sulfhydryl group provides the unique reactivity in the coordina-
tion environment [85]. The importance of amino acids in the coordination
sphere is shown in Figure 2.5b. A role of hydrogen bonds in the interactions
of zinc-bound hydroxide ion and His ligands with other amino acids was
suggested.
His plays a significant role in the coordination of metals in biological envi-
ronment such as superoxide dismutase, the prion protein, amyloid β-peptides,
and histones [73, 86-90]. Metal-binding sites in proteins may also include the
phenol ring of Tyr [91]. Peroxynitrometal complexes have also been evoked
in hemoglobin and myoglobin [92]. Furthermore, the interaction of a metal
ion with an amino acid ligand increases significantly with a decrease in the
solvent polarity of the media [70]. The following sections represent selected
examples of complex formation of metals with amino acids and proteins, which
have shown roles in generating reactive species and in mimicking structures
of metalloenzymes [93].
2.2.2.1 Iron. Complexes of iron have significant relevance in natural and
biological environments [94-96]. Marine siderophores have a high affinity
for the ferric iron, which are produced when a demand of iron arises [97].
Studies on iron-catecholate complexes and iron-peptide of sequence Ac-Ala-
DOPA-Thr-Pro-CONH 2 (DOPA = 3,4-dihydroxyphenylalanine) have been
performed [98]. DOPA-containing peptide mimics mussel adhesive proteins.
The complexes with iron were Fe(L) n ( n = 1-3), which could react with oxygen
to yield organic radicals. Such complexes may also react with H 2 O 2 to result
in reactive species (see Chapter 6). The coordination chemistry of the Fe(III)
complex with alterobactin A, a siderophore from the marine bacterium
Alteromonas luteoviolacea , has also been evaluated [97]. High affinity of
Fe 3+ ion with alterobactin A was suggested based on the estimation of high
stability constants for the formation of Fe(III)-alterobactin A complex. Com-
plexation of Fe(III) with the biologically important ligand, cystine, has also
been studied [99]. This study showed the existence of FeL + and FeL − species
in at least 30% of total Fe. Free iron and complex iron have shown different
reactivities with reactive species (Chapter 4). Complexation of Fe(III) with
poly(aminecarboxylate) ligands has been studied in detail to understand their
rates with reactive species (e.g., Chapter 4) [94].
Formation of high-valent species (ferryl and perferryl species) in Fenton
reaction and Fenton-type reactions is related to complexation of low-valent
iron species (Chapter 4). Structural studies on high-valent iron complexes