ACID–BASE PROPERTIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 63

1.0

H 2 A

A 2-

0.8

0.6

0.4

0.2

H 3 A +

HA -

0.0

0 2 4 6 8 10 12 14

pH

Figure 2.4. Calculated species distribution of cysteine at I = 0.0 (solid lines) and I = 4.0
(dashed lines).

TABLE 2.3. Speciation of Selected Amino Acids at pH 7.4
Amino Acid α H3A α H2A α HA α A

Gly - 1.1 × 10 −5 0.9937 0.0063

Pro -

His 5.5 × 10 −8 0.0378 0.9488 0.0134

Cys 4.7 × 10 −6 0.8576 0.1423 2.2 × 10 −4

Met - 5.5 × 10 −6 0.9939 0.0061

Trp -

9.2 × 10 −6

0.9891 0.0108

4.3 × 10 −6 1.0 × 10 −5 Tyr 0.9800 0.0200

Phe - 7.5 × 10 −6 0.9886 0.0114

2.2.2 Metal Complexes

Many proteins and enzymes contain metals ions in which peptide bonds (or
amino acid groups) are coordinated with metal ions [73]. The nature of coor-
dination depends on the protonation of amino acids (see discussion in the
previous section). For example, two kinds of electron-pair donors in α-amino
acids, nitrogen of amino groups ( NH + and NH 2 ) and oxygen of carboxylate
(COO − ), can form mono , bis , or tris complexes with Cu(II) [74]. Because of
the role of coordination in the redox chemistry of metals in enzymes [75], the
solution equilibria and structures of metal-amino acid and metal-peptide