The tanning process in the manufacture of leather relies upon the interaction of Cr³⁺ with the fibrous protein collagen. Although glycine and L-proline are the most important amino acids (see Table 29.2) in collagen, glutamic acid (pK_a = 3.8) and aspartic acid (pK_a = 4.2) are also present. During tanning, the pH of an aqueous solution of Cr(OH)SO₄ is lowered from ≈2.8 to 3.8. 

(a) What chromium(III) ion is present in aqueous solution at very low pH and why is H⁺ needed to stabilize this species?

(b) In the absence of collagen, aqueous solutions of Cr³⁺ at pH 3.8 contain linear, tri- and tetranuclear species. Suggest structures for these species. 

(c) A study [A.D. Covington et al. (2001) Polyhedron, vol. 20, p. 461] of the interaction of chromium(III) with collagen states that, at pH 3.8, carboxylate groups should compete with hydroxido ligands for chromium. The study concludes that the predominant chromium-containing species bound to leather is a linear oligomer with nuclearity 2 or 3. Using this information, suggest how Cr³⁺ interacts with collagen and indicate how Cr³⁺ may be involved in cross-linking of collagen fibres. 

(d) A major concern in the tanning industry is to prevent toxic waste arising from the oxidation of Cr(III) to Cr(VI). Chromium(VI) may be present as [Cr₂O₇]²⁻ and two other anions, depending upon the pH. Write equilibria to show how the three Cr(VI) species are interrelated, and suggest a method to remove them from waste water.

Table 29.2.

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Name of amino acid L-Alanine L-Arginine L-Asparagine L-Aspartic acid L-Cysteine Abbreviation for amino acid residue (abbreviation used in sequence specification) Ala (A) Arg (R) Asn (N) Asp (D) Cys (C) Structure H₂N H₂N H₂N H H₂N H H₂N Me CO₂H CO₂H H₂N NH₂ CO₂H OH CO₂H SH CO₂H H NH Acidic, neutral or basic Neutral Basic Neutral Acidic Neutral