Ionization of the peptide hydrogens

The copper(II)-promoted hydrolysis of glycylglycine has been studied in some detail.120 Copper(II) ions catalyze the hydrolysis of glycylglycine in the pH range 3.5 to 6 at 85 °C.120 The pH rate profile has a maximum at pH 4.2, consistent with the view that the catalytically active species in the reaction is the carbonyl-bonded complex. The decrease in rate at higher pH is associated with the formation of a catalytically inactive complex produced by ionization of the peptide hydrogen atom. This view has subsequently been confirmed by other workers,121 in conjunction with an IR investigation of the structures of the copper(II) and zinc(II) complexes in D20 solution.122 Catalysis by cobalt(II),123 and zinc(II), nickel(II) and manganese(II) has also been studied.124-126... 

Figure 11. Proposed ionization of the amine hydrogen causing the yellow-to-red color change when base is added to Cu(lll)-peptide complexes...

The shift of the peptide carbonyl stretching band from 1625 (IV) to 1610 (V) cm" may indicate the ionization of the peptide NH hydrogen, since such an ionization results in the resonance of the O—C—N system, as indicated by the dotted line in structure V. Kim and MartelP also studied the triglycine and tetraglycine Cu(ll) systems. 

The structure of chloro(glycyl-L-methionato)platinum(n) monohydrate has been studied by. iV-rays and ND. The power of ND to locate hydrogen atoms even in the presence of very heavy atoms has been used to determine the state of internal ionization of the peptide. 

At different hydrogen ion concentrations different amounts of the amino acids and of the peptide exist as zwitterions K in the above e( uation pertains only to the equilibrium between the zwitterions. The useful cjuantity is the total degree of synthesis from the total amounts of amino acids it is a quantity that will vary according to the pH and ionization constants of the amino acids and dipeptide. The effect of varying pH can be calculated as follows. Representing the total ionigen concentration as S[I] that of the cation as [1+] that of the zwitterion as [I ] and that of the anion as [I ] then... 

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