Charge-Transfer Interactions of Aliphatic Amino Acids with Metal Ions

The effect of various non-SH-containing amino acids on an electron spin resonance spectrum as well as on an optical absorption spectrum of an aqueous copper (II) solution was investigated. The data obtained indicate that a charge-transfer complex is formed with amino acids acting as electron donors. A plot of the wavelength of the CT-band vs. the pK values of the carboxyl group exhibits a straight line with the larger pK values appearing at shorter wavelengths. Similar results were obtained when the intensity was plotted vs. the pK values of the amino group; again, higher pK values correspond to lower intensities. Thus, the method allows the determination of the pK values of prosthetic groups of amino acids. The data obtained are in good agreement with the CT-theory.