The biological significance of histidine residue as a metal binding site in proteins has prompted many workers to investigate copper(II) histidine complexes. It has been found that elucidate of the complex formation between Cu(II) and histidine is not easy since histidine involves a bulky imidazole ring and has three of four coordination sites, and Cu(II) requires a tetragonal coordination [1–3]. The probable electrostatic interactions giving rise to stereoselective aspects of the problem have been mainly investigated using potentiometric, spectral and magnetic measurements [4]. No polarographic studies are reported in the literature. In the present communication the parent and ternary complexes of D- and L-histidinato copper(II) with some L-aminoacids, viz. phenylalanine, tryptophan, valine, proline, methionine, leucine, lysine, serine, threonine, alanine, glutamic acid and aspartic acid have been studied in aqueous 0.5 - M KNO 3 at the dropping mercury electrode under varying experimental conditions with a view to calculate the kinetic parameters. Besides half wave potential (E 1 2 , transfer coefficient (α), the formal rate constant (Kr), the activation energy of the rearrangement of the depolizer ( Q e) and the activation energy of diffusion ( Q D) have been evaluated for each system. Electronic spectra (in solution) of these complexes have also been recorded. Experimental Experimental details are described in several earlier communications from this laboratory [5]. The characteristics of dme for one set of measurements are given below: ▪ Results Polarographic characteristics for some of these ternary complexes (1:1:1 ratio) are given in Table I. All these complexes are observed to undergo diffusion-controlled single-step two electron irreversible electro-reduction at dme. A perusal of the Table shows that Kr and also i d increase with increase in temperature in the case of Cuhistidinealanine/ or serine, indicating an easier reduction, which is also supported by the shift of E 1 2 to comparatively more positive potentials with the rise of temperature. However, in the case of Cuhistidineaspartic and/or threonine, such trend in E 1 2 is disturbed. The significance of results will be discussed in detail. t001 Polarographic Characteristics of some Mixed-Aminoacid Complexes of Copper(II) at Different Temperatures: h = 40 cm, [Cu 2+] = 1 m M, KNO 3 = 0.5 M, gelatin = 0.005%, pH = 6.5 ± 0.1. Complexes Temp. (° C) − E 1 2 (V vs. SCE) i d (μA) D × 10 7 (cm 2 sec −1) Kr × 10 4 (cm sec −1) Q D (kcal) Q e (kcal) Cuhistidineaspartic acid 25 0.189 0.90 0.2766 2.086 6.195 30 0.170 0.76 0.3405 1.488 9.743 7.889 7.5733 35 0.178 1.02 0.2724 2.680 8.768 Cuhistidinealanine 25 0.183 0.845 0.3575 1.840 7.979 30 0.178 0.920 0.3367 2.181 9.455 5.2690 4.4329 35 0.178 1.28 0.3347 4.221 12.78 Cuhistidinethreonine 25 0.183 0.89 0.2570 2.041 6.616 30 0.193 1.20 0.2647 3.710 7.413 10.474 2.3312 35 0.194 1.40 0.2431 5.049 8.230 Cuhistidineserine 25 0.203 0.96 0.2483 2.383 2.374 4.793 30 0.200 1.10 0.2425 3.118 5.779 6.487 3.9574 35 0.197 1.31 0.2674 4.442 7.519