After discovery of the anticancer properties of cisplatin, special attention was devoted to evaluation of gold(III) complexes as potential antitumor agents due to the fact that both Pt(II) and Au(III) ions possess the same d8 electronic configuration and preferentially form square-planar complexes. The possible involvement of gold(III)complexes in cancer treatment initiated an interest in the area of gold(III) interactions with different biologically important ligands such as amino acids, peptides and proteins. However, the mechanism of antitumor activity of gold(III) complexes is not yet completely understood and further investigation of this subject is required. A better knowledge of the gold(III) reactions with peptides contributes to the future development of gold(III) complexes as potential antitumor agents and also has importance in relation to the severe toxicity of gold-based drugs. The Introduction part of this thesis highlights recent findings in the field of gold(III) complexes with amino acids, peptides and proteins. The first part of this chapter provides an overview of the gold(III) reactions with amino acids, such as glycine, alanine, histidine, cysteine and methionine, while the second part is mainly focused on the results achieved in the mechanistic studies of the reactions between gold(III) and different peptides and structural characterization of gold(III)-peptide complexes as the final products in these reactions. Finally, the last part of Introduction deals with the reactions of gold(III) complexes with proteins as primary targets for cytotoxic gold compounds (B. Đ. Glišić, U. Rychlewska and M. I. Djuran, Dalton Trans., 41 (2012) 6887-6901). The Results and Discussion deals with spectroscopic and electrochemical investigations of gold(III) reactions with dipeptides containing no heteroatom in the side chain, and with histidine- and methionine-containing dipeptides. Furthermore, resulting gold(III)-peptide complexes were characterized by application of X-ray crystallography. The proton NMR spectroscopy was applied to study the reactions of the dipeptides glycylglycine (Gly-Gly) and glycyl-L-alanine (Gly-L-Ala) with hydrogen tetrachloridoaurate(III) (HAuCl4). All reactions were performed in the pH range 2.00 – 5.00 and at 40ºC. When these reactions were carried out at pH 2.00 and 3.00, the formations of the Au(III)-peptide complexes containing tridentate coordinated Gly-Gly and Gly-L-Ala dipeptides were obsereved. The Au(Gly-Gly-N,N’,О)Cl and Au(Gly-L-Ala-N,N’,О)Cl complexes, as the major products in the investigated reactions, were characterized by 1H and 13C NMR spectroscopy. From the obtained spectroscopic data, it was concluded that coordination of the dipeptides to Au(III) ion occurred through the nitrogen atom of the terminal amino group, the deprotonated peptide nitrogen and the oxygen atom of carboxyl group. The fourth coordination place in these square-planar complexes was occupied by the chloride ion. However, when the reactions of the Gly-Gly and Gly-L-Ala dipeptides with AuCl4￣ were performed at pH > 3.00 (pH 4.00 and 5.00), reduction of Au(III) occured during time and the complete reaction mixture was immediately dark from elemental gold. The Au(Gly-Gly-N,N’,O)Cl complex formation was two times faster (k2 = (1.63 0.07) x 10-7 M-1s-1) than Au(Gly-L-Ala-N,N’,O)Cl (k2 = (0.71 0.06) x 10-7 M-1s-1). Difference in the reactivity between these two peptides was attributed to the steric hindrance of the methyl group of L-alanine. However, the reactions of AuCl4– with Gly-Gly and Gly-L-Ala were one hundred times slower than with glycyl-L-histidine (Gly-L-His) (k2 = (124.00 0.30) x 10-7 M-1s-1) (B. Đ. Glišić, S. Rajković, M. D. Živković and M. I. Djuran, Bioorg. Chem., 38 (2010) 144-148). In the second part of this chapter, two forms of gold(III) complex with Gly-L-His, hydrated Au(Gly-L-His-N,N’,N’’)ClNO3 .1.25H2O and unhydrated Au(Gly-L-His- N,N’,N’’)ClNO3, as well as Au(L-Ala-L-His-N,N’,N’’)ClNO3 .2.5H2O complex were synthesized and characterized by 1H NMR spectroscopy and X-ray crystallography. In these complexes, L-histidine-containing dipeptides were coordinated to Au(III) ion through N3 nitrogen atom of the imidazole ring, deprotonated amide nitrogen and terminal amino group of glycine or L-alanine, while the fourth coordination place was occupied by the chloride ion. As a consequence of the Au…Cl interactions the square-planar coordination of Au(III) ion in Au(Gly-L-His-N,N’,N’’)ClNO3 .1.25H2O, Au(Gly-L-His- N,N’,N’’)ClNO3 and Au(L-Ala-L-His-N,N’,N’’)ClNO3 .2.5H2O complexes is completed to an elongated octahedron by two more distant chloride ions belonging to the neighbouring complexes (U. Rychlewska, B. Warżajtis, B. Đ. Glišić, M. D. Živković, S. Rajković and M. I. Djuran, Dalton Trans., 39 (2010) 8906-8913). Furthermore, the reactions of the monofunctional Au(dien)ClCl2 complex, in which dien is diethylenetriamine tridentate coordinated to Au(III) ion, with L-histidinecontaining dipeptides, namely L-histidyl-glycine (L-His-Gly) and glycyl-L-histidine (Gly- L-His), were studied by application of 1H and 13C NMR spectroscopy. All reactions were performed in aqueous solution in the pD range 3.50 – 5.50 and at ambient temperature. When the reaction of this complex with N-terminal histidine-containing dipeptide, L-His- Gly, was performed in the investigated pD range, strongly selective coordination of the dipeptide through N3 imidazole nitrogen atom to Au(III) was observed, with formation of Au(dien)(L-His-Gly-N3)3+ complex. However, in the reaction with the dipeptide containing C-terminal histidine Gly-L-His, tridentate coordination through the N3, deprotonated amide and amino nitrogen atoms with concomitant detachment of the dien ligand was occured. No coordination of the carboxylate oxygen atom to Au(III) was observed by 13C NMR spectroscopic measurements for investigated reactions under above mentioned experimental conditions (B. Đ. Glišić, S. Rajković and M. I. Djuran, J. Coord. Chem., accepted for publication). Finally, the proton NMR spectroscopy was applied to study the reactions of the dipeptide glycyl-D,L-methionine (Gly-D,L-Met) and its N-acetyl derivative (Ac-Gly-D,LMet) with HAuCl4 complex. The corresponding dipeptide and AuCl4￣ were reacted in 1:1, 2:1 and 3:1 molar ratios. Аll reactions were performed at pH 2.00 in 0.01 mol/dm3 DCl in D2O as solvent and at 25ºC. It was found that the first step of these reactions was very fast coordination of Au(III) to the thioether sulfur with formation of the gold(III)- peptide complex AuCl3(R-Gly-Met-S) (R = H or Ac). This intermediate further reacts with an additional methionine residue to generate the R-Gly-Met chlorosulfonium cation as the second intermediate product, which readily undergoes hydrolysis to give the R-Gly- Met sulfoxide as the final product of this redox process. The oxidation of the methionine residue in the reaction between Gly-D,L-Met and AuCl4– was five times faster (k2 = 0.363 0.074 M-1s-1) in comparison to the same process with N-acetylated derivative of this peptide (k2 = 0.074 0.007 M-1s-1). The difference in the oxidation rates between these two peptides can be attributed to the free terminal amino group of Gly-D,L-Met dipeptide. The mechanism of the reaction between the Gly-D,L-Met dipeptide and AuCl4– was additionally investigated by UV-Vis and cyclic voltammetry (CV) techniques. From these measurements, it was shown that the AuCl2– complex formed in the reaction with equimolar amounts of the reactants showed a strong tendency to disproportionate to AuCl4– and metallic gold. However, in the presence of excess of the dipeptide, the resulting polynuclear Gly-D,L-Met-S-Au(I) and Ac-Gly-D,L-Met-S-Au(I) complexes showed themselves to be quite stable products (B. Đ. Glišić, S. Rajković, Z. Stanić and M. I. Djuran, Gold Bull., 44 (2011) 91-98). The structure of the resulting product of gold(III)- induced oxidation of Gly-D,L-Met dipeptide was confirmed by X-ray crystallography. The crystal structure of H+Gly-D,L-Met sulfoxideAuCl4 consists of discrete square-planar AuCl4anions and glycylmethionine sulfoxide cations. It was shown that the cations are disordered at the methionine side chain due to the presence, at the same crystal site, of two diastereomers differing in their configuration at the triply bonded S atom. The crystallization process of H+Gly-D,L-Met sulfoxideAuCl4 complex is partially diastereoselective, leading to a substantial excess of one of two possible diastereoisomers (U. Rychlewska, B. Warżajtis, B. Đ. Glišić, S. Rajković and M. I. Djuran, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 66 (2010) 51-54). The results of this thesis can contribute to the development of new gold(III) complexes as potential antitumor agents and also can be important in relation to the severe toxicity of gold-based drugs. Nakon otkrića antitumorske aktivnosti cisplatine, posebna pažnja je posvećena ispitivanju kompleksa zlata(III) kao potencijalnih antitumorskih agenasa, zbog činjenice da su Au(III) i Pt(II) izoelektronski joni (d8 elektronska konfiguracija) i da formiraju kvadratno-planarne komplekse. Usled potencijalne primene kompleksa zlata(III) u lečenju tumora, u toku poslednjih nekoliko decenija,intenzivno su izučavane reakcije Au(III) jona sa biološki važnim ligandima, kao što su aminokiseline, peptidi i proteini. Mehanizam antitumorskog delovanja kompleksa zlata(III) još uvek nije u potpunosti razjašnjen i predstavlja predmet intenzivnih izučavanja. Ispitivanje reakcija kompleksa zlata(III) sa peptidima je od velikog značaja za definisanje mehanizma antitumorskog i toksičnog delovanja ovih kompleksa. U Opštem delu ove doktorske disertacije dat je pregled najnovijih rezultata postignutih u oblasti ispitivanja reakcija kompleksa zlata(III) sa aminokiselinama, peptidima i proteinima. U prvom delu ovog poglavlja prikazani su rezultati ispit