Adduct formations of Pt(II) complexes containing an aromatic diimine (DA) and an L-amino acid (A) with an aromatic carboxylate (AR) or a mononucleotide (NMP) has been studied by synthetic, structural, spectroscopic, and calorimetric methods. Several adducts between Pt(II) complexes, [Pt(DA)(L-A)] (charges are omitted; DA=2,2'-bipyrimidine (bpm); A=L-arginine (L-Arg), L-alaninate (L-Ala), and AR (=indole-3-acetate (IA), gentisate (GA)) or GMP were isolated as crystals and structurally characterized by the X-ray diffraction method. GMP in [Pt(bpm)(Arg)](GMP).5 H(2)O was revealed to be bound through the pi-pi stacking and guanidinium-phosphate hydrogen bonds. The [Pt(DA)(A)]-AR and -NMP systems in aqueous solution exhibited NMR upfield shifts of the aromatic ring proton signals due to stacking. The stability constants (K) for the adducts were determined by absorption and NMR spectra and calorimetric titrations. The log K values were found to be in the range 1.40-2.29 for AR and 1.8-3.3 for NMP, the order for NMP being GMP>AMP>CMP>UMP. The DeltaH degrees values were negative for all the systems studied, and the values for AR (=IA and GA) were more negative than those for NMP, indicating that ARs are stronger electron donors than NMPs. Comparison of the log K values for [Pt(bpm)(L-Arg)] and [Pt(bpm)(L-Ala)] (Ala=alaninate) indicated that the Arg moiety further stabilized the adducts by the guanidinium-carboxylate or -phosphate hydrogen bonds. The combined effects of weak interactions on the stability of the adducts in solution are discussed on the basis of the thermodynamic parameters and solid state structures.

Ternary Cu(II) complexes containing an aromatic diimine (DA = di(2-pyridylmethyl)amine (dpa), 4,4'-disubstituted 2,2'-bipyridine (Y 2 bpy; Y = H (bpy), Me, Cl, N(Et) 2 , CONH 2 , or COOEt), or 2,2'-bipyrimidine) and an aromatic amino acid (AA = p-substituted L-phenylalanine (Xphe; X = H (phe), NH 2 , NO 2 , F, Cl, or Br), L-tyrosine, L-tryptophan (trp), or L-alanine) were studied by X-ray diffraction, spectroscopic, and potentiometric measurements.The structures IntroductionNoncovalent or weak interactions involving aromatic rings attract much attention for their importance in molecular recognition, stabilization of protein structures, and supramolecular architecture in chemistry and biology. [1][2][3][4] Studies on benzene and porphyrin dimers showed that the edge-to-face and offset face-to-face interactions between the aromatic molecules are more effective than the face-to-face interactions, 5,6 and a survey of protein structures revealed that the former type of interactions are most common for phenylalanine residues. 3 Cozzi and Siegel reported that the intensities of face-to-face interactions are influenced by ring substituents;electron-withdrawing groups strengthen the interactions while electron-donating groups weaken them. 7 On the other hand, cation-π interactions have been revealed for various systems including proteins, where the cationic groups of amino acid side chains such as guanidinium and ammonium groups were found to be located close to the aromatic rings of aromatic amino acid residues. 8Both gas-phase experiments and theoretical calculations indicated that alkali metal ions bind strongly to aromatic rings in the gas phase. 8,9 Metalation of porphyrin is known to enhance the π-π interaction between two porphyrin molecules due to intramolecular polarization of the metal ion and the porphyrin. 5,10 We have been studying aromatic ring stacking interactions in ternary Cu(II) and Pd(II) complexes, [M(DA)(AA)] (M = Cu(II) or Pd(II); DA = aromatic diimines such as 1,10-phenanthroline (phen); AA = aromatic amino acids such as L-tyrosine (tyr)); we found that metal-coordinated DA effectively stacks with the side chain aromatic ring of coordinated AA 11 and that the ring substituent in the interacting ring of AA has influence on the stacking. 11,12 While 2N1O-donor tripod-like ligands containing one pyridine and one phenol ring and a pendent indole ring were found to undergo only weak intramolecular indole-pyridine interactions in CH 3 CN, their Pd(II) complexes exhibited much stronger interactions as evidenced by the 1 H NMR upfield shifts due to the ring current effect and the methylene proton signals showing a fixed side chain conformation. 13 The adduct formation between planar Pt(II) complexes, Pt(DA)(L') (L' = 4 ethylenediamine or amino acids), and mononucleotides was revealed to be enthalpically driven mainly through stacking interactions by the relevant thermodynamic parameters 14 and decrease the electron density of the Pt(II) center as seen from and the downfield shift of the 195 Pt NMR s...

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