The structures and protonation energies of aminoguanidine and aminoguanidinium ion were obtained from quantum chemical calculations by using the density functional method B3-LYP and the standard basis set 6 -31 G(d ). The energy differences between the various forms of the two possible tautomers, the endiamine and the imidamide form, were investigated. The endiamine was found to be more stable than the most stable form of the imidamide by 5 kcal/mol in the gas phase. The proton affinity of am inoguanidine was found to be 241.4 kcal/mol (B3 -LYP/6 -31 G(d )) in the gas phase. The liquidation pKa2 for amininoguanidine and the guanidinium ion was determined to be 11.5 ± 0.1. The reference values for guanidine free base and the guanidinium ion are 235.7 kcal/mol and 13.6, respectively. The difference in the order of basicity in the gas phase and in aqueous solution between aminoguanidine and guanidine is discussed in terms of symmetry, resonance, delocalisation, rehybridisation of nitrogen atoms, and electrostatic repulsion.

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The iron(III) complex formation of entacapone, a novel catechol O-methyltransferase (COMT) inhibitor, has been studied at 25 degrees C in aqueous 0.1 mol/L NaCl solution by using the electromotive force titration method. Entacapone functions as a bidentate ligand chelating through the catecholate oxygen atoms and forms stable iron(III) complexes with the formation constant of a tris complex: log beta-613 ([FeL3(3-)][H]6+/[Fe3+][H2L]3) = -6.9 +/- 0.1. Distribution curves show that entacapone is highly effective for iron(III) in moderately dilute solution (10(-3) mol/L) whereas in very dilute solution (10(-6) mol/L) the iron hydroxo complexes together with FeL3(3-) dominate under physiological pH 7.4. Comparison of iron(III) species distribution in a competitive two-ligand entacapone-catechol system reveals that the complexation of entacepone is favored at high and low dilution.

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The first study on the crystal and molecular structures of basic forms of bis(amidinohydrazones) is reported. The structures of the free base and the monohydrochloride salt of the antileukemic agent glyoxal bis(amidinohydrazone) (GBG) were determined by single crystal X-ray diffraction and were refined to R-values of 0.038 and 0.040, respectively. These structures are of special interest because recent results indicate that, in contrast to previous assumptions, the free base may be the actual antileukemic form of bis(amidinohydrazones) and that the monocation form is the predominant species of antileukemic bis(amidinohydrazones) at physiological conditions. In the crystals of the free base as well as in those of the monohydrochloride salt, GBG was found to exist solely in the all-trans configuration of the chain and to consist of one of the three possible geometrical isomers only. In the solid state, GBG free base consists solely of the endiamine tautomer instead of the 'classical’ carboximidamide tautomer, as does also the non-protonated part of GBG monocation in the monohydrochloride salt. Proton NMR measurements indicated that the free base consists of the endiamine tautomer also in dimethyl sulfoxide solution. In the solid state, both of the compounds studied consist of stacks of planes. In the case of the free base, the stacks are crisscross to each other. The distance between the mean planes of the molecules or the monocations is approximately 3.5 Å. The crystals of the monohydrochloride salt contain one molecule of water per each GBG monocation. In both compounds studied, the molecules in the crystals are held together by very extensive hydrogen bond networks and by the interaction of delocalized π-electrons. The crystal of C4H10N8 is monoclinic, space group C2/c with a = 15.874(6), b = 6.972(4), c = 7.8l3(5)Å, β = 90.34(4)° and Z = 4. The crystal of C4H13N8OC1 is monoclinic, space group P21/n with α = 7.010(3), b = 22.307(9), c = 7.028(3)Å, β = 66.33(3)° and Ζ = 4.