The segmental ligands bis{1-alkyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl}methane (alkyl = methyl (L(5)), ethyl (L(6))) react with lanthanide perchlorates (Ln = La, Eu, Gd, Tb) in acetonitrile to yield the f-f dinuclear homotopic triple-stranded helicates [Ln(2)(L(i)())(3)](6+) (i = 5, 6) under thermodynamic control. The crystal structure of [Tb(2)(L(6))(3)](ClO(4))(3)(MeCN)(2)(THF)(0.5)(EtOH)(0.5) (11a, C(124)H(145)N(26)O(31)Cl(6)Tb(2), triclinic, P&onemacr;, Z = 2) shows the wrapping of the ligands about a pseudo-C(3) axis passing through the metal ions. The Tb ions are 9-coordinate in facial pseudo-tricapped trigonal prismatic sites and are separated by 9.06 Å. (1)H-NMR and ES-MS data establish that the triple helical structure is maintained in solution. Spectrophotometric titrations (Ln = La, Eu) indicate log beta(23) = 24-25 and the formation of a 2:2 complex [Ln(2)(L(5))(2)](6+) (log beta(22) = 19-20). Quantum yield determination in acetonitrile shows that the terminal N,N-diethylcarboxamide groups in L(5) favor efficient intramolecular L(5) --> Eu(III) energy transfers leading to strong Eu-centered red luminescence, 50 times as intense as the luminescence observed when the carboxamide groups are replaced by substituted benzimidazole units in [Eu(2)(L(4))(3)](6+). Resistance toward hydrolysis also results from the use of carboxamide groups, and no quenching of luminescence is observed for [Eu(2)(L(5))(3)](6+) in moist acetonitrile up to 2.5 M water. The crucial role played by carboxamide groups for the control of structural, electronic, and photophysical properties is discussed. Replacing perchlorates by triflates allows the isolation of the dinuclear double-stranded helicate [Eu(2)(L(6))(2)(CF(3)SO(3))(4)(H(2)O(2))(2)](CF(3)SO(3))(2)(MeOH)(2)(H(2)O)(5)(.5), whose crystal structure (13a, C(85)H(106)Eu(2)F(18)N(16)O(30)S(6), monoclinic, C2/m, Z = 2) reveals a side-by-side arrangement of the two strands and 9-coordinate Eu ions linked through hydrogen-bonded water molecules.

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Abstractx Ray crystallography is currently the most favoured technique for structure determination of proteins and biological macromolecules. Increasingly, those interested in all branches of the biological sciences require structural information to shed light on previously unanswered questions. Furthermore, the availability of a protein structure can provide a more detailed focus for future research. The extension of the technique to systems such as viruses, immune complexes, and protein-nucleic acid complexes serves only to widen the appeal of crystallography. Structure based drug design, site directed mutagenesis, elucidation of enzyme mechanisms, and specificity of protein-ligand interactions are just a few of the areas in which x ray crystallography has provided clarification.