Metal-modified nucleobase pairs and triplets as cytosine receptors

A preorganized cationic receptor 2 for cytosine (C) is described which is c omposed of trans-a(2)Pt(II) (a = NH3 or CH3NH2) cross-linked modules with a denine (A), guanine (G), and uracil (U) or thymine (T) model nucleobases. T he functions of these three modules are as follows: i) Adenine orientates t he two other bases at right angles, thus producing the L-shape of the recep tor. ii) Guanine is the primary receptor, iii) Uracil or thymine act as cor eceptors. Compared with the normal Watson-Crick pair between G and C, the a ssociation constant between 2 and C increases by a factor of 3 (in DMSO). A s deduced from H-1 NMR spectroscopy and confirmed by the X-ray crystal stru cture of the C adduct 4b, cytosine is fixed through five hydrogen bonds to the receptor, one of which involves the aromatic H(5) of C. A comparison of C binding is made with a structurally related linkage isomer receptor as w ell was the precursor molecule trans-[a(2)PtAG](2). The potential of modula r, cationic receptors is illustrated.