Base quartets

S. Metzger, B. Lippert, A metalated guanine,cytosine base quartet with a novel GC pairing pattern involving h(5) of c. J. Am. Chem. Soc. 118, 12467-12468 (1996)... 

The simplest quadruplex-based higher order structure is a double-quadruplex, in which two tetramolecular quadruplexes associate to form an octaplex. One may distinguish two types of dimers (a) those based on simple stacking of terminal quartets and (b) those involving interlocked base quartets. 

Figure 4. Schematic potential energy surface for the reaction of FeO" " with methane. The sohd line indicates the sextet surface the quartet surface is shown with a dotted line, in each case leading to the production of Fe + CH3OH. The dashed line leads to formation of FeOET + CH3. The pathway leading to the minor FeCH2" + H2O channel is not shown. Schematic structures are shown for the three minima the [OFe CHJ entrance channel complex, [HO—Fe—CH3] insertion intermediate, and Fe" (CH30H) exit channel complex. See text for details on the calculations on which the potential energy surface is based.

Recently, Moskaleva et al. have proposed a new mechanism based on electronic structure calculations." Earlier experimental studies by Kasdan et al. determined that methyne (HC) has a doublet ground state and with a doublet-quartet energy splitting (AEdq) of 71.5 + O.SkJ/mol." Moskaleva et al. noted that the initially proposed mechanism (for HCN and N(" S) atom formation) is therefore spin-forbidden, and they also proposed a more favorable and spin-allowed reaction on the doublet surface. This new route on the doublet energy surface proceeds through the formation of an NCN intermediate, with concomitant formation of (doublet) hydrogen atom. 

Metal-modified base pairs have been reported some time ago for identical (119), complementary (120), and non-complementary bases (59,121-126). These may assemble into a type I quartet through dimerization, as seen for trassociation constant of 59.1 M 1 in d6-DMSO (123). Rather unusual is the involvement of the aromatic C-H5 proton in the hydrogen bond-... 

Fig. 10.13 The guanine-silylated conjugate 5 self-associates in the presence of alkali-metal cations to form hexagonal nanorod hybrids, presumably formed by G-quartet-based interactions.

Sreenivasachary, N. Lehn, J.-M. Gelation-driven component selection in the generation of constitutional dynamic hydrogels based on guanine-quartet formation. Proc. Natl. Acad. Sci. U.S.A. 2005,102, 5938-5943. 

A comparison of these nucleobase-derived metallacalixarenes with natural nucleobase-derived tetrands may be advisable. In the non-natural quartets formed by interconnection of uracil, purine or its derived nucleobases with coordinative metals, the cationic character lies on the metals in the periphery and anions are bound in the cavity. In natural tetrands of these nucleobases, the hydrogen bonds interconnect the bases and the structures are stabilized by a cation located in the centre whereas phosphate groups located in the periphery carry negative charge (2005AGE668). As against the existence of only natural... 

Lee et al. evolved a comprehensive analytical-theoretical treatment, based on the solution of the reorientational isotropic diffusion equation, for an ensemble of high-spin systems under motion. These authors developed an analytical expression for the slow-tumbling motional region that relates the orientational-motion correlation time t (in s), or the corresponding tumbling rate t, with the step separation bB, of the ESR fine structure of a quartet by Eq. 8,... 

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