No abstract

COMMUNICATIONStemperature with occasional agitation. The supernatant was separated by ultrafiltration (Centricon, Amicon, cut-off 30 kDa, 6 h at 15 "C). The nanotubes were then washed with water (0.6 mL), and the separation was repeated. In a control experiment (to check for adsorption of DNA onto the membrane filter), DNA solution was added to the ultrafiltration tube in the absence of nanotuhes, and the separation and washing steps carried out as before. DNA concentrations were determined from the absorption at 260 nm. The recovery of DNA in the control experiment was >87%. A second set of experiments was carried out with 0.1 M NaCIO, to wash the tubes, and then melting curves for the desorbed DNA were determined by UV. Molecular rnodelmg: A bifunctional Pt adduct was constructed on one strand of a B-DNA model with the HyperChem program [19] and restrained energy minimization, followed by docking of the ammine ligands in an appropriate geometry. This produced a model of the bifunctional adduct similar to those obtained with Newton-Raphson energy minimization and a force field developed for PtiDNA interactions [20]. Iodine atoms were then added to the 5-positions of each cytidine base.

An extremely long CC single bond of 1.713 Å is found in the [2+2] cycloadduct of ortho‐didehydrobenzene and tetradehydrodianthracene. The force constant of this central bond is reduced to only 30% of that of the CC bond in ethane. The corresponding CC stretching vibration (687 cm−1) in the Raman spectrum is red‐shifted 300 cm−1 relative to the bands of “normal” hydrocarbons.

Presumably C12-C20 are 50:50 mixtures of diastereomers, as established for lower homologs C 4 -G . However. in each case only a single set of NMR signals is observed. Other spectroscopic properties. and redox behavior. are also presumed to be identical. We have not yet been able to obtain single crystals. perhaps in part due to the presence of two diastereomers.

A n p w .