Chiral Tetraarylmethane Derivative with Metal-Coordinating Ability

Phenyl-pyrazinyl-2-pyridyl-2-pyrimidinylmethane (1) has been synthesized in four steps from 2-(chloromethyl)pyridine. This compound is chiral and expected to show metal-coordinating ability because three of four aryl groups have the nitrogen atom at ortho-position with respect to the central carbon atom. The X-ray crystallographic analysis of rac-1 unambiguously revealed the tetraarylmethane framework, but the nitrogen atoms could not be assigned. The optical resolution of 1 was achieved by chiral HPLC. Besides the CD spectra of the two fractions exhibited opposite signs as expected, the solvent effect on the CD spectra was also observed. According to the calculated CD curve based on time-dependent density functional theory (TDDFT) method, the first eluted fraction is the R isomer in terms of absolute configuration. It should be noted that the CD spectrum of 1 was also changed by the addition of the transition metal ions because of the formation of the metal complexes of 1. The Job plot and the electrospray ionization (ESI) mass spectrum of the solution of rac-1 in the presence of Cu2+ ion revealed that the stoichiometry of 1 and Cu2+ was 2 : 1

DNA as supramolecular scaffold for porphyrin arrays on the nanometer scale

Tetraphenyl porphyrin substituted deoxyuridine was used as a building block to create discrete multiporphyrin arrays via site specific incorporation into DNA. The successful covalent attachment of up to 11 tetraphenyl porphyrins in a row onto DNA shows that there is virtually no limitation in the amount of substituents, and the porphyrin arrays thus obtained reach the nanometer scale (~10 nm). The porphyrin substituents are located in the major groove of the dsDNA and destabilize the duplex by Tm 5-7 °C per porphyrin modification. Force-field structure minimization shows that the porphyrins are either in-line with the groove in isolated modifications or aligned parallel to the nucleobases in adjacent modifications. The CD signals of the porphyrins are dominated by a negative peak arising from the intrinsic properties of the building block. In the single strands, the porphyrins induce stabilization of a secondary helical structure which is confined to the porphyrin modified part. This arrangement can be reproduced by force-field minimization and reveals an elongated helical arrangement compared to the double helix of the porphyrin-DNA. This secondary structure is disrupted above ~55 °C (Tp) which is shown by various melting experiments. Both absorption and emission spectroscopy disclose electronic interactions between the porphyrin units upon stacking along the outer rim of the DNA leading to a broadening of the absorbance and a quenching of the emission. The single-stranded and double-stranded form show different spectroscopic properties due to the different arrangement of the porphyrins. Above Tp the electronic properties (absorption and emission) of the porphyrins change compared to room temperature measurements due to the disruption of the porphyrin stacking at high temperature. The covalent attachment of porphyrins to DNA is therefore a suitable way of creating helical stacks of porphyrins on the nanometer scale