No abstract

The luminescence of DNA-bound [Ru(phen)(2)dppz](2+) is shown to be highly sensitive to environmental conditions such as ionic strength, temperature, and the sequence and secondary structure of the nucleic acid, although not to bulky DNA substituents in the major groove. Each enantiomer has two characteristic lifetimes with any polynucleotide and their relative amplitudes vary as a function of binding ratio. For [poly(dA-dT)](2) as a model sequence, the longer lifetime for Δ-[Ru(phen)(2)dppz](2+) has been assigned to canted intercalation of the complex and the shorter lifetime is ascribed to symmetric intercalation. At a fixed binding ratio, the longer lifetime amplitude increases with increasing ionic strength, without significant change in lifetimes. Increasing temperature has a similar effect, but also affects lifetimes. In general, emission is strongest with AT-rich polynucleotides and with higher-order secondary structures, with intensity increasing as single-stranded < duplex < triplex. However, sequence-context and secondary duplex structure also influence the photophysics since emission with [poly(dA)]·[poly(dT)] is significantly higher than with [poly(dA-dT)](2) or [poly(rA)]·[poly(rU)]. The strong influence of different environmental conditions on the emission of nucleic acid-bound [Ru(phen)(2)dppz](2+) reflects subtle heterogeneities that are inherent elements of DNA recognition by small molecules, amplified by large changes in photophysics caused by differential exposure of the dppz nitrogens to groove hydration.

The bi-exponential emission decay of [Ru(L)(2)dppz](2+) (L = N,N'-diimine ligand) bound to DNA has been studied as a function of polynucleotide sequence, enantiomer, and nature of L (phenanthroline vs. bipyridine). The lifetimes (τ(i)) and pre-exponential factors (α(i)) depend on all three parameters. With [poly(dA-dT)](2), the variation of α(i) with [Nu]/[Ru] has little dependence on L for Λ-[Ru(L)(2)dppz](2+) but a substantial dependence for Δ-[Ru(L)(2)dppz](2+). With [poly(dG-dC)](2), by contrast, the Λ-enantiomer α(i) values depend strongly on the nature of L, whereas those of the Δ-enantiomer are relatively unaffected. DNA-bound linked dimers show similar photophysical behaviour. The lifetimes are identified with two geometries of minor-groove intercalated [Ru(L)(2) dppz](2+), resulting in differential water access to the phenazine nitrogen atoms. Interplay of cooperative and anti-cooperative binding resulting from complex-complex and complex-DNA interactions is responsible for the observed variations of α(i) with binding ratio. [Ru(phen)(2)dppz](2+) emission is quenched by guanosine in DMF, which may further rationalise the shorter lifetimes observed with guanine-rich DNA.

An optically trapped birefringent microparticle is rotated by a circularly polarized beam in a confined gaseous medium. By recording the terminal rotation velocity and the change in polarization of the incident trapping beam, we determine the viscosity by probing a picoliter volume of air, carbon dioxide, and argon in the vicinity of the microparticle. We also characterize the optical force acting on a trapped particle in air using the generalized Lorenz-Mie theory taking into account the aberrations present. This opens up a new potential application of optical tweezers for the accurate measurement of gas viscosity in confined geometries.