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Rotational and translational diffusion coefficients in aqueous solution of a series of three B duplex oligonucleotides 8, 12, and 20 base pairs in length are measured by depolarized and polarized dynamic light scattering, respectively. Theoretical relations for the diffusion coefficients of short rods by Tirado and Garcia de la Torre are used in combination with the experimental data to obtain the dimensions of the molecules. The theoretical relations are self-consistent through the series and give a hydrodynamic diameter for DNA of 20±1.5 Å. The results, furthermore, show that the combination of depolarized and polarized dynamic light scattering provides a powerful method for obtaining hydrodynamic dimensions of short rodlike particles and may therefore be used in applications which follow molecular sizes and size changes.

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ABSTRACT:The dynamics of three synthetic oligonucleotides d(CG)4, d(CG)6, and d(CGCGTTGTTCGCG) of different length and shape were studied in solution by depolarized dynamic light scattering (DDLS) and time-resolved nuclear Overhauser effect cross-relaxation measurements. For cylindrically symmetric molecules the DDLS spectrum is dominated by the rotation of the main symmetry axis of the cylinder. The experimental correlation times describe the rotation of the oligonucleotides under hydrodynamic stick boundary conditions. It is shown that the hydrodynamic theory of Tirado and Garcia de la Torre gives good predictions of the rotational diffusion coefficients of cylindrically symmetric molecules of the small axial ratios studied here. These relations are used to calculate the solution dimensions of the DNA fragments from measured correlation times. The hydrodynamic diameter of the octamer and dodecamer is 20.5 f 1.0 A, assuming a rise per base of 3.4 A. The tridecamer, d(CGCGTTGTTCGCG), adopts a hairpin structure with nearly spherical dimensions and a diameter of 23.0 f 2.0 A. The DDLS relaxation measurements provide a powerful method for distinguishing between different conformations of the oligonucleotides (e.g., DNA double-helix versus hairpin structure). Furthermore, the rotational correlation times are a very sensitive probe of the length of different fragments. The N M R results reflect the anisotropic motion of the molecules as well as the amount of local internal motion present. The experimental correlation time from N M R is determined by the rotation of both the short and long axes of the oligonucleotide. The contributions of the various correlation times are given by the relative orientation of the internuclear vector with respect to the main axis and the correlation function for internal motion, if it is assumed that the librational motions decay much faster than the overall reorientational motion. Our results are compared with those from N M R relaxation measurements on other short oligonucleotides with lengths of up to 20 base pairs. Various dynamic models for the reorientation of the internuclear vector are applied to their interpretation. The extent of local internal motion is apparently independent of the length and base sequence of the DNA fragments and gives a significant contribution to the effective correlation time measured by the cross-relaxation rate.

The present contribution presents the single-step preparation and characterization of poly(N-isopropyl acrylamide)-co-polystyrene core-shell microgels with varying polystyrene content. The swelling behavior of the particles is investigated using dynamic light scattering and differs significantly from the swelling behavior of poly(N-isopropyl acrylamide) homopolymer particles. The lower critical solution temperature is found to be shifted to lower temperatures upon increasing the polystyrene content of the particles. The core-shell structure of the particles is revealed by means of small angle neutron scattering (SANS) using the method of contrast variation. Additionally, the formation of mesoscopic crystals of these particles is investigated by means of scanning electron microscopy and also by SANS. The particles seem to have preferable properties with respect to crystallization compared to homopolymer microgels.

A novel picosecond laser approach is used to investigate the intrinsic fluorescence of several oligonucleotides. All biomolecules are excited at 283 nm with laser pulses of typically 80 ps duration and an energy of 250 µJ; concentrations were on the order of 10 -5 M. Detection of the resulting fluorescence behind a spectrometer with a streak camera permits the simultaneous acquisition of spectral and lifetime information in two-dimensional images. In a systematic study, the fluorescence spectra and the associated temporal decays are analyzed with respect to monomer and potential excimer components. For this, the nucleotides AMP, CMP, GMP, and TMP are studied as well as homo-oligonucleotides of the type d(X) n with variable sequence length of n ) 2-15, enabling a comparison of the emission characteristics of these single-stranded compounds under physiologic conditions in solution at room temperature. Also, the influence of conformational changes on the fluorescence response is investigated using mixtures of complementary oligonucleotides d(X) 15 ×d(Y) 15 with the combinations X ) A, Y ) T and X ) G, Y ) C. These structures, which form double helices, differ in flexibility and stacking geometry from the single-stranded compounds. From experiments with self-complementary variants with alternating base sequences of the type d(XY) 8 with XY ) AT and GC, information on exciplex formation tendencies is obtained for these compounds, which also form double helices in solution. Preliminary results of time-dependent fluorescence anisotropy measurements with this direct picosecond laser approach are discussed.

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