A multifaceted study on the interaction of the cationic surfactant CTAB with calf thymus DNA was carried out by using different techniques. The measurements were done at different molar ratios X = [CTAB]/[DNA]. Results show the conformational change that DNA suffers due to the interaction with surfactant molecules at low molar ratios: the condensation of the polynucleotide, from an extended coil state to a globular state. The effect observed at the higher molar ratios is worth noting: the decondensation of DNA, that is, the transition from a compact state to a more extended conformation. Experimental data obtained confirm that this latter state is not exactly the same as that found in the absence of the surfactant. Attractive interactions between different parts of the molecule by ion correlation effects are the driving force to produce both the compaction and decompaction events. Results also show the importance of choosing both a proper system for the study and the most seeming measuring technique to use. The study demonstrates that, in some cases, the use of several techniques is desirable in obtaining reliable and accurate results.
The photoassisted degradation of n-butyltin chlorides
in air-equilibrated aqueous TiO2 suspension
has
been studied. Several factors affecting the degradation rate, such
as the extent of adsorption on the surface
of catalyst particles and the effect of pH values, were examined.
Particular interest is focused on the
TiO2-assisted photodegradation of n-butyltin
species. The simultaneous photodeposition of tin
oxides/metallic tin on the TiO2 surfaces is also addressed as a
route to form surface-complexed semiconductors.
A mechanism consisting of interfacial trapping of a photogenerated
electron−hole pair can explain the
observed photoassisted degradation of n-butyltin species and
the simultaneous photodeposition of tin onto
the TiO2 surface. The results are compared with those,
previously reported, on the photolytic degradation
of these species in water.
An Electrochemical Scanning Tunneling Microscopy (EC-STM) study of the adsorption of adenine on Au(111) electrodes was undertaken in the pH range between 1 and 7, aiming at achieving a deeper knowledge on the structure and organization of adenine chemisorbed on gold, and at confirming previous conclusions obtained from combined electrochemical and insitu IR spectroscopy measurements. This study confirms that chemisorption of adenine induces the lifting of the Au(111) surface reconstruction. Furthermore, the 4% excess gold atoms of the reconstructed surface which are expelled during lifting of the reconstruction do not diffuse away from the reconstruction rows. We observe, in contrast, the formation of nanometric islands arranged forming chains along the directions previously followed by the reconstruction solitons. Chemisorbed adenine adlayers consist of short chains of adenine molecules roughly aligned along the three main crystallographic directions of the substrate and stabilized by stacking. These chains tend to align parallel to each other, forming very small domains and yielding an adlayer with a very short-range order. The same adlayer structure is observed at all the studied pH values. The STM results also confirm that, in very acidic media, and at low potentials, adenine adsorbs very weakly on the reconstructed Au(111) surface.
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