The competition between dimensional and domain effects for the packing of amphiphilic passivants on CdSe quantum dot (QD) surfaces is examined using temperature-dependent attenuated total reflection infrared spectroscopy. The results indicate that at very small particle sizes (r < 22 Å) disordered packing of the organic passivant arises due to the lack of an available thermodynamically stable packing domain for the passivant on the QD facets. At particle sizes r > 22 Å, a stable chain domain can be established where the chain packing thermodynamics is governed by purely dimensional effects related to the ratio of the chain length (〈L〉) to the particle radius (r). Using a mean field statistical mechanical calculation, we find the theoretical calculations predict the observed 〈L〉/r passivant layer stabilization for QD systems.
We have used DNase I footprinting to study the binding strength and DNA sequence selectivity of novel derivatives of the quinoxaline bis-intercalator TANDEM. Replacing the valine residues in the cyclic octadepsipeptide with lysines does not affect the selectivity for TpA but leads to a 50-fold increase in affinity. In contrast, replacing both of the quinoxaline chromophores with naphthalene rings abolishes binding, while changing a single ring decreases the affinity, and footprints are observed at only the best binding sites (especially TATATA). By using fragments with different lengths of [(AT) n ], we demonstrate that these ligands bind best to the center of the longer (AT) n tracts.
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