NMR spectroscopy and computer modeling were used to characterize tiopronin monolayer-protected gold
clusters (MPCs). These MPCs contain gold cores with a distribution of radii ranging from 0.4 to 2.6 nm.
NOESY and HMQC spectra yielded assignments for all NMR sensitive nuclei in the tiopronin ligands. DOSY
and T
2 experiments provided information about the particle size distribution as a function of proton frequency
shift. Further information was obtained from hole-burning and amide-exchange experiments. The spectroscopic
data reveal two classes of ligands, a network of hydrogen bonds, and considerable inhomogeneous and
homogeneous line broadening. The methyl and methine protons clearly exhibit two components with separations
that decrease strongly with the number of bonds separating the proton from the gold core. Spin−echo
experiments clearly show that a range of T
2 values is associated with each resonance frequency in both the
upfield and downfield components for each type of proton but that the most probable value is larger for the
upfield component. Various models that may be consistent with the NMR data and the properties of reported
crystal structures were considered. It is suggested that bimodal frequency distributions result from chemical
shifts that are associated with a mixture of primarily two gold cluster structure types that differ in the mode
of core packing. It is suggested that the Knight shift contributes to the large downfield shift observed for the
methine protons in the larger particles.
The structure of thrombin-binding DNA aptamer complexed with a single Sr2+ ion (Sr2+:TBA complex) has been determined using NMR spectroscopy and restrained molecular dynamics simulations. The quadruplex structure for the Sr2+:TBA complex is similar in topology, but distinct in structure, from that previously reported for the K+:TBA complex. The inter-tetrad distance of the Sr2+:TBA complex is 3.8 angstroms, or 0.7 angstroms larger than in the K+:TBA complex. This substantial difference can be attributed to a different binding site for Sr2+ in the Sr2+:TBA complex than for K+ in the K+:TBA complex. The Sr2+:TBA complex assumes a 1:1 stoichiometry, and it is very likely that the Sr2+ ion simultaneously interacts with the eight O6 atoms of the two G-tetrads. The results indicate that quadruplex DNA structures are highly sensitive to the presence of specific metal ions. The binding of specific metal ions may modulate the biological activity of quadruplex DNA structures in vivo.
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