Three pairs of isomeric, iron−sulfur core dendrimers were prepared. Each isomer pair was distinguished by a 3,5-aromatic substitution pattern (extended) versus 2,6-aromatic substitution pattern ...(backfolded). Several observations were made that supported the hypothesis that the iron−sulfur cluster cores were encapsulated more effectively in the backfolded isomers as compared to their extended isomeric counterparts. The backfolded isomers were more difficult to reduce electrochemically, consistent with encapsulation in a more hydrophobic microenvironment. Furthermore, heterogeneous electron-transfer rates for the backfolded molecules were attenuated compared to the extended molecules. From diffusion measurements obtained by pulsed field gradient spin−echo NMR and chronoamperometry, the backfolded dendrimers were found to be smaller than the extended dendrimers. Comparison of longitudinal proton relaxation (T 1) values also indicated a smaller, more compact dendrimer conformation for the backfolded architectures. These findings indicated that the dendrimer size was not the major factor in determining electron-transfer rate attenuation. Instead, the effective electron-transfer distance, as determined by the relative core position and mobility in a dendrimer, is most relevant for encapsulation.
As a model for riboflavin, lumiflavin was investigated using density functional theory methods (B3LYP/6-31G* and B3LYP/6-31+G**) with regard to the proposed cascade of intermediates formed after ...excitation to the triplet state, followed by electron-transfer, proton-transfer, and radical−radical coupling reactions. The excited triplet state of the flavin is predicted to be 42 kcal/mol higher in energy than the singlet ground state, and the π radical anion lies 45.1 kcal/mol lower in energy than the ground-state flavin and a free electron in the gas phase. The former value compares to a solution-phase triplet energy of 49.8 kcal/mol of riboflavin. For the radical anion, the thermodynamically favored position to accept a proton on the flavin ring system is at N5. A natural population analysis also provided spin density information for the radicals and insight into the origin of the relative stabilities of the six different calculated hydroflavin radicals. The resulting 5H-LF· radical can then undergo radical−radical coupling reactions, with the most thermodynamically stable adduct being formed at C4 ‘. Vibrational spectra were also calculated for the transient species. Experimental time-resolved infrared spectroscopic data obtained using riboflavin tetraacetate are in excellent agreement with the calculated spectra for the triplet flavin, the radical anion, and the most stable hydroflavin radical.
A correlation is found between the fragment ion yield and the degree of branching for oligosaccharides analyzed by matrix assisted laser desorption ionization/Fourier transform mass spectrometry ...(MALDI/FTMS). The most branched oligosaccharides produced the least amount of fragment ion. Another relationship is found between the size of the alkali metal ion and the yields of fragment ions. The smallest alkali metal ion produced the greatest amount of fragment ions. The presence of the quasimolecular ion is dependent on both the size of the metal ion and the size of the sugar. A minimum number of saccharide residues are necessary to stabilize the complex and produce a quasimolecular ion. Molecular mechanics calculations were performed and structures obtained consistent with the observed experimental behavior. The site of coordination plays a dominant role in the yield and types of fragment ions observed.
Vases and kites as cavitands Moran, John R; Ericson, John L; Dalcanale, Enrico ...
Journal of the American Chemical Society,
07/1991, Letnik:
113, Številka:
15
Journal Article
A diphenylacetylene containing two different end groups (isonitrile and thioacetate) was synthesized, showing that the chemistry used to install each end group is compatible with that of the others. ...The isonitrile group binds preferentially to platinum, and the thiol group binds preferentially to gold. However, the selectivity was different when nanoparticles were compared to planar substrates.
