James G. Pavlovich

Isolation of the heterofullerene C59N as its dimer (C59N)(2)

The heterofullerene ion C59N+ is formed efficiently in the gas phase during fast atom bombardment mass spectroscopy of a cluster-opened N-MEM (N-methoxyethoxy methyl) ketolactam. This transformation is shown to occur also in solution in the presence of strong acid, affording biazafullerenyl (C59N)2 in good yield. It is proposed that the azafullerene dimer is formed upon in situ reduction of the highly reactive azafulleronium ion. The isolation and characterization of biazafullerenyl opens a viable route for the preparation of other heterofullerenes in solution.

Magic Numbers in DNA-Stabilized Fluorescent Silver Clusters Lead to Magic Colors

DNA-stabilized silver clusters are remarkable for the selection of fluorescence color by the sequence of the stabilizing DNA oligomer. Yet despite a growing number of applications that exploit this property, no large-scale studies have probed origins of cluster color or whether certain colors occur more frequently than others. Here we employ a set of 684 randomly chosen 10-base oligomers to address these questions. Rather than a flat distribution, we find that specific color bands dominate. Cluster size data indicate that these “magic colors” originate from the existence of magic numbers for DNA-stabilized silver clusters, which differ from those of spheroidal gold clusters stabilized by small-molecule ligands. Elongated cluster structures, enforced by multiple base ligands along the DNA, can account for both magic number sizes and color variation around peak wavelength populations.

Silver (I) as DNA glue: Ag + -mediated guanine pairing revealed by removing Watson-Crick constraints

Metal ion interactions with DNA have far-reaching implications in biochemistry and DNA nanotechnology. Ag+ is uniquely interesting because it binds exclusively to the bases rather than the backbone of DNA, without the toxicity of Hg2+. In contrast to prior studies of Ag+ incorporation into double-stranded DNA, we remove the constraints of Watson-Crick pairing by focusing on homo-base DNA oligomers of the canonical bases. High resolution electro-spray ionization mass spectrometry reveals an unanticipated Ag+-mediated pairing of guanine homo-base strands, with higher stability than canonical guanine-cytosine pairing. By exploring unrestricted binding geometries, quantum chemical calculations find that Ag+ bridges between non-canonical sites on guanine bases. Circular dichroism spectroscopy shows that the Ag+-mediated structuring of guanine homobase strands persists to at least 90 °C under conditions for which canonical guanine-cytosine duplexes melt below 20 °C. These findings are promising for DNA nanotechnology and metal-ion based biomedical science.

Biosynthesis of arachidonic acid metabolites in Limulus polyphemus amebocytes: analysis by liquid chromatography-electrospray ionization mass spectrometry

Abstract Eicosanoid metabolites were generated by isolated granular amebocytes of the primitive arthropod, Limulus polyphemus, when stimulated by the calcium ionophore A23187 and/or exogenous arachidonic acid. the metabolites were isolated, identified, and the major metabolite was quantified using reverse-phase high pressure liquid chromatography (RP-HPLC) coupled to electrospray ionization mass spectrometry (ESI-MS). Qualitative examination revealed putative metabolites and the major product, 8-hydroxyeicosatetraenoic acid (8-HETE), which was quantified using standard curves generated from extracted ion profiles of the molecular ion. Electrospray ionization of the HETEs in negative ion mode produces a base peak for all isomers which corresponded to the molecular ion [(MH)−: m/z 319]. The molecular ion was accompanied by the neutral loss of water and carbon dioxide [(MH −H2O)−: m/z 301; (MH −H2O −CO2)−: m/z 257], as well as daughter ions which were dependent upon the position of hydroxy substitution. Standard curves were generated in full scan mode for standards ranging from 6.25 to 100 ng, whereas selected ion recording was used for the lower levels of 0.8 to 6.25 ng.

Polyimide Dendrimers Containing Multiple Electron Donor–Acceptor Units and Their Unique Photophysical Properties†

A high-yielding synthesis of a series of polyimide dendrimers, including decacyclene- and perylene-containing dendrimer D6, in which two types of polyimide dyes are present, is reported. In these constructs, the branching unit is represented by trisphenylamine, and the solubilizing chains by N-9-heptadecanyl-substituted perylene diimides. The photophysical properties of the dendrimers have been studied by absorption, steady-state, and time-resolved emission spectroscopy and pump-probe transient absorption spectroscopy. Photoinduced charge-separated (CS) states are formed on the femtosecond timescale upon visible excitation. In particular, in D6, two different CS states can be formed, involving different subunits that decays independently with different lifetimes (ca. 10-100 ps).