Partial poly(glutamic acid) <--> poly(aspartic acid) exchange in layer-by-layer polyelectrolyte films. Structural alterations in the three-component architectures

Layer-by-layer (LBL) polyelectrolyte films were constructed from poly(L-glutamic acid) (PGA) and poly(L-aspartic acid) (PAA) as polyanions, and from poly(L-lysine) (PLL) as the polycation. The terminating layer of the films was always PLL. According to attenuated total reflection Fourier transform infrared measurements, the PGA/PLL and PAA/PLL films, despite their chemical similarity, had largely different secondary structures. Extended beta-sheets dominated the PGA/PLL films, while alpha-helices and intramolecular beta-sheets dominated the PAA/PLL films. The secondary structure of the polyelectrolyte film affected the adsorption of human serum albumin (HSA) as well. HSA preserved its native secondary structure on the PGA/PLL film, but it became largely deformed on PAA/PLL films. Both PGA and PAA were able to extrude to a certain extent the other polyanion from the films, but the structural consequences were different. Adding PAA to a (PGA/PLL)5-PGA film resulted in a simple exchange and incorporation: PGA/PLL and PAA/PLL complexes coexisted with their unaltered secondary structures in the mixed film. The incorporation of PGA into a (PAA/PLL)5-PAA film was up to 50% and caused additional beta-structure increase in the secondary structure of the film. The proportions of the two polyanions were roughly the same on the surfaces and in the interiors of the films, indicating practically free diffusion for both polyanions. The abundance of PAA/PLL and PGA/PLL domains on the film surfaces was monitored by the analysis of the amide I region of the infrared spectrum of a reporter molecule, HSA, adsorbed onto the three-component polyelectrolyte films.     

A pulsed field gradient NMR diffusion investigation of enkephalin peptide-sodium dodecyl sulfate micelle association

Pulsed field gradient NMR (PFG-NMR) diffusion experiments were used to investigate the binding of leucine and methionine enkephalin peptides to anionic sodium dodecyl sulfate (SDS) micelles. The study was undertaken to investigate the mechanism of interaction between enkephalin peptides and SDS micelles and to determine if NMR-derived association constants, K(eq), can predict the elution order in electrokinetic chromatography (EKC). In EKC, peptides are separated on the basis of their interactions with micelles. The Leu-enkephalin peptide-micelle association constant increased from 130 +/- 8 to 1459 +/- 57 and 1744 +/- 64 M(-1), respectively, when an Arg or Lys was added to the C-terminus. The association constant of Leu-enkephalinamide was approximately equal to that of Leu-enkephalin-Arg. Substitution of Phe4 with a Trp or Gly2 with an Ala in the Leu-enkephalin peptides also increased the micelle binding affinity. These results confirm that the interaction of Leu-enkephalin peptides with SDS micelles is largely electrostatic and that the non-polar amino acid side chains interact with the hydrophobic micelle core. The peptide-micelle association constants for the cationic Met-enkephalin peptides were also greater than their zwitterionic counterparts. For example, the Met-enkephalin K(eq) value was 162 +/- 9 M(-1), while the association constants for Met-enkephalin-Arg, Met-enkephalin-Lys, and Met-enkephalinamide were, respectively, 674 +/- 31, 426 +/- 23, and 453 +/- 27 M(-1). In both Met-enkephalin and Met-enkephalinamide, replacing Gly2 with an Ala did not significantly increase the association constant. These results confirm that with the Met-enkephalin peptides, there was little or no interaction of the amino acid side chains with the micelle core. For both the Leu-enkephalin and Met-enkephalin peptides, the association constants were consistent with EKC results, in that the peptides with smaller K(eq) values were found to elute before those with larger association constants.     

Disruption of protein-protein interactions using nanoparticles: inhibition of cytochrome c peroxidase

Functionalized gold nanoparticles bind selectively to cytochrome c or cytochrome c peroxidase and inhibit enzyme turnover.     

Synthesis and biological evaluation of aromatic analogues of conduritol F, L-chiro-inositol, and dihydroconduritol F structurally related to the amaryllidaceae anticancer constituents

Pancratistatin is a potent anticancer natural product, whose clinical evaluation is hampered by the limited natural abundance and the stereochemically complex structure undermining practical chemical preparation. Fifteen aromatic analogues of conduritol F, l-chiro-inositol, and dihydroconduritol F that possess four of the six pancratistatin stereocenters have been synthesized and evaluated for anticancer activity. These compounds serve as truncated pancratistatin analogues lacking the lactam ring B, but retaining the crucial C10a-C10b bond with the correct stereochemistry. The lack of activity of these compounds provides further insight into pancratistatin's minimum structural requirements for cytotoxicity, particularly the criticality of the intact phenanthridone skeleton. Significantly, these series provide rare examples of simple aromatic conduritol and inositol analogues and, therefore, this study expands the chemistry and biology of these important classes of compounds.     

Non-linear effects in the quenching of fluorescent quantum dots by nitroxyl free radicals

Quenching of quantum dot luminescence by nitroxides is extremely non-linear and dependent on nanoparticle size, being most effective for the smaller particles; it is proposed to involve an electron exchange mechanism.     

Polyhydroxylated sapphyrins: multisite non-metallic catalysts for activated phosphodiester hydrolysis

Enhanced hydrolysis rates for the cleavage of bis(4-nitrophenyl)phosphate (BNPP), a model phosphodiester, may be achieved by using appropriately designed ditopic receptors containing the known phosphate-binding nucleus, sapphyrin, attached covalently to suitably oriented polyhydroxyl subunits. Evidence for the interaction between sapphyrin and BNPP comes from solid-state X-ray diffraction analysis of a diprotonated dihydroxylated sapphyrin-BNPP complex and from solution-phase (31)P NMR spectroscopic binding studies. The sapphyrins described in this paper may have a role to play as oligonucleotide cleavage agents.     

Creating polymer structures of tunable electric functionality by nanoscale discharge-assisted cross-linking and oxygenation

We report the creation of polymeric micro/nanostructures which exhibit distinct chemical and physical characteristics from the matrix poly(N-vinyl carbazole) (PVK). The structure formation is based on atomic force microscopy (AFM) facilitated cross-linking and oxygenation. The reaction of PVK with AFM lithographically induced nanoscale discharge produces raised structures in which bridge oxygen links neighboring carbazole groups. The cross-linking by bridge oxygen converts the initially insulating PVK matrix to chemically modified conducting patterns through the formation of extended pi-conjugations. A comprehensive AFM, PES (photoelectron spectroscopy), FTIR (Fourier transform infrared spectroscopy), and DFT (density functional theory) analysis is presented to address the chemophysical identity of the patterned structures. Our results demonstrate new capabilities of AFM nanolithography in generating heterogeneous functional structures in a polymer matrix.     

Efficient catalysis of ammonia borane dehydrogenation

In the presence of an iridium pincer complex, dehydrogenation of ammonia borane (H3NBH3) occurs rapidly at room temperature in tetrahydrofuran to generate 1.0 equivalent of H2 and [NH2BH2]5. A metal borohydride complex has been isolated as a dormant form of the catalyst which can be reactivated by reaction with H2.     

Diindolylquinoxalines: effective indole-based receptors for phosphate anion

The synthesis of a new series of 2,3-diindol-3'yl quinoxalines (DIQ), as well as a comparison of their anion recognition properties to those of our previously reported pyrrole based sensors, 2,3-dipyrrol-2'yl quinoxalines, is reported. To the best of our knowledge, this new DIQ system represents the first example of a free-standing indole-based small molecule receptor for which evidence of anion binding is available both in solution and in the solid-state. It also provides one of the few structurally characterized neutral receptor-dihydrogen phosphate complexes. This work thus serves to demonstrate the utility of indoles as an anion recognition motif.