The reversible blockage of synthetic pores formed by rigid-rod beta barrels, either by substrates or products, was used to sense a variety of enzymatic reactions in high-throughput format with "naked-eye" fluorescent detection. Improvement of sensor sensitivity beyond three orders of magnitude by straightforward internal mutations underscores the functional plasticity of rigid-rod beta barrels. Such detectors of enzyme activity with the aforementioned characteristics are needed in areas as diverse as proteomics and environmentally benign organic synthesis.
The effects of binding and conformational changes induced by the neutral amphiphilic ligand [5-(alkoxy)naphthalen-1-amine] with different alkyl chain lengths on bovine serum albumin (BSA) have been studied using UV-visible and fluorescence spectroscopic methods. The BSA fluorescence exhibits appreciable bathochromic shift along with a reduction in fluorescence intensity and fluorescence lifetime upon binding with ligands. Ligand quenches the fluorescence of BSA in a concentration-dependent manner and deviates positively from the linear Stern-Volmer equation. The calculated quenching rate constants and binding constants were shown to depend entirely on the alkyl chain length of the ligands. After binding of probes with protein, the distance between the donor and acceptor was calculated using Forster theory. Ligands bind near Trp-134 in the subdomain IA of the native BSA and they become accessible to Trp-212 when BSA gets unfolded. The spectral data well supports the idea that BSA changes its three-dimensional conformation incrementally during its unfolding.
We report the characterization of multifunctional rigid-rod β-barrel ion channels with either internal aspartates or arginine-histidine dyads by planar bilayer conductance experiments. Barrels with internal aspartates form cation selective, large, unstable and ohmic barrel-stave (rather than toroidal) pores; addition of magnesium cations nearly deletes cation selectivity and increases single-channel stability. Barrels with internal arginine-histidine dyads form cation selective (P K ϩ/P Cl Ϫ = 2.1), small and ohmic ion channels with superb stability (single-channel lifetime > 20 seconds). Addition of "protons" results in inversion of anion/cation selectivity (P Cl Ϫ/P K ϩ = 3.8); addition of an anionic guest (HPTS) results in the blockage of anion selective but not cation selective channels. These results suggest that specific, internal counterion immobilization, here magnesium (but not sodium or potassium) cations by internal aspartates and inorganic phosphates by internal arginines (but not histidines), provides access to synthetic multifunctional pores with attractive properties.
We have synthesized a new indole functionalized rhodamine derivative L(1) which specifically binds to Cu(2+) in the presence of large excess of other competing ions with visually observable changes in their electronic and fluorescence spectral behavior. These spectral changes are significant enough in the NIR and visible region of the spectrum and thus enable naked eye detection. The receptor, L(1), could be employed as a resonance energy transfer (RET) based sensor for detection of Cu(2+) based on the process involving the donor indole and the acceptor Cu(2+) bound xanthene fragment. Studies reveal that L(1)-Cu complex is selectively and fully reversible in presence of sulfide anions. Further, fluorescence microscopic studies confirmed that the reagent L(1) could also be used as an imaging probe for detection of uptake of these ions in HeLa cells.
Dedicated to Prof. Wolf-Dietrich Woggon on the occasion of his 60th birthday Intermediate internal charge repulsion (ICR) is required to create synthetic pores with large, stable, transmembrane, and variably functionalized space. This conclusion is drawn from maximal transport and, in one case, catalytic activity of p-octiphenyl b-barrel pores with internal lysine, aspartate, and histidine residues around pH 7, 6, and 4.5, respectively. pK a Simulations corroborate the experimental correlation of intermediate ICR with activity and suggest that insufficient ICR causes pore −implosion× and excess ICR pore −explosion×. Esterolysis experiments support the view that the formation of stable space within multifunctional p-octiphenyl b-barrels requires more ICR in bilayer membranes than in H 2 O. Multivalency effects are thought to account for p-octiphenyl b-barrel expansion with increasing number of b-sheets, and proximity effects for unchanged pH profiles with increasing b-sheet length. Q-TOF-nano-ESI-MS barrel-denaturation experiments indicate that contributions from internal counterion effects are not negligible. The overall characteristics of p-octiphenyl bbarrel pores with internal lysine, aspartate, and histidine residues, unlike de novo −a-barrels× and similarly to certain biological channels, underscore the usefulness of rigid-rod molecules to preorganize complex multifunctional supramolecular architecture. Fig. 2. Axial view and peptide sequences of the p-octiphenyl b-barrels 1 ± 6 from Fig. 1 (8-stranded) and Fig. 3 (6stranded). One-letter abbreviations are used for amino acids (L Leu, K Lys, H His, D Asp; G ÀOCH 2 COÀ ). n number of acid/bases per supramolecule. In molecular models , internal distances are a % 5 ä, b % 4 ä (1: ca. 12 ä), c % 7 ä (1: ca. 23 ä), c' % 14 ä, and d % 7 ä.
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