A water-soluble polythiophene-Au nanoparticle composite for pH sensing

In this paper, we report the development of a reversible pH sensor in aqueous medium based on the fluorescence properties of a polythiophene-gold nanoparticle (Au NP) composite. The composite was synthesized in water by simultaneous reduction of HAuCl(4) to Au NPs and polymerization of thiophene in the presence of no additional reagents. It was stable for weeks and had characteristic emissions, which changed in the pH range of 3.0 to 6.0, thus providing a mean for probing the pH of an aqueous solution. Measurement of the pH could be performed over several cycles of titrations, pointing to the robustness of the materials for such sensing applications. The mass spectra of the composite at two extreme pH values were identical, indicating that the primary structure of the polymer was not affected due to changes in pH of the medium. Transmission electron microscopic (TEM) measurements indicated the presence of small sized Au NPs with the polymer in the milieu. The composite could be titrated by acid (or base) and considering the acid-base equilibria at different pHs, we have been able to calculate the pK(eq) of the composite, which was further used in calculating the pH of an aqueous solution from the emission spectrum of the composite. Our approach took advantage of redox chemistry in synthesizing the water-soluble composite and the optical behavior of a conjugated polymer in developing an important pH sensor, which may form the basis of further development of versatile pH or other sensors by suitably modifying the backbone of the monomer.     

The first metamagnetic thiocyanato-bridged one-dimensional nickel(II) complex

A novel single thiocyanato-bridged one-dimensional polymer, [NiL(NCS)(micro-NCS)]n (L = N1-(1-pyridin-2-yl-ethylidine)propane-1,3-diamine) has been synthesized and characterized by X-ray crystallography and a variable temperature magnetic study; the compound is a metamagnet with a Neél temperature (T(N)) of 3.5 K.     

Organization and dynamics of NBD-labeled lipids in membranes analyzed by fluorescence recovery after photobleaching

Lateral diffusion of membrane constituents plays an important role in membrane organization and represents a central theme in current models describing the structure and function of biological membranes. Fluorescence recovery after photobleaching (FRAP) is a widely used approach that provides information regarding dynamic properties and spatial distribution of membrane constituents. On the basis of the unique concentration-dependent fluorescence emission properties of a fluorescently labeled cholesterol analogue modified at the tail region, 25-[N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-methyl]amino]-27-norcholesterol (25-NBD-cholesterol), we have previously shown that it exhibits local organization even at very low concentrations in membranes. In this paper, we address aspects regarding the molecular size and dynamics of such an organized assembly of 25-NBD-cholesterol by monitoring its lateral diffusion characteristics using FRAP. To obtain a comprehensive understanding of the organization and dynamics of 25-NBD-cholesterol in the membrane, we compare its diffusion properties to that of a fluorescent phospholipid analogue 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-(1,3-benzoxadiazol-4-yl)) (NBD-PE). Our results indicate significant differences in the membrane dynamics of these NBD-labeled lipids. Importantly, on the basis of a novel wavelength-selective FRAP approach, our results show that the organization of 25-NBD-cholesterol is heterogeneous, with the presence of fast- and slow-diffusing species which could correspond to predominant populations of monomers and dimers of 25-NBD-cholesterol. The potential application of the wavelength-selective FRAP approach to monitor the organization and dynamics of molecules in membranes therefore represents an exciting possibility.     

Steric strain release-directed regioselective functionalization of meso-methyl Bodipy dyes

Starting from some meso-methyl Bodipy dyes, the corresponding meso-styryl derivatives were synthesized by regioselective Knoevenagel-type condensation with different aromatic aldehydes. The reactions were driven by the alleviation of the structural strain of the alkyl substituted Bodipys that could override the differential acidities of the methyl protons at the pyrrole ring of the Bodipy moiety.     

Simultaneous parallel and antiparallel self-assembly in a triazole/amide macrocycle conformationally homologous to D-,L-α-amino acid based cyclic peptides: NMR and molecular modeling study

A 1,4-linked triazole/amide based peptidomimetic macrocycle, synthesized from a triazole amide oligomer of cis-furanoid sugar triazole amino acids, possesses a conformation resembling the D-,L-α-amino acid based cyclic peptides despite having uniform backbone chirality. It undergoes a unique mode of self-assembly through an antiparallel backbone to backbone intermolecular H-bonding involving amide NH and triazole N2/N3 as well as parallel stacking via amide NH and carbonyl oxygen H-bonding, leading to the formation of a tubular nanostructure.     

Interaction of alcohols with 2-fluoro- and 4-fluorophenylacetylenes: infrared-optical double resonance spectroscopic and computational investigation

Alcohol complexes of 4-fluorophenylacetylene and 2-fluorophenylacetylene were investigated using IR-UV double resonance spectroscopy. Methanol forms a cyclic complex with both the fluorophenylacetylenes incorporating C-H···O and O-H···π hydrogen bonds, the structure of which is similar to that of the corresponding water complex but different from that of a phenylacetylene-methanol complex. The anti conformer of ethanol also binds in a similar fashion to both the fluorophenylacetylenes. Additionally, the gauche conformer of ethanol binds to 2-fluorophenylacetylene in a distinctly different structural motif that incorporates C-H···F and O-H···π hydrogen bonds. The OH group of trifluoroethanol interacts primarily with the π electron density of the C≡C bond. The π electron density of the C≡C bond is the principal point of interaction between the alcohols and both the fluorophenylacetylenes. The present results are indicative of the fact that fluorine substitution on the phenyl ring is sufficient to eliminate the subtle hydrogen bonding behavior of phenylacetylene.     

In situ reversible tuning of photoluminescence of Mn2+-doped ZnS quantum dots by redox chemistry

Herein we report the development of a new method for in situ reversible tuning of photoluminescence properties of quantum dots (Qdots) by partial oxidation of population of the emitting species and subsequent chemical reduction of the oxidized form. The concept has been demonstrated using Mn(2+)-doped ZnS Qdots stabilized by acetyl acetonate. Treatment of an aqueous solution of the Qdots (with Mn(OAc)(2) being the source of Mn used for the synthesis of the Qdots) by potassium peroxodisulfate (KPS) led to reduction of intensity of emission due to Mn(2+) ((4)T(1)-(6)A(1)). Subsequent treatment of the solution containing KPS-treated Qdots with NaBH(4) led to regaining of intensity, thus providing reversibility to the tuning, which was possible for more than one cycle. Electron spin resonance (ESR) spectroscopic investigations revealed reduction of the population of Mn(2+) upon treatment with KPS, whereas it went back up upon further treatment with NaBH(4). Interestingly, a mixed population of oxidation states of Mn was indicated to be present in the Qdots prepared using KMnO(4) as the source of Mn. The fluorescence intensity of the Qdots so prepared increased upon treatment with NaBH(4) following synthesis, which was not possible when the source of Mn was Mn(OAc)(2). Transmission electron microscopic and X-ray diffraction studies indicated that oxidation and reduction did not change the sizes of Qdots significantly.     

Development of a highly selective cell-permeable ratiometric fluorescent chemosensor for oxorhenium(V) ion

A novel 6-(2-pyridinyl)-5,6-dihydrobenzimidazo[1,2-c]quinazoline (HL) serves as a first-time highly selective and sensitive ratiometric fluorescent chemosensor probe for oxorhenium (ReO(V)) ion in acetonitrile : water = 9 : 1 (v/v) at 25 °C. The decrease in fluorescence at 410 nm and increase in fluorescence at 478 nm with an isoemissive point at 444 nm in the presence of ReO(V) ion is accounted for by the formation of mononuclear [ReOL(2)Cl] complex, characterized by physico-chemical and spectroscopic tools. The fluorescence quantum yield of the chemosensor (HL) was only 0.198 at 410 nm, and it increased more than 3-fold in the presence of 2 equiv. of the ReO(V) ion at 478 nm. Interestingly, the introduction of other metal ions and relevant anions caused the fluorescence intensity at 478 nm to be either unchanged or weakened. The fluorescence-response fits a Hill coefficient of 2.088 indicates the formation of a 1 : 2 stoichiometry for the L-ReO(V) complex. In the concentration range of 0-20 μM of oxorhenium(V) species calibration graph was linear with correlation coefficient (R) of 0.99994 and the calibration sensitivity was found to be 4.0 × 10(-7) M. The cellular image in the confocal microscope clearly indicated the presence of ReO(V) in Candida albicans cells using this chemosensor (HL).     

A highly selective fluorescent chemosensor for zinc ion and imaging application in living cells

A new 2,6-bis(5,6-dihydrobenzo[4,5]imidazo[1,2-c]quinazolin-6-yl)-4-methylphenol (1) serves as a highly selective and sensitive fluorescent probe for Zn(2+) in a HEPES buffer (50 mM, DMSO:water = 1:9 (v/v), pH = 7.2) at 25 °C. The increase in fluorescence in the presence of Zn(2+) is accounted for by the formation of dinuclear Zn(2+) complex [Zn(2)(C(35)H(25)N(6)O)(OH)(NO(3))(2)(H(2)O)] (2), characterized by X-ray crystallography. The fluorescence quantum yield of the chemosensor 1 is only 0.019, and it increases more than 12-fold (0.237) in the presence of 2 equiv of the zinc ion. Interestingly, the introduction of other metal ions causes the fluorescence intensity to be either unchanged or weakened. By incubation of cultured living cells (A375 and HT-29) with the chemosensor 1, intracellular Zn(2+) concentrations could be monitored through selective fluorescence chemosensing.     

Stereodivergent route to the carbocyclic core of 2',3'-olefinic carbanucleosides: toward the synthesis of (L)-(+)- and (D)-(-)-carbovir

(R)-2,3-Cyclohexylideneglyceraldehyde (1) has been elegantly exploited for a stereodivergent construction of the potential precursors (11a and 11b) of (L)-(+)- and (D)-(-)-carbovirs, respectively. The key steps in this approach were Luche's allylation of formaldehyde with allylic bromide 4c to produce 5 and ring-closing metathesis of 10b using Grubbs' first-generation catalyst to obtain 11. The moderate stereoselectivity of Luche's allylation reaction resulted in attaining stereodivergence in this approach which could be realized finally through easy chromatographic separation of the two isomers of the metathesis product to obtain homochiral precursors 11a and 11b in good amounts.