Self-assembled pi-nanotapes as donor scaffolds for selective and thermally gated fluorescence resonance energy transfer (FRET)

Self-assembled nanotapes of a few tailor-made oligo(p-phenylenevinylene)s (OPVs) have been prepared and used as supramolecular donor scaffold to study the fluorescence resonance energy transfer (FRET) to a suitable acceptor. In nonpolar solvents, FRET occurs with nearly 63-81% efficiency, exclusively from the self-assembled OPVs to entrapped Rhodamine B, resulting in the quenching of the donor emission with concomitant formation of the acceptor emission at 625 nm. The efficiency of FRET is considerably influenced by the ability of the OPVs to form the self-assembled aggregates and hence could be controlled by structural variation of the molecules, and polarity of the solvent. Most importantly, FRET could be controlled by temperature as a result of the thermally reversible self-assembly process. The FRET efficiency was significantly enhanced (ca. 90%) in a xerogel film of the OPV1 which is dispersed with relatively less amount of the acceptor (33 mol %), when compared to that of the aggregates in dodecane gel. FRET is not efficient in polar solvents due to weak self-organization of the chromophores. These results indicate that energy transfer occurs exclusively from the self-assembled donor and not directly from the individual donor molecules. The present study illustrates that the self-assembly of chromophores facilitates temperature and solvent controlled FRET within pi-conjugated nanostructures.     

The chemical nature of the 2'-substituent in the pentose-sugar dictates the pseudoaromatic character of the nucleobase (pKa) in DNA/RNA

We here show that the pKa (error limit: 0.01 to 0.03 pKa unit) of a nucleobase in a nucleotide can be modulated by the chemical nature of the 2'-substituent at the sugar moiety. This has been evidenced by the measurement of nucleobase pKa in 47 different model nucleoside 3',5'-bis- and 3'-mono-ethylphosphates. The fact that the electronic character of each of the 2'-substituents (Fig. 1) alters the chemical shift of the H2' sugar proton, and also alters the pKa of the nucleobase in the nucleotides has been evidenced by a correlation plot of pKa of N3 of pyrimidine (T/C/U) or pKa of N7 of 9-guaninyl with the corresponding deltaH2' chemical shifts at the neutral pH, which shows linear correlation with high Pearson's correlation coefficients (R = 0.85-0.97). That this modulation of the pKa of the nucleobase by a 2'-substituent is a through-bond as well as through-space effect has been proven by ab initio determined pKa estimation. Interestingly, experimental pKas of nucleobases from NMR titration and the calculated pKas (by ab initio calculations utilizing closed shell HF 6-31G** basis set) are linearly correlated with R = 0.98. It has also been observed that the difference of ground and protonated/de-protonated HOMO orbital energies (DeltaHOMO, a.u.) for the nucleobases (A/G/C/T/U) are well correlated with their pK(a)s in different 2'-substituted 3',5'-bis-ethylphosphate analogs suggesting that only the orbital energy of HOMO can be successfully used to predict the modulation of the chemical reactivity of the nucleobase by the 2'-substituent. It has also been demonstrated that pKa values of nucleobases in 3',5'-bis-ethylphosphates (Table 1) are well correlated with the change in dipole moment for the respective nucleobases after protonation or de-protonation. This work thus unambiguously shows that alteration of the thermodynamic stability (Tm) of the donor-acceptor complexes [ref. 20], as found with various 2'-modified duplexes in the antisense, siRNA or in triplexes by many workers in the field, is a result of alteration of the pseudoaromatic character of the nucleobases engineered by alteration of the chemical nature of the 2'-substitution.     

On the homotopy invariance of torsion for covering spaces

We prove the homotopy invariance of torsion for covering spaces, whenever the covering transformation group is either residually finite or amenable. In the case when the covering transformation group is residually finite and when the cohomology of the covering space vanishes, the homotopy invariance was established by Lück. We also give some applications of our results.

     

Epoxide-initiated electrophilic cyclization of azides: a novel route for the stereoselective construction of azabicyclic ring systems and total synthesis of (+/-)-indolizidine 167B and 209D

[reaction: see text] A novel and general method for the stereoselective construction of 5-hydroxymethyl azabicyclic ring skeletons based on epoxide initiated electrophilic cyclization of azides has been developed and applied in the synthesis of (+/-)-indolizidine 167B and 209D with an overall yield of 16.5% and 17.8%, respectively. The efficiency of this methodology is further exemplified in the synthesis of azepine skeleton via tandem cation-olefin-azide cyclization.     

X-ray structure analysis of 4-cyano-4′-<Emphasis Type="Italic">n</Emphasis>-undecyloxybiphenyl

The structure of a liquid crystal of 4-cyano-4′-n-undecyloxybiphenyl (C₂₄H₃₁NO) is determined by X-ray diffraction. The compound crystallizes in the monoclinic crystal system (space group C2/c) with the unit cell parameters a = 84.108(7) Å, b = 7.159(2) Å, c = 6.922(2) Å, and β = 91.6(4)°. The structure has been solved by direct methods and refined to the residual index R₁ = 0.067. Both phenyl rings are almost planar, and the dihedral angle between these rings is 31.5(6)°.     

Preparation, characterization, and X-ray structure analysis of diethylenetriammonium tetrathiomolybdate

The interaction of H₂S gas with aqueous ammonium heptamolybdate solution in the presence of diethylenetriamine (dien) led to the formation of diethylenetriammonium tetrathiomolybdate crystals. The elemental analyses, UV-Visible, IR, and ¹H NMR spectra suggested the presence of organic cation and (MoS₄)^2– moiety. The ¹H NMR revealed the presence of two sets of symmetrical A₂X₂ multiple patterns. The compound crystallizes in the monoclinic system with space group Pn, a = 7.196(6), b = 7.532(3), c = 11.525(10) Å, = 90°, = 90.17(3)°, = 90°, V = 624.78(8) ų, and Z = 2, wR ₁ = 0.0261 and wR ₂ = 0.0698. The structure consists of tetrahedral tetrathiomolybdate anions, which form an extended three-dimensional network in the solid state with Mo–SH–N hydrogen bonding with the organic cation.     

DTA studies on the thermal oxidation and crosslinking reactions of carboxyl-terminated polybutadiene

Studies on the thermal oxidation of carboxyl-terminated polybutadiene in the presence of antioxidants have been carried out by dynamic DTA. Bis-thioacetylacetonato nickel(II) compounds are found to be effective in inhibiting the air oxidation reaction in the polymer. The crosslinking reaction of the polymer through the double bonds present in the polymer molecule is desensitized by the antioxidants and the effect is more with N-phenyl-1-naphthylamine. An exothermic peak formed at 270°C in the presence of tris(2-methylaziridinyl-1) phosphine oxide has been identified as the curing reaction. The infrared spectra of CTPB in the presence of MAPO at various temperatures confirm the various stages of reaction.     

Tunable diode laser measurements of spectral parameters of HCN at room temperature

A tunable infrared diode laser was used to record 17 fully resolved vibration-rotation transitions in the v₁ fundamental band of HCN at 3μ. The experiments were conducted in an absorption cell on room temperature mixtures of HCN diluted by N₂ and Ar. The v₁ fundamental band strength of HCN was determined to be 267±8 cm^-2 atm^-1 at 273.2 K. Small but significant reductions in the residual errors were obtained by using the Galatry profile rather than the Voigt profile to fit the experimentally recorded line shapes. Collisional broadening and narrowing parameters were determined simultaneously from Galatry profile fits to the data. The collision-broadened linewidths of HCN lines in N₂ and Ar were determined as a function of rotational quantum number of transitions ranging from P(14) to R(14) (3268.22-3353.29 cm^-1). The optical diffusion coefficients of HCN in N₂ and Ar at 300 K were determined from the collisional narrowing parameters and were 0.074±0.01 and 0.016±0.03 cm²s^-1 respectively.