Ultrafast nonlinear processes in quantum-dot optical amplifiers

Ultrafast gain dynamics in quantum-dot optical amplifiers has been studied by using the pump-probe and four-wave mixing (FWM) techniques. It was found that there are at least three nonlinear processes, which are attributed to carrier relaxation to the ground states, phonon scattering, and carrier capture from the wetting layers into the quantum dots (QDs). The relevant time constants were evaluated to be ～90 fs, ～260 fs, and ～2 ps, respectively, under a 50 mA bias condition. The compressed gain recovered to 3% of its initial value in 4 ps, and no recovery component slower than 2 ps could be seen in the temporal range tested. This is quite different from the feature in quantum wells, where a very slow component (> 50 ps) exists. This suggests a possibility of enhancing the operation speed of semiconductor optical amplifiers by using QDs as an active layer. The third-order optical susceptibility (χ⁽³⁾) has been evaluated by means of both nonlinear transmission and FWM experiments. The results show that the nonlinearity expressed by χ⁽³⁾/g ₀ is quite similar to that of bulk and quantum wells, which can be explained by the similar relaxation times.     

RA‐XXV and RA‐XXVI,Bicyclic Hexapeptides from Rubia cordifolia L.: Structure,Synthesis, and Conformation

Two RA‐series bicyclic hexapeptides, RA‐XXV ( 4 ) and RA‐XXVI ( 5 ), which have no N‐methyl group at Tyr‐5, were isolated from the roots of Rubia cordifolia L. Their amino acid compositions and sequences were determined by interpretation of MS, and 1D and 2D NMR data and their relative structures were elucidated by XRD analysis of 4 and RA‐XXVI acetate ( 6 ). The absolute stereochemistry of 4 was established by the total synthesis of 4 , and that of 5 , by the chemical correlation with 4 . Peptides 4 and 5 exhibited cytotoxicity toward human promyelocytic leukemia HL‐60 (IC₅₀=0.062 and 0.066 μm , respectively) and human colonic carcinoma HCT‐116 (IC₅₀=0.028 and 0.051 μm , respectively) cell lines. Analysis of the conformational structures of 4 and 6 in the crystalline state and those of 4 and 5 in solution revealed that the N‐methyl group at Tyr‐5 functions to make this series of peptides preferentially adopt the active conformation.     

Studies toward the total synthesis of gambieric acids, potent antifungal polycyclic ethers: convergent synthesis of a fully elaborated GHIJ-ring fragment

A stereocontrolled synthesis of a fully elaborated GHIJ-ring fragment of gambieric acids, which are potent antifungal polycyclic ether natural products, has been accomplished. The synthesis features convergent assembly of the tetracyclic polyether skeleton through aldol coupling/cyclodehydration/reductive etherification processes and stereoselective construction of the J-ring side chain by a CeCl₃-promoted Julia-Kocienski olefination.     

Studies on rotational barriers of N-C axially chiral compounds: increase in the rotational barrier by aromatization

The rotational barriers in axially chiral quinolin-2-one and quinazolin-2-one possessing N-(ortho-tert-butyl)phenyl group were found to significantly increase in comparison with those of corresponding dihydroquinolin-2-one and dihydroquinazolin-2-one. Analysis of transition state structure during N-Ar bond rotation based on DFT calculation indicates that the increase in the rotational barrier is due to considerable distortion of the nitrogen-containing heterocyclic part.     

Theoretical study on thermochemistry of solvated lithium-cation with propylene carbonate

We report the theoretical analysis results of thermochemical properties of solvated Li⁺ ion in propylene carbonate (PC), which is one of the most popular solvents used in the lithium-ion battery composite. In the theoretical calculation, we employed the density functional theory method with the 6-31G basis set using the Gaussian03 package. It has been made clear that the solvation with four PC molecules around a Li⁺ ion is most favorable. Detailed results of the conventional quantum chemical analyses for these materials will also be presented. Thermochemical properties such as the standard (that is at 298.15 K and 101325 Pa) enthalpy, entropy, and Gibbs energy changes upon the formation of Li⁺ complexes solvated with PC molecules have been numerated and discussed. Furthermore, we will afford the features of desolvation of the solvated Li⁺ ion complexes when they interact with the carbon electrode modeled by ovalene molecules.     

Structure elucidation of a novel analog of sildenafil detected as an adulterant in a dietary supplement using LC-UV and LC/MS

A sildenafil-related compound was detected in a dietary supplement marketed as an aphrodisiac. The compound was detected during analysis of the dietary supplement using LC-UV and LC/electrospray ionization-MS. The structure of the compound was established using high resolution MS, NMR spectrometry, and X-ray crystal structure analysis. The compound was identified as 5-(5-((3,5-dimethylpiperazin-1-yl)sulfonyl)-2-ethoxyphenyl)-l-methyl-7-((1-methyl-4-nitro-1H-imidazol-5-yl)thio)-3-propyl-1H-pyrazolo[4,3-d] pyrimidine. Based on this structure, the compound was named nitroprodenafil. The dietary supplement was found to contain 90 mg nitroprodenafil/capsule. This article describes the structural characterization of a new sildenafil-related compound. The compound was detected during analysis of a dietary supplement using LC-UV and LC/electrospray ionization (ESI)-MS. The structure was established using high resolution MS (HRMS), NMR spectrometry, and X-ray crystal structure analysis. The structures of methisosildenafil, thiomethisosildenafil, and this new analog, named nitroprodenafil (21), are shown in Figure 1. In the Demizu et al. report, the compound is named mutaprodenafil instead ofnitroprodenafil. Considering the naming right, the authors of this paper think the use of mutaprodenafil is appropriate as the compound name, although nitroprodenafil is used.     

Hydrogen bonding of water in 3-methylpyridine studied by O 1s X-ray emission and absorption spectroscopy

O 1s X-ray emission and X-ray absorption spectroscopy is applied to probe hydrogen bonding of water (D(2)O) in 3-methylpyridine. Owing to element selectivity of X-ray spectroscopies the electronic structure of water in the binary mixture was observed selectively. Based on the observed spectral changes associated with hydrogen bonding in O 1s X-ray emission and X-ray absorption spectra, we have investigated the hydrogen bond of the mixture sample over a wide range of D(2)O concentrations (X(D(2)O) = 0.02-1.0) at room temperature under atmospheric pressure.     

Facile interstrand migration of the hydrocarbon moiety of a dibenzo[a,l]pyrene 11,12-diol 13,14-epoxide adduct at N2 of deoxyguanosine in a duplex oligonucleotide

When a synthesized deoxyribonucleotide duplex, 5'-CCATCGCTACC-3'.5'-GGTAGCGATGG-3', containing a trans 14R dibenzo[a,l]pyrene (DB[a,l]P) adduct, corresponding to trans opening of the (+)-(11S,12R)-diol (13R,14S)-epoxide by N (2) of the central G residue, was allowed to stand for 2-6 days at ambient temperature in neutral aqueous solution, three new products were observed on denaturing HPLC. One of these corresponded to loss of the DB[a,l]P moiety from the original adducted strand to give an 11-mer with an unmodified central dG. The other two products resulted from a highly unexpected migration of the hydrocarbon moiety to either dG5 or dG7 of the complementary strand, 5'-GGTAG5CG7ATGG-3'. Enzymatic hydrolysis of the two 11-mer migration products followed by CD spectroscopy of the isolated adducted nucleosides indicated that, in both cases, the hydrocarbon moiety had undergone configurational inversion at C14 to give the cis 14S DB[a,l]P dG adduct. MS/MS and partial enzymatic hydrolysis showed that the major 11-mer had the hydrocarbon at dG7. Two 11-mer oligonucleotides were synthesized with a single cis 14S DB[a,l]P dG adduct either at G7 or at G5 and were found to be chromatographically identical to the major and minor migration products, respectively. Although HPLC evidence suggested that a small extent of hydrocarbon migration from the trans 14S DB[a,l]P dG diastereomer also occurred, the very small amount of presumed migration products from this isomer precluded their detailed characterization. This interstrand migration appears unique to DB[a,l]P adducts and has not been observed for their fjord-region benzo[c]phenanthrene or bay-region benzo[a]pyrene analogues.