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Fluorescence Emission Maximum (fluorescence + emission_maximum)
Selected AbstractsSynthesis, photoluminescence, and electrochromic properties of wholly aromatic polyamides bearing naphthylamine chromophoresJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 20 2006Guey-Sheng Liou Abstract A series of novel polyamides with pendent naphthylamine units having inherent viscosities of 0.15,1.02 dL/g were prepared via direct phosphorylation polycondensation from various diamines and a naphthylamine-based aromatic dicarboxylic acid, 1-[N,N -di(4-carboxyphenyl)amino]naphthalene. These amorphous polyamides were readily soluble in various organic solvents and could be cast into transparent and tough films. The aromatic polyamides had useful levels of thermal stability associated with high glass-transition temperatures (268,355 °C), 10% weight loss temperatures in excess of 480 °C, and char yields at 800 °C in nitrogen higher than 60%. These polymers showed maximum ultraviolet,visible absorption at 350,358 nm and exhibited fluorescence emission maxima around 435,458 nm in N -methyl-2-pyrrolidinone solutions with fluorescence quantum yields ranging from 0.4 to 15.0%. The hole-transporting and electrochromic properties were examined with electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the polyamide films cast onto an indium tin oxide coated glass substrate exhibited one oxidative redox couple around 1.08,1.16 V (oxidation onset potential) versus Ag/AgCl in an acetonitrile solution and revealed good stability of the electrochromic characteristics, with a color change from colorless to green at applied potentials ranging from 0 to 1.6 V. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6094,6102, 2006 [source] The relationship between thermal stability and pH optimum studied with wild-type and mutant Trichoderma reesei cellobiohydrolase Cel7AFEBS JOURNAL, Issue 5 2003Harry Boer The major cellulase secreted by the filamentous fungus Trichoderma reesei is cellobiohydrolase Cel7A. Its three-dimensional structure has been solved and various mutant enzymes produced. In order to study the potential use of T. reesei Cel7A in the alkaline pH range, the thermal stability of Cel7A was studied as a function of pH with the wild-type and two mutant enzymes using different spectroscopic methods. Tryptophan fluorescence and CD measurements of the wild-type enzyme show an optimal thermostability between pH 3.5,5.6 (Tm, 62 ± 2 °C), at which the highest enzymatic activity is also observed, and a gradual decrease in the stability at more alkaline pH values. A soluble substrate, cellotetraose, was shown to stabilize the protein fold both at optimal and alkaline pH. In addition, unfolding of the Cel7A enzyme and the release of the substrate seem to coincide at both acidic and alkaline pH, demonstrated by a change in the fluorescence emission maximum. CD measurements were used to show that the five point mutations (E223S/A224H/L225V/T226A/D262G) that together result in a more alkaline pH optimum [Becker, D., Braet, C., Brumer, H., III, Claeyssens, M., Divne, C., Fagerström, R.B., Harris, M., Jones, T.A., Kleywegt, G.J., Koivula, A., et al. (2001) Biochem. J.356, 19,30], destabilize the protein fold both at acidic and alkaline pH when compared with the wild-type enzyme. In addition, an interesting time-dependent fluorescence change, which was not observed by CD, was detected for the pH mutant. Our data show that in order to engineer more alkaline pH cellulases, a combination of mutations should be found, which both shift the pH optimum and at the same time improve the thermal stability at alkaline pH range. [source] Unexpected fluorescence emission of poly(,,,- L -malic acid) in aqueous mediumJOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2007Yaofeng Fan Abstract Unexpected fluorescence of poly(,,,- L -malic acid) (,,,-PMA) without traditional fluorophore was observed firstly. This fluorescent polymer was synthesized via melt polycondensation of L -malic acid. The polymer was characterized by gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), thermogravimetry (TG), Fourier transform infrared spectroscopy (IR), Fourier transform Raman spectroscopy (Raman), and X-ray powder diffractometry (XRD). The high molecular weight ,,,-PMA was synthesized by the optimum polycondensation at 130°C for 15 h, followed by fractional precipitation with diethyl ether and petroleum ether. The degree of branching of ,,,-PMA was from 10% to 20% according to the reaction condition. Terminal group of ,,,-PMA was mainly hydroxycarboxylic group companied with a few CHCHCOOH groups owing to dehydration of a normal terminal during the melt polycondensation. A fluorescence emission maximum of ,,,-PMA in water appeared at 420 nm when it was excited at 340 nm. Further study indicated that the fluorescence intensity was concentration-dependent, pH-dependent, and molecular-weight-dependent. The fluorescence formation may result from multichain aggregations, which was formed readily in aqueous solution due to intermolecular hydrogen bonds between branched ,,,-PMA. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007 [source] Environmental Effects on the Photochemistry of A2-E, a Component of Human Retinal Lipofuscin,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 3 2001Laura Ragauskaite ABSTRACT Several retinal dystrophies are associated with the accumulation of lipofuscin, a pigment mixture, in the retinal pigment epithelium (RPE). One of the major fluorophores of this mixture has been identified as the bis-retinoid pyridinium compound, A2-E. Because this compound absorbs incident radiation that is transmitted by the anterior segment of the human eye, photophysical and photochemical studies were performed to determine if A2-E could photosensitize potentially damaging reactions. Steady-state fluorescence measurements indicate that the fluorescence emission maximum and quantum yield are very sensitive to the chemical environment and a correlation between these two parameters and the solvent dielectric constant is observed. Time-resolved absorption experiments of A2-E in pure organic solvents showed no formation of transient species on the timescale of our experiments. However, when these measurements were repeated for A2-E in Triton X-100 micelles, a short-lived (,, 14 ,s), weak absorption was observed. This species is quenched by oxygen (k= 2 × 109M,1 s,1) and by the addition of the antioxidants, cysteine and N,N,N,,N, -tetramethylphenylenediamine. Quenching of this species by 2,3,5-trimethylhydroquinone results in the formation of the 2,3,5-trimethylsemiquinone free radical and an increase in yield of the A2-E,derived species. Sensitization of the A2-E triplet excited state indicates that the species observed in micelles upon direct excitation is not consistent with the triplet excited state. Based on these data we tentatively assign this absorption to a free radical. In the RPE these initial processes can ultimately lead to damage to the tissue through the formation of peroxides and other oxidized species. [source] | ||||||||||