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51.
Selected hydrophilic interaction chromatography (HILIC) columns packed with bare silica, bridge-ethyl hybrid silica, or an amide sorbent chemistry were utilized for an investigation of chromatographic behavior and separation selectivity of tryptic peptides. Retention model was proposed allowing for retention prediction of peptides with correlation coefficient R(2)~0.92-0.97 for various columns. The values of optimized amino acid retention coefficients were compared to those obtained for reversed-phase liquid chromatography (Gilar et al., Anal. Chem. 2010, 82, 265-275) and used to elucidate the impact of different amino acid on peptide HILIC retention. In contrast to reversed-phase chromatography, where presence of Phe, Trp, Ile, and Leu amino acid residues in sequence strongly promoted, and presence of hydrophilic His, Lys and Arg residues strongly reduced peptide retention, the effects of these amino acid residues in HILIC were opposite (His, Lys and Arg promote, Phe, Trp, Ile and Leu demote peptide retention in HILIC). Retention coefficient optimized for pH experiments illustrated the impact of silanols on HILIC retention. 相似文献
52.
Rebane A Drobizhev M Makarov NS Beuerman E Haley JE Krein DM Burke AR Flikkema JL Cooper TM 《The journal of physical chemistry. A》2011,115(17):4255-4262
We investigate two-photon absorption (2PA) in a series of fluorenyl-based 9,9-diethyl-2-ethynyl-7-((4-R-phenyl)ethynyl)-9,9a-dihydro-4aH-fluorene chromophores with R being various electron donating (ED) and electron withdrawing (EW) groups. We use wavelength-tunable femtosecond laser pulses to measure the 2PA cross sections in the lowest dipole-allowed transition and show that the substituents with stronger ED or EW character enhance the peak 2PA cross section (up to σ(2) ~ 60-80 GM) while the neutral substituents lead to smaller cross sections, σ(2) < 10 GM. We apply two-level approximation to establish a quantitative relation between the 2PA in the pure electronic transition (0-0) and the corresponding change of the permanent electric dipole moment upon the excitation (Δμ). This relation is elucidated by comparing Δμ values obtained from the 2PA measurements with quantum-chemical calculations and with measurements of solvatochromic shifts in a series of solvents. We show that the calculated Δμ correlate well with the values obtained from the 2PA spectroscopy. The Δμ values obtained from the solvatochromic shifts agree well with the above two methods for the chromophores with neutral or weak EW or ED substituents. On the other hand, stronger EW or ED end groups give much larger Stokes shifts, which lead to an overestimation of the Δμ values. We tentatively attribute this effect to the excitation-induced electronic density change occurring predominantly at the substituent side of the molecule, which causes the effective point dipole associated with the Δμ to interact more strongly with the surrounding solvent. 相似文献
53.
Dolashka P Moshtanska V Dolashki A Velkova L Rao GS Angelova M Betzel C Voelter W Atanasov B 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,83(1):67-73
The native form of Cu/Zn-superoxide dismutase, isolated from fungal strain Humicola lutea 103 is a homodimer that coordinates one Cu(2+) and one Zn(2+) per monomer. Cu(2+) and Zn(2+) ions play crucial roles in enzyme activity and structural stability, respectively. It was established that HLSOD shows high pH and temperature stability. Thermostability of the glycosylated enzyme Cu/Zn-SOD, isolated from fungal strain H. lutea 103, was determined by CD spectroscopy. Determination of reversibility toward thermal denaturation for HLSOD allowed several thermodynamic parameters to be calculated. In this communication we report the conditions under which reversible denaturation of HLSOD exists. The narrow range over which the system is reversible has been determined using the strongest test of two important thermodynamic independent variables (T and pH). Combining both these variables, the "phase diagram" was determined, as a result of which the real thermodynamic parameters (ΔC(p), ΔH(exp)°, and ΔG(exp)°) was established. Because very narrow pH-interval of transitions we assume they are as result of overlapping of two simple transitions. It was found that ΔH(o) is independent from pH with a value of 1.3 kcal/mol and 2.8 kcal/mol for the first and the second transition, respectively. ΔG(o) was pH-dependent in all studied pH-interval. This means that the transitions are entropically driven, these. Based on this, these processes can be described as hydrophobic rearrangement of the quaternary structure. It was also found that glycosylation does not influence the stability of the enzyme because the carbohydrate chain is exposed on the surface of the molecule. 相似文献
54.
55.
Haubrich J Loffreda D Delbecq F Sautet P Jugnet Y Krupski A Becker C Wandelt K 《Physical chemistry chemical physics : PCCP》2011,13(13):6000-6009
The prediction of a reaction mechanism and the identification of the corresponding chemical intermediates is a major challenge in surface science and heterogeneous catalysis, due to a complex network of elementary steps and surface species. Here we demonstrate how to overcome this difficulty by tracking the temperature dependent formation of the initial reaction intermediates and identifying the decomposition pathways in the case of prenal, an α,β-unsaturated aldehyde, on the Pt(111) model catalyst surface by combining vibrational spectroscopy, thermal reaction/desorption spectroscopy (TPRS) experiments and detailed theoretical analysis. TPRS characterization of this reaction up to 600 K shows a series of desorption states of H(2) (~280 K, 410 K and 473 K) and CO (~414 K), giving valuable insights into the sequence of elementary steps suggesting that the loss of hydrogen and the carbonyl functions are among the first elementary steps. HREELS experiments recorded after annealing to specific temperatures result in complex spectra, which can be assigned to several subsequently formed and transformed surface intermediates. Starting from stable prenal adsorption structures, complementary DFT calculations allow the determination of the most likely reaction pathway for the initial decomposition steps and the identification of the corresponding intermediates by comparison with HREELS. The decomposition occurs from the strongly bonded prenal adsorption structures via a dehydro-η(3)-triσ(CCC)-H1 intermediate to the highly stable η(1)-isobutylidyne species at high temperatures. 相似文献
56.
Aleksander Yevtushenko Michal Kuciej Olena Yevtushenko 《Archive of Applied Mechanics (Ingenieur Archiv)》2011,81(6):715-726
The transient temperature field and corresponding quasi-static thermal stresses are analysed in a system consisting of a semi-space
and a strip. The strip is heated on its outer surface by a heat flux with the intensity equal to the specific power of friction
during braking with a uniform retardation. The evolution and distribution in depth from a surface of friction for temperatures
and thermal stresses were investigated for the metal-ceramic FMK-11 material of the strip. 相似文献
57.
58.
Renard L Elhamzaoui H Jousseaume B Toupance T Laurent G Ribot F Saadaoui H Brötz J Fuess H Riedel R Gurlo A 《Chemical communications (Cambridge, England)》2011,47(5):1464-1466
Self-assembled nanoporous tin-based hybrid thin films prepared by the sol-gel method from organically-bridged ditin hexaalkynides detect hydrogen gas from 50 to 200 °C at the 200-10,000 ppm level. This finding opens a fully new class of gas-sensing materials as well as a new opportunity to integrate organic functionality in gas sensing metal oxides. 相似文献
59.
60.
Madhuri Mukhopadhyay Debi Banerjee Aleksander Koll Aleksander Filarowski Samaresh Mukherjee 《Chemical physics letters》2006,420(4-6):316-320
The effect of neutral, cationic and anionic micellar environments on the ground and excited state proton transfer reactions of salicylidine-3,4,7-methyl amine (SMA) in water has been studied by steady state and time resolved fluorescence spectroscopy. In the ground state, the formation of the primary form of SMA is enhanced at the expense of the zwitterionic species due to micellization. In the excited state, anion formation decreases both in the presence of Triton-X and cetyl trimethyl ammonium bromide (CTAB). However, in the presence of sodium dodecyl sulphate (SDS), the anionic emission increases after reaching a certain micellar concentration. The lifetime of the anion is significantly reduced in CTAB compared to that in the bulk water and also in the presence of Triton-X. It is proposed that the destabilization and modification of SMA anion occurs due to the different electrostatic environments produced by micellization. 相似文献