Fluorescence emission from dendrimers and its pH dependence

A strong fluorescence emission was observed from different kinds of dendrimers under acidic condition. There was a remarkable difference in fluorescence properties between second and fourth generation NH2-terminated poly(amido amine) dendrimers. It can be assumed that the backbone of the dendrimer played the key role in forming the novel fluorescent center.     

Structural changes of purple membrane and bacteriorhodopsin during its denaturation induced by high pH

Bacteriorhodopsin (bR) trimers naturally form two-dimensional hexagonal crystals in purple membrane (PM), which make it very stable. However, the dnaturation of bR was found to occur during a very narrow pH range when the pH was increased above 12.0, as indicated by inactivation of the photochemical cycle observed by flash photolysis kinetic spectra. Here, atomic force microscopy was used to study the surface structural changes of PM during the denaturation process induced by high pH. Together with the absorption and fluorescence spectra, it was found that the structural changes could be divided into three steps. First, some hydrophobic amino acids of bR become exposed to the aqueous environment and PM loses its 2D crystalline structure, transforming into the so-called "nonisland" structure. Second, bR molecules are extracted out of membrane and form protrusions on the surface like islands in the sea; therefore, the "nonisland" structure transforms into the "island" structure. Finally, most bRs break off from the membrane and form large depositions.     

光(热)致变色现象

在光照或加热的情况下,某些物质的颜色会发生可逆性的变化,称之为光（热）致变色现象。此类物质的种类不同,其变色机理不同,所呈现的颜色也不同。此现象首先发现于无机物,并已有130余年的历史。但不论是在合成与理论研究上,还是在实际应用上的迅速发展,却是在近30余年来更显得卓有成效。光（热）致变色物质目前已在图像显示、光记录材料、彩色录像、照像、印刷照排、化妆品、染料、装饰品、油漆、玩具、护目镜等方面得到了不同程度的应用。     

Studies on the riboflavin-resorcinol interaction in an aqueous medium and its pH dependence

The equilibrium constants, K', for the formation of the molecular association complex between riboflavin and resorcinol in the pH range 6-8 were found to be in the range 5-25 mol(-1)dm(3) by difference absorption spectroscopy. The equilibrium constants from emission spectroscopy were estimated to be in the range 38-55 mol(-1)dm(3) over the same pH range. Both sets of K' show a remarkable dependence on pH. The equilibrium constants K(A) and K(B) describing the interaction of riboflavin with neutral and ionic forms of resorcinol, respectively, obtained by resolving K', shows that K(B) is much greater than K(A). Molecular modeling studies suggest a stacked conformation of the two components in the complexed form. The interaction energies obtained from modeling studies also suggest a stronger interaction between the ionic form of resorcinol and riboflavin.     

Thermochromism of polypropylene gels

It is found that polypropylene gels in some solvents of benzene-derivatives show a striking change of colour. The colour changes from blue to yellow as the temperature rises from the melting point of the solvent to around 70–80 °C. The apparent characteristic of the phenomenon resembles the thermochromism of cholesteric liquid crystals although polypropylene itself is colourless and has no asymmetric carbons in the molecules. Polypropylene flakes swelled in these solvents also show similar thermochromism.     

Thermochromism and Solvatochromism in Solution

Summary.  The thermochromic behaviour of various coordination compounds in solution is discussed, with a special focus on cyclic diamine chelates. Thermally induced spin-crossover phenomena of iron(II) complexes are also considered. The solvatochromic behaviour of mixed-ligand complexes is presented in detail. Received April 2, 2001. Accepted (revised) April 19, 2001     

Thermochromism and Solvatochromism in Solution

 The thermochromic behaviour of various coordination compounds in solution is discussed, with a special focus on cyclic diamine chelates. Thermally induced spin-crossover phenomena of iron(II) complexes are also considered. The solvatochromic behaviour of mixed-ligand complexes is presented in detail.     

Viscoelastic study of the adsorption of bovine serum albumin on gold and its dependence on pH

Using a quartz crystal resonator system operating at 5 MHz the shear wave propagating properties of bovine serum albumin (BSA) have been monitored as it is adsorbed on a gold surface from a phosphate buffered saline (PBS) solution. Employing a 2-layer model for the combined BSA layer and PBS solution, the viscoelasticity of the BSA layer may be determined in real time as the adsorption on gold proceeds. The viscoelasticity is found to depend on the pH of the PBS solution and changes gradually over long times. It is suggested that at the low frequency of the measurement, large-scale molecular motions are being monitored which are a consequence of the structural changes in the protein molecules undergoing adsorption. Such low-frequency molecular motions are difficult to examine by any other technique. The results and their interpretation in viscoelastic terms demonstrate the considerable potential of the quartz crystal resonator system for assessing the stability of proteins on surfaces and their suitability as coatings for prosthetic materials.     

All-trans to 13-cis retinal isomerization in light-adapted bacteriorhodopsin at acidic pH

The flash photolysis kinetic spectra of the intermediate M(412) of bacteriorhodopsin were monitored during the process of acid titration. In the light-adapted state, the maximum peak amplitude of M(412) absorbance of bacteriorhodopsin decreased (pK(a)=3.40+/-0.05) as the pH decreased from 7.3 to 1.9. In the dark-adapted state, the maximum peak amplitude of M(412) absorbance of bacteriorhodopsin increased as the pH decreased from 6.9 to 4.1, and then decreased (pK(a)=2.85+/-0.05) as the pH dropped to 2.1. These different trends in the change in the maximum peak amplitude suggested that not only the transition of purple membrane to blue membrane had taken place in both light and dark-adapted states, but also the fraction of all-trans-bR had changed during the acid titration. The pH-dependent absorption changes at 640 nm of bacteriorhodopsin in both light- and dark-adapted states were also observed. The pK(a)-values of the purple-to-blue transition were 3.80+/-0.05 in light-adapted state and 3.40+/-0.05 in dark-adapted state, respectively. According to Balashov's method, the fraction of all-trans-bR was assayed as the pH decreased. All these results indicated that the purple-to-blue transition of light-adapted bacteriorhodopsin was accompanied by an all-trans to 13-cis retinal isomerization at acidic pH.     

Thermochromism of bithiophenes and internal aromatic chain rotation

The relation between thermochromism and thermal motion of aromatic rings on model bithiophene and 3-butylbithiophene molecules was studied by molecular dynamics (MD) simulations based on semiempirical AM1 hamiltonian. Theoretical optical spectra at various temperatures are generated from the conformations obtained from MD calculations. Alkyl chains have a tendency to tilt bithiophene fragments from co-planarity (connected with blue shift) whereas the unsubstituted systems prefer the higher degree of planarity (connected with red shift) upon the temperature increase. The slope of the linear temperature dependence of the reciprocal maximal wavelengths consists of the above electronic and of the vibrational contribution that (according to Wien’s displacement law) causes always the increase of the above mentioned slope by reciprocal Wien’s constant. This model explains the temperature dependence of experimental UV–vis spectra for dibutylheptathiophene and 3′,3,4′,4-tetrahexylhexathiophene.     

联二炔囊泡的光致聚合、热致变色及应用

物理化学实验为各大高校本科化学教学的主干实验课程之一。虽然各高校的物理化学实验内容已十分丰富,但仍然缺乏将不同章节知识点融会贯通以及与学科前沿密切联系的综合型实验内容。我们根据南开大学郭东升教授课题组发表的一篇有关联二炔程序性响应检测胆碱酯酶的工作设计了"联二炔囊泡的光致聚合、热致变色及应用"的物理化学实验,涉及到联二炔缔合胶体的制备、联二炔的紫外光致聚合反应和聚联二炔的热致变色,进而显示其在防伪领域的潜在应用价值。希望通过此实验,使学生对物理化学中的缔合胶体、光化学、热力学、动力学等章节有更深一步的贯通性掌握,并了解自组装、光聚合、变色材料和防伪等相关研究领域的前沿动态。     

pH dependence of the electronic structure of glycine

The carbon, nitrogen, and oxygen K-edge spectra were measured for aqueous solutions of glycine by total electron yield near-edge X-ray absorption fine structure (TEY NEXAFS) spectroscopy. The bulk solution pH was systematically varied while maintaining a constant amino acid concentration. Spectra were assigned through comparisons with both previous studies and ab initio computed spectra of isolated glycine molecules and hydrated glycine clusters. Nitrogen K-edge solution spectra recorded at low and moderate pH are nearly identical to those of solid glycine, whereas basic solution spectra strongly resemble those of the gas phase. The carbon 1s --> pi*(C=O) transition exhibits a 0.2 eV red shift at high pH due to the deprotonation of the amine terminus. This deprotonation also effects a 1.4 eV red shift in the nitrogen K-edge at high pH. Two sharp preedge features at 401.3 and 402.5 eV are also observed at high pH. These resonances, previously observed in the vapor-phase ISEELS spectrum of glycine, have been reassigned as transitions to sigma* bound states. The observation of these peaks indicates that the amine moiety is in an acceptor-only hydrogen bond configuration at high pH. At low pH, the oxygen 1s --> pi*(C=O) transition exhibits a 0.25-eV red shift due to the protonation of the carboxylic acid terminus. These spectral differences indicate that the variations in electronic structure observed in the NEXAFS spectra are determined by the internal charge state and hydration environment of the molecule in solution.     

pH dependence of adsorption of n-dodecyl-beta-D-maltoside on solids

Adsorption of surfactants on solids plays an important role in industrial operations such as separation, lubrication, flotation, dispersion, chemical mechanical polishing, and enhanced oil recovery. In this work, adsorption of a typical biodegradable nonionic surfactant, n-dodecyl-beta-d-maltoside, on solids was studied to explore its potential applications. Even though it is a nonionic surfactant, significant pH-dependence was revealed for the adsorption on alumina in the range from pH 4 to 7. The adsorption density was found to be proportional to the concentration of surface AlOH group among Al(OH(2))(+) and AlO(-) groups. The equilibriums among the surface species are governed by pH through surface ionization reactions. The surface AlOH group evidently determines the formation of hydrogen bonding between the surfactant molecules and the solid surface and thus the adsorption. Similar correlation was also found in the case of hematite. The results help to understand the mechanism of adsorption of sugar-based surfactant on solids.     

A Fourier-transform Raman study of the temperature dependence of chromophore conformation in light-adapted and dark-adapted bacteriorhodopsin

Fourier-transform Raman spectra are reported for bacteriorhodopsin (BR) in its light-adapted and dark-adapted states at room and elevated temperatures. Qualitative changes are observed in the FT-Raman spectrum above the midpoint temperature of the predenaturational transition indicating structural changes and inhomogeneous protein populations. Dark-adapted BR shows an increased all-trans component at 78°C, while light-adapted BR at 78°C is found to contain 13-cis BR, possibly as a result of a reduced extent of light adaptation or a change in protein—chromophore interactions.     

Thermochromism of poly-1-butene gels

Poly-1-butene gels in some solvents of benzene-derivatives show a colouring phenomenon. The colour changes from blue to yellow under irradiation of natural light as the temperature rises from the melting point of the solvent to the sol-gel transition temperature. The colouring phenomenon is due to selective scattering, but not to optical absorption. The apparent characteristics of the phenomenon resemble the thermochromism of cholesteric liquid crystals, although poly-1-butene itself is colourless and has no asymmetric carbons.     

pH dependence of fluorescence and absorbance spectra of free sulphonated aluminium phthalocyanine and its conjugate with monoclonal antibodies.

Water-soluble phthalocyanines and phthalocyanines linked to targeting monoclonal antibodies are considered to be one of the most promising photosensitizers in photodynamic therapy. Here the spectrum characteristics of sulphonated aluminium phthalocyanine and its protein conjugate in the pH range 1.8-12.0 in solution have been studied. The pK values are determined.     

pH and concentration dependence of luminescent characteristics in glass-encapsulated Eu-complex

In order to improve the stability of red phosphor with efficient and high color-purity characteristics under ultraviolet excitation, we encapsulated Eu(TTA)₃phen(tris(2-thenoyltrifluoroacetonato)(1,10-phenanthroline)europium(III)) by sol–gel derived silica glasses and studied their photoluminescence (PL) characteristics. The dependence of pH values in the starting solution revealed that the process condition should be within the modest region with the optimum pH value around 6–7. The PL intensity increased with increasing the concentration of Eu(TTA)₃phen initially, then it showed saturating tendency above 0.6 mol%. An improvement of reliability by the glass encapsulation was exemplified by monitoring the long-term PL intensity under constant UV irradiation. We consider that the glass network carefully prepared by sol–gel process is effective for preventing free oxygen and water from attacking Eu(TTA)₃phen molecule without loss of PL output.     

Vibrational spectroscopic study on pH dependence of some diazines

The infrared and Raman spectra of several pyrazines and pyridazines were measured in a wide acidity range. Methods of measurements were elaborated for extreme acidity conditions. The approximate assignments of some spectra were determined. The spectra show bands with pH depending frequencies and intensities and give information on the solute—solvent interaction.     

Thermochromism of perylenes: dynamics in aromatics

A thermal equilibrium between planar and skew perylene derivatives induces thermochromism and may modulate molecular recognition. Thus, the long-lasting discussion about the geometry of perylene could be solved in terms of this equilibrium. Extension to other aromatics and practical applications such as for non contact fluorescence thermometers are discussed.     

Preparation of a cross-linked gelatin/bacteriorhodopsin film and its photochromic properties

Bacteriorhodopsin (BR) is a photochromic membrane protein isolated from a strain of halobacteria.Embedment of BR into a polymeric matrix enables the application of the photoactive protein as an optical material.In this work,a chemically crosslinked BR/gelatin film was prepared.The cross-linked film was found to be highly stable even under extreme alkaline or detergent circumstance while BR maintained its bioactivity.The treatments of base and detergents also led to dramatic prolongation of the lifetime of M...