Effect of axial ligation or pi-pi-type interactions on photochemical charge stabilization in "two-point" bound supramolecular porphyrin-fullerene conjugates

Two types of structurally well-defined, self-assembled zinc porphyrin-fullerene conjugates were formed by "two-point" binding strategies to probe the effect of axial ligation or pi-pi-type interactions on the photochemical charge stabilization in the supramolecular dyads. To achieve this, meso-tetraphenylporphyrin was functionalized to possess one or four [18]crown-6 moieties at different locations on the porphyrin macrocycle while fullerene was functionalized to possess an alkyl ammonium cation, and a pyridine or phenyl entities. As a result of the crown ether-ammonium cation complexation, and zinc-pyridine coordination or pi-pi-type interactions, stable zinc porphyrin-fullerene conjugates with defined distance and orientation were formed. Evidence for the zinc-pyridine complexation or pi-pi-type interactions was obtained from the spectral and computational studies. Steady-state and time-resolved emission studies revealed efficient quenching of the zinc-porphyrin singlet excited state in these dyads, and the measured rates of charge separation, k(CS) were found to be slightly better in the case of the dyads held by axial coordination and crown ether-cation complexation. Nanosecond transient absorption studies provided evidence for the electron transfer reactions, and these studies also revealed charge stabilization in these dyads. The lifetimes of the radical ion pairs were found to depend upon the type of porphyrins utilized to form the dyads, that is, porphyrin possessing the crown ether moiety at the ortho position of one of the phenyl rings yielded prolonged charge stabilized states. Addition of pyridine to the supramolecular dyads eliminated the zinc-pyridine coordination or pi-pi-type interactions of the "two-point" bound systems due to the formation of a new zinc-pyridine axial bond thus giving a unique opportunity to probe the effect of axial coordination or pi-pi interactions on k(CS) and k(CR). Under these conditions, the measured electron transfer rates revealed faster k(CS) and slower k(CR) as compared to those obtained in the absence of added pyridine. The evaluated lifetimes of the radical ion-pairs were found to be hundreds of nanoseconds and were longer in the presence of pyridine.     

On the preparation of carbohydrate-protein conjugates using the traceless Staudinger ligation

[reaction: see text] The nature of a linker used for preparing glycoconjugate vaccines is of utmost importance as it may lead to immunogenic biomolecules. We report the conjugation of carbohydrate haptens to protein carriers leading to potential vaccines using the traceless Staudinger ligation. The ligation relies on the selective transfer of a phosphane substituent to an azide to form a native amide bond in the final product upon release of an oxidized phosphane byproduct. We designed new phosphino-functionalized cross-linkers suitable for protein carrier derivatization. We evaluated their utility in preparing conjugates using both synthetic and purified bacterial carbohydrates. The use of a borane-protected phosphane which is deprotected at the time of the ligation reaction led to the best results observed thus far in terms of stability toward oxidation and reactivity.     

Site-directed photochemical disruption of the actin cytoskeleton by actin-binding Rose Bengal-conjugates

The in situ light-induced, non-enzymatic digestion of cytoskeletal actin by a xanthene dye conjugated to heavy meromyosin, anti-actin antibodies and/or anti-myosin antibodies is reported. The dye Rose Bengal was conjugated to either anti-actin antibodies, anti-myosin antibodies or heavy meromyosin. Under our experimental conditions, visible light induced the non-enzymatic breakdown of cytoskeletal actin when mammalian tissue culture cells were probed either with Rose Bengal-conjugated anti-actin and/or anti-myosin antibodies. Similar results were obtained when tissue culture cells were probed with Rose Bengal-conjugated heavy meromyosin before irradiation with visible light. The in situ photochemical reaction depended on the presence of actin-binding Rose Bengal-conjugates.     

The effect of environment on cystine disruption by ultraviolet light

When cystine is irradiated at pH 1 by 254-nm u.v. the following yields are observed: 4 cystines → 5.2 cysteines + 2.8NH₃. Thus, SH production accounts for only 0.65 of the cystine destruction; further C-S breakage to give alanine or serine is not efficient. The yields for cystine and glutathione destruction are essentially the same at pH 1. However the presence of the glutamic and glycine residues stabilize the cystine in glutathione so that NH₃ is not lost until the peptide bonds are hydrolyzed. Increasing the pH from 1 to 8.6 increases the yield of cystine destruction in glutathione by 50 per cent. The yield of cystine destruction is greater in both compounds when O₂ is present during irradiation (e. g. the cysteic acid yield in glutathione is increased by 50 times). The overall production of SH varies by a factor of 2 in the four proteins-insulin, RNase, trypsin and lysozyme. The present data further support the earlier observation that radiation damage is quite non-random in RNase: at least two and perhaps three of the four constituent cystines must be disrupted before activity is lost: i.e. the most radiosensitive cystines are not critical for enzymic activity. Similarly, in both trypsin and lysozyme the integrity of the most radiosensitive cystines also does not appear to be critical for the retention of enzymic potential. In insulin, however, all three cystines appear to be crucial for activity and to have approximately equal radiosensitivities. These differences in sensitivity of cystines in different proteins must depend specifically upon energy transfer and/or chemical interactions between the chromophoric groups. If yields are calculated on the basis of those quanta absorbed only in the cystines, values about 5 to 8 times greater than those in the model compounds cystine and oxixized glutathione are obtained. The yields of cystine destruction are much higher in those protiens which contain trypotophan.     

Water catalysis and anticatalysis in photochemical reactions: observation of a delayed threshold effect in the reaction quantum yield

The possible catalysis of photochemical reactions by water molecules is considered. Using theoretical simulations, we investigate the HF-elimination reaction of fluoromethanol in small water clusters initiated by the overtone excitation of the hydroxyl group. The reaction occurs in competition with the process of water evaporation that dissipates the excitation and quenches the reaction. Although the transition state barrier is stabilized by over 20 kcal/mol through hydrogen bonding with water, the quantum yield versus energy shows a pronounced delayed threshold that effectively eliminates the catalytic effect. It is concluded that the quantum chemistry calculations of barrier lowering are not sufficient to infer water catalysis in some photochemical reactions, which instead require dynamical modeling.     

The effect of spin multiplicity on the photochemical reaction of a 2-vinylnaphthalene-dichloromaleic anhydride system

Research on Chemical Intermediates - The photochemical reaction of a 2-vinylnaphthalene — dichloromaleic anhydride system has been studied in order to elucidate the correlation between the...     

The effect of annealing on the local structure of glassy polycarbonate

Annealing of polycarbonate glasses at temperatures belowTg leads to an increase in yield stress and a drop in the impact strength. Although such behaviour may be related to the corresponding reduction in free volume upon annealing, variations in the wide-angle X-ray scattering curves indicate some modification to the local structure. The area of an intrachain peak at s 0.7 Å^–1 is monitored with respect to annealing temperature and time. It is proposed that the variations may be described by an increasing level of interlocking or (nesting) between neighbouring chain segments, a process which is a natural consequence of the molecular shape of polycarbonate.     

The effect of gamma irradiation on the structural properties of olivine

Journal of Radioanalytical and Nuclear Chemistry - Gamma irradiation studies of (Mg0.905Fe0.095)2SiO4 olivine were performed using X-ray fluorescence method, X-ray diffraction, Raman and...     

The effect of environment on local dynamics of macromolecules

The role of local environment on mobility of chain molecules is illustrated by considering the case of a miscible mixture of two polymers. The historical background, observed dynamic responses to mixing, and theoretical explanations for these responses are outlined. Two miscible blend systems: poly(isoprene) [PI]/poly(vinyl ethylene) [PVE] and poly(ethylene oxide) [PEO]/poly(methyl methacrylate) [PMMA] are reviewed in detail, with available data consolidated in a set of figures. Unusual features of the PEO/PMMA blend are discussed.     

Stabilizing effect of 1,1,5-trihydroperfluoropentanol on photochemical degradation of polycaproamide

The effect of fluorinated alcohols on the photochemical degradation of polycaproamide was studied. It was shown that, even when a small amount (0.035 wt %) of 1,1,5-trihydroperfluoropentanol is added, the rate of photodegradation of polyamide fibers decreases. Electron microscopic examination showed that, in the presence of the fluorinated alcohol, the test fibers are characterized by a reduced number of defects. The simultaneous introduction of microquantities of 1,1,5-trihydroperfluoropentanol and N,N-bis(2,2,6,6-tetramethyl-4-piperidinyl) 1,3-benzenedicarbamide is responsible for the synergistic photostabilizing effect.     

Photochemical ligation of DNA conjugates through anthracene cyclodimer formation and its fidelity to the template sequences

Anthracene readily forms photoadducts, anthracene dimers, and this photodimerization reaction has been well characterized. In general, however, the reaction requires close proximity and certain spatial alignment of both reaction partners. DNA could provide an ideal scaffold for accelerating the photocyclic addition. We synthesized a number of anthracene-DNA conjugates. The sequences of the conjugates, 5'AntODNn and 3'AntODNn (the length of methylene linkers: n = 3 or 6), were designed to bind adjacent sequences of the template with the anthracene units directed such that they stacked with each other. The conjugates were only dimerized in the presence of the template by light irradiation. The efficiency was affected by one-base displacement in the template sequence.     

The effect of organic additives on the local dynamics of polyelectrolyte-surfactant complexes

The effect of organic additives on the local dynamics of complexes of poly(N-ethyl-4-vinylpyridinium bromide) with sodium dodecyl sulfate is studied with the use of the solubilization of butanol and Triton X-100 as an example. It is shown that, irrespective of the chemical nature of the solubilizate, the addition of organic compounds gives rise to an increase in the local mobility of the micellar phase. In this case, the segmental mobility of the polyelectrolyte in complexes remains unchanged.     

The effect of filler content on the lower yield stress of polymer composites

The effect of the concentration of the dispersed elastic filler on the lower yield stress of matrix composites based on plastic polymers is studied. As the matrix polymers, LDPE-HDPE and LDPE-(medium-density PE) are used. The elastic filler is rubber crumb prepared by roll grinding of worn tires or by deformation grinding of ethylene-propylene-diene rubber. Irrespective of the type of filler particles and their adhesion to the polymer matrix, the lower yield stress σ_d of the composite is described by the linear law σ_d = σ_dm(1 ? V _f ), where σ_dm is the lower yield stress of the polymer matrix and V _f is the volume content of the filler. Analysis of the published data shows that this relationship is quite general and describes the effect of rigid inorganic particles on the lower yield stress when adhesion between the filler particles and the matrix is poor.     

Photoquenching: The dependence of the primary quantum yield of a monophotonic laser-induced photochemical process on the intensity and duration of the exciting pulse

Excited state absorption in large molecules leads to a decrease of the primary quantum yield of a photochemical or a photophysical process. Since then the quantum yield decreases with increasing light intensity this effect is called photoquenching.Kinetic analysis of the excitation in a general level scheme of a large molecule yields expressions for the quentum yield of a laser-induced photochemical process. Calculation of the quantum yield for various combinations of molecular parameters and laser pulse characteristics shows quenching of the photochemical process due to excited state absorption, at laser intensities for which bleaching effects and other nonlinear processes are negligeable. The applicability of the steady state approximation in analyzing laser-induced processes is discussed. Experiments are reported, which confirm the calculated intensity-dependent quantum yield function. Previous measurements of intensity-induced quenching can now be discussed quantitatively. Care should be taken in interpreting laser-induced photochemical yields, especially at mode locked laser intensities; correct values can only be obtained by extrapolation to zero laser intensity     

The photochemical reactions of the halogenated N-benzylanilines. Mechanism of the reactions

Several N-(2-halobenzyl)anilines and N-benzyl-2-haloanilines have been synthesized and their photochemical reactions studied. Upon irradiation, the aqueous acetonitrile solution of N-benzyl-2-chloroaniline was cyclized and reduced to give phenanthridine, 5,5′,6,6′-tetrahydro-6,6′-biphenanthridyl (THBP), N-ben-zylaniline, and bibenzyl. Similar products were produced in the photochemical reactions of other halo-genated N-benzylanilines, except iodo-substituted N-benzylanilines. No dimer (THBP) was produced from the iodo-substituted N-benzylanilines. Both singlet and triplet states are involved in the photochemical reactions of the haloarenes.     

Effect of the structural nonequilibrium of solid samples on the low temperature limit of the photochemical reaction rate

The results from two cryogenic experiments at helium temperatures are compared. Widely separated and adjacent aggregations formed by vapor deposition are considered. It is shown that upon the photochlorination of methane in a solid structural nonequilibrium mixture prepared from the gas phase at liquid helium temperatures, the quantum yield of chlorine consumption at the low temperature limit falls to zero while the effective activation energy in the Arrhenius region rises due to achieving a state of supersaturation with vacancies and a vibrational mode of sintering. It is demonstrated that upon the photobromination of ethylene under conditions of disequilibrium decay, accompanied by halogen-olefin self-organization of the condensate structure (complexation), the quantum yield of bromine consumption in the athermal region rises while the activation energy in the Arrhenius region falls.