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Bile salt aggregates are supramolecular systems containing two different binding sites. The effect of the addition of acetonitrile on the specificity and dynamics of guest binding to the two binding sites of cholate aggregates was studied. The protection of guests included in the aggregate from interaction with ions in the aqueous phase was evaluated from quenching of the singlet and triplet excited states of guest molecules bound to the cholate aggregates. The dynamics of guest binding to the primary and secondary binding sites of the cholate aggregates were determined at increasing acetonitrile mole fractions. The structure of the aggregates was not significantly altered provided the cholate concentrations were higher than 20 mM and the acetonitrile mole fraction did not exceed 0.033 (9.1% v/v). These results show that acetonitrile can be used to modulate the solubility of guests in the aggregates and to manipulate the residence time of guests in the primary and secondary binding sites.

The reactivity of excited benzophenone (Bp) and 4,4‘-dimethylbenzophenone (DMBp) in the presence of sodium cholate (NACh) aggregates was studied by following the kinetics of the excited triplet states and the ketyl radicals of both ketones. At low NACh concentrations only primary aggregates are present in solution. The ketyl radicals were formed from the reaction of the triplet ketones bound to the primary sites. The decay of the ketyl radicals occurred primarily by the reaction of these radicals in water. Some long-lived triplets included in the primary aggregate were also observed. A different reactivity was observed at higher NACh concentrations where secondary aggregates are present. The binding process associated with the secondary binding sites is much faster than for the primary site. The hydrogen abstraction reaction in the secondary binding site is too slow to compete with the exit process, but self-quenching competes efficiently, leading to a shortening of the triplet lifetimes for both ketones. From the dynamic results it was concluded that only a small number of NACh molecules (8−13) are needed to define the primary and secondary binding sites.

We tested the hypothesis that unanesthetized rats exhibit ventilatory long-term facilitation (LTF) after intermittent, but not continuous, hypoxia. Minute ventilation (V˙e) and carbon dioxide production (V˙co2) were measured in unanesthetized, unrestrained male Sprague-Dawley rats via barometric plethysmography before, during, and after exposure to continuous or intermittent hypoxia. Hypoxia was either isocapnic [inspired O2fraction (FiO2) = 0.08–0.09 and inspired CO2fraction (FiCO2) = 0.04] or poikilocapnic (FiO2= 0.11 and FiCO2= 0.00). Sixty minutes after intermittent hypoxia, V˙eorV˙e/V˙co2was significantly greater than baseline in both isocapnic and poikilocapnic conditions. In contrast, 60 min after continuous hypoxia,V˙eandV˙e/V˙co2were not significantly different from baseline values. These data demonstrate ventilatory LTF after intermittent hypoxia in unanesthetized rats. Ventilatory LTF appeared similar in its magnitude (after accounting for CO2feedback), time course, and dependence on intermittent hypoxia to phrenic LTF previously observed in anesthetized, vagotomized, paralyzed rats.

The complexation dynamics of xanthone to substituted β-cyclodextrins (CDs) and of thioxanthone to β-CD was studied by directly following the relocation of the excited triplet-state ketones from the CD cavity to the aqueous phase. Equilibrium constants for the ground-state complexation of both ketones with CDs were determined from fluorescence studies. The complexation efficiency and dynamics of thioxanthone with β-CD were the same as previously observed for the xanthone/β-CD system. Substitution of some of the hydrogens on the alcohol groups by methyl moieties at both entrances to the β-CD cavity increased the equilibrium constant for ground-state xanthone, but decreased that for the excited triplet state, leading to a larger difference for the binding behavior of these two electronic states of the guest. Addition of one pendant arm containing a carboxyl moiety to the rim where xanthone is bound does not change the binding efficiency of the ground state or the binding dynamics of triplet xanthone. In contrast, when this arm contains an ethyl ester moiety, the binding efficiencies of both the ground and excited triplet states are increased. The higher binding constant for triplet xanthone is due to an increase of the association rate constant and a decrease for the dissociation rate constant.

By rapid heat treatments surface layers with properties that are distinctly different from those of the initial microstructure can be generated. In case of high nitrogen steels the rapid heat treatment aims at generating a hard surface with compressive residual stresses and high corrosion resistance. Here, the microstructure, the residual stresses and especially the corrosion behaviour of the rapid heat treated high nitrogen steel X30CrMoN15 1 and the reference steel X39CrMo17 1 are presented in dependence of the maximum heat treatment temperature and the heating rate.

Stereoselective binding of the 1-naphthyl-1-ethanol (1-NpOH) and 2-naphthyl-1-ethanol (2-NpOH) enantiomers with β-cyclodextrin (β-CD) was investigated by photophysical and theoretical studies. The latter are based on structural calculations of the complexes and their respective induced circular dichroism spectra. For the enantiomers of 1-NpOH, where the complexes with β-CD have a 1:1 (guest:CD) stoichiometry, a mixture of structures with different geometries was observed for each enantiomer. No stereoselectivity was apparent for these complexes. (R)- and (S)-2-NpOH form complexes with β-CD that have both 1:1 and 2:2 stoichiometries. Structural calculations indicate that the naphthyl moiety of both enantiomers is preferentially included in the CD cavity for the 1:1 complex. In the case of the complexes with 2:2 stoichiometry, a mixture of complexes is present that either have the naphthyl moieties at a distance or in close proximity. The former geometry explains the observation of long-lived excited triplet states for 2-NpOH, whereas the latter geometry is responsible for the excimer emission. The stereoselectivity observed for the monomer to excimer intensity ratios when (R)- and (S)-2-NpOH are complexed to β-CD is due to the different contributions of 2:2 complexes with different geometries.

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The dynamics of purified poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO−PPO−PEO) block-copolymer micellization and phase separation in aqueous solutions were studied using the iodine laser temperature-jump and stopped flow techniques. The changes in the micellar solutions were followed by either light scattering or fluorescence of 1,6-diphenyl-1,3,5-hexatriene (DPH), which is a probe located in the micelle interior. Three different relaxation processes were observed for the temperature range covering the micro- and macrophase separation of the EO13PO30EO13 (Pluronic L64) block-copolymer. The fastest process corresponds to the incorporation of unimers into micelles which leads to larger micelles that are not thermodynamically stable. This process is followed by a relaxation with negative amplitude during which the micellar core is dehydrated and a redistribution of micellar sizes is achieved. The third relaxation process corresponds to the clustering of micelles into larger aggregates which is associated with the initial step of macrophase separation. Other PEO−PPO−PEO block-copolymers, like EO19PO43EO19 (Pluronic P84) and EO27PO61EO27 (Pluronic P104), were investigated to provide additional information concerning the second relaxation process. Mixed micelles containing sodium dodecyl sulfate were studied to support the assignment of relaxation processes involving micellar collisions. This study of the dynamics of purified PEO−PPO−PEO block-copolymers clarifies several controversial points because the dynamics were investigated over a wide temperature and concentration range and avoid impurity effects.

The photodecomposition of -guaiacoxyacetoveratrone (GAV) in homogeneous solvents and in aqueous cyclodextrin solutions was studied by following the fluorescence emission of the photoproducts formed. The same qualitative behavior as previously observed for the photodegradation quantum yield measurements was reproduced in the fluorescence studies in dry or wet methanol and acetonitrile. The yield for GAV photodegradation in water is smaller than in the organic solvents. Complexation of GAV to - and -cyclodextrins leads to an increase in the photodecomposition yield, especially during the early part of the photodecomposition kinetics. The transients formed in the photolysis of GAV in water are similar to those observed in acetonitrile, and the lifetime of triplet GAV in water is 130 ns. In addition, solvated electrons were formed when GAV was photolyzed in water. Complexation of GAV to cyclodextrins leads to an increase in the triplet yield compared to the radical yield and a moderate shortening of the triplet lifetime.Key words: -guaiacoxyacetoveratrone, cyclodextrin, excited singlet states, excited triplet states, radicals.

The dynamics of the micellization of Pluronic^® L64 (EO13PO30EO13, PEO/PPO = 2/3) was studied by employing the iodine laser temperature-jump technique. These studies show a more complex behavior than previously described. Three relaxation processes were observed, depending on the temperature at which experiments were performed. The first two of these relaxation processes are related to the unimer incorporation into micelles and the redistribution of micelles after incorporation of the unimers. The third relaxation process, which had not been previously observed, occurs at high temperatures and is related to the clustering of micelles into larger aggregates. In addition, the presence of impurities and hydrophobic components was shown for the first time to significantly affect the dynamics of L64 micellization.

The photochemistry of several methyl-substituted p-methoxy-β-phenylpropiophenones (1–5) has been examined by nanosecond laser flash photolysis using 337-nm laser excitation. The ketone triplets decay by intramolecular β-aryl quenching of the carbonyl in a well-known process that is believed to involve charge transfer. Surprisingly, methyl substitution at the β-methylene group, which is not a participant in the deactiviation mechanism, causes a dramatic decrease in the triplet lifetime; the effect is attributed to different conformational preferences in ketones 1–3and is rationalized on the basis of MMX calculations. Methyl substitution in the β-aryl ring affects the ease of oxidation of this ring and as a result induces changes in the kinetics for intramolecular charge transfer. This conclusion is further supported by electrochemical studies of ketones 1, 4, and 5. Keywords: laser flash photolysis, triplet ketones, intramolecular quenching, charge transfer.

The structural aspects for the complexation of xanthone and fluorenone to α- as well as β-cyclodextrins (CDxs) were explored by combining absorption, fluorescence, and induced circular dichroism studies with theoretical calculations of the optimized structures for the ketone−CDx complexes and their predicted ICD spectra. Detailed information on the similarities and differences for the complexes of these ketones with CDxs was obtained. For both CDxs, the ketones are bound to the rim of the cavity, and deeper penetration is observed for the complexes with β-CDx. In addition, a 1:2 complex involving α-CDx was only observed in the case of fluorenone. Although the two ketones have similar structures, their molecular recognition properties and the resulting structures of the CDx complexes show distinct differences.

The complexation behavior of 1-naphthyl-1-ethanol (1-NpOH) and 2-naphthyl-1-ethanol (2-NpOH) with β-cyclodextrin (β-CD) was studied by employing several spectroscopic techniques. In the case of 1-NpOH, only a complex with 1:1 stoichiometry is formed with β-CD, which has an equilibrium constant that is smaller than that observed for the 1:1 complex between β-CD and 2-NpOH. Excimer emission was observed in the presence of β-CD for solutions containing high 2-NpOH concentrations. This excimer emission was ascribed to a complex with 2:2 β-CD/2-NpOH stoichiometry. In addition, ¹H NMR data suggest that 2-NpOH is axially incorporated into the β-CD cavity. Only in the case of 2-NpOH was a broadening of the signals corresponding to the aromatic protons observed in the presence of β-CD. This broadening was attributed to the formation of the 2:2 complex. The dynamics of NpOH complexation was investigated by using the quenching methodology for triplet states. The entry rate constants for the 1:1 complex of 1-NpOH and 2-NpOH are (4.7 ± 1.9) × 10⁸ M^-1 s^-1 and (2.9 ± 1.6) × 10⁸ M^-1 s^-1, respectively, whereas the exit rate constants for the two compounds are (4.8 ± 1.8) × 10⁵ s^-1 and (1.8 ± 0.7) × 10⁵ s^-1. In the case of 2-NpOH, we were able for the first time to estimate the rate constant for the dissociation of a β-CD 2:2 complex ((0.2−2.5) × 10³ s^-1), showing that the dynamics for complexes including more than one cyclodextrin are remarkably slower than the dynamics observed for 1:1 complexes.

Elementary reactions have been studied quantitatively in the complex overall process catalyzed by horseradish peroxidase whereby isobutyraldehyde and molecular oxygen react to form triplet state acetone and formic acid. The rate constant for the reaction of the enol form of isobutyraldehyde with compound I of peroxidase is (8 +/- 1) X 10(6) M-1 s-1 and with compound II (1.3 +/- 0.3) X 10(6) M-1 s-1. Neither the enolate anion nor the keto form is reactive. The reactivity of enols with peroxidase parallels that of unionized phenols and a common mechanism is proposed. The overall catalyzed reaction of isobutyraldehyde and oxygen consists of an initial burst followed by a steady state phase. The burst is caused by the following sequence: 1) an initial high yield of compound I is formed from reaction of native enzyme with the autoxidation product of isobutyraldehyde, a peracid and 2) compound I rapidly depletes the equilibrium pool of enol which is present. After this burst a steady state phase is observed in which the rate-limiting step is the conversion of the keto to the enol form of the aldehyde catalyzed by phosphate buffer. The rate constant for the keto form reacting with phosphate is (8.7 +/- 0.6) X 10(-5) M-1 s-1. All constants were measured in dilute aqueous ethanol at 35 degrees C, pH 7.4, and ionic strength 0.67 M. Both the initial burst of light and the steady state emission from triplet acetone can be observed with the naked eye. Since the magnitude of the burst is a measure of the equilibrium amount of enol, the keto-enol equilibrium constant is readily calculated and hence also the rate constant for conversion of enol to keto. The keto-enol equilibrium constant is unaffected by phosphate which therefore acts as a true catalyst.

It has been established that the horseradish peroxidase/O2/isobutyraldehyde (IBAL) system leads to triplet acetone and formic acid formation followed by phosphorescence of the triplet acetone (see, for example, Bechara, E.J.H., Faria Oliveira, O.M.M., Durán, N., Casadei de Baptista, R., and Cilento, G. (1979) Photochem. Photobiol. 30, 101-110). In this paper many of the mechanistic details are established. The reaction is initiated by the autoxidation of IBAL to form the peracid (CH3)2CHC = O(OOH). The peracid converts horseradish peroxidase into compound I which in turn is converted into compound II by abstracting the alcoholic hydrogen atom from the enol form of IBAL. This creates a free radical with two resonance forms. (Formula: see text) Addition of molecular oxygen to the latter resonance form creates a peroxy radical which abstracts a hydrogen atom near the active site of the enzyme. The newly formed alpha-peroxide in turn forms a dioxetane-type of intermediate which rapidly decomposes into triplet acetone and formic acid. Compound II reacts with the enol by the same pathway as compound I. Thus native horseradish peroxidase is regenerated. The hydrogen atom abstraction near the enzyme active site may occur directly from ethanol, present to solubilize IBAL or from a group on the enzyme, in which case ethanol participates in a repair mechanism. Phosphate buffer is necessary because it catalyzes the keto-enol conversion of IBAL. Thus horseradish peroxidase participates in a normal peroxidatic cycle. The only chain reaction is the uncatalyzed autoxidation of IBAL, most of which occurs prior to the mixing of IBAL with the oxygenated horseradish peroxidase solution.

Fluorescence quenching by nitromethane of pyrene-labeled poly(N-isopropylacrylamides) (PNIPAMs) which contain alkyl chains was employed to investigate the accessibility of the quencher to the pyrene moieties. The quenching efficiency for monomeric pyrene or excimers was decreased in PNIPAMs when compared to the quenching efficiency of pyrene fluorescence in the aqueous phase, showing that pyrene is always located within the polymeric micelles. The contribution of static quenching was evaluated by comparing the quenching efficiency determined from steady-state measurements with that obtained from time-resolved data. Static quenching was present for all PNIPAMs studied. The decrease of the quenching efficiency was related to the polymeric micelle structure. The highest degree of protection was observed for the excimer emission of the polymer containing randomly-substituted alkyl chains and with the pyrene bound to the same polymeric unit as the alkyl chain.

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