Relaxations in poly(vinyl alcohol) and in poly(vinyl acetate) detected by fluorescence emission of 4-aminophthalimide and prodan

Steady-state and time-resolved emission spectroscopy (TRES) of the medium-sensitive probes 4-aminophthalimide (4-AP) and 6-propionyl-2-(dimethylamino)naphthalene (Prodan) were performed at 77 and 298 K in vacuum-sealed thin films of poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc). The two probes show similar red-edge effect in steady state emission and a red shift with time in TRES in PVA. In PVAc the red shifts are much smaller and the spectral shift for 4-AP is slower. 4-AP locates in highly polar environments in PVA, where H-bond interaction with the polymer is important. Prodan locates in less polar environments, as evidenced by the position of the emission maximum with respect to reference solvents. Consequently, the observed monoexponential spectral red shift with time of 4-AP in PVA and in PVAc is attributed to relaxation of the interaction of the probe with the hydroxy and acetate moieties, respectively. The more intense interaction of the lighter -OH moiety with the probes explains the greater and faster spectral shift observed in PVA compared to PVAc. The lifetime of this monoexponential spectral shift is independent of temperature in PVA and takes place with a highly negative activation entropy. This fact is attributed to a collective rearrangement of -OH groups to better interact with the excited state. This relaxation nevertheless does not account for the complete accommodation of the excited state. Prodan shows a linear variation of the spectral shift with time that can be explained by microheterogeneity. In PVA, the width at half-maximum of the emission spectra does not change with time for Prodan and it decays with a lifetime similar to the lifetime of the spectral shift in the case of 4-AP. The differences in the behavior of the probes are attributed to their different average location in the polymer matrix.     

Rigid spacer-mediated synthesis of bis-spiroketal ring systems: stereoselective synthesis of nonsymmetrical spiro disaccharides

The application of the "rigid spacer-mediated linkage between nonreacting centers" concept to the preparation of nonsymmetrical bis-spiroketal structures is demonstrated by the stereoselective synthesis of the bis-spiro fructodisaccharide 1, a minor component of industrial caramels. An o-xylylene bridge has been used to limit the conformational space during the intramolecular glycosylation-spirocyclization reaction of a difructopyranose precursor, thus controlling both the ring size and the stereochemistry at the spiro centers. [Structure: see text]     

Electrophilic properties of nitroheterocyclic compounds. Potential hypoxic cells radiosensitizers

Investigation of the reduction potential and calculation of the partition coefficient n-octanol/water allow the assessment of the potential suitability of nitropirydine N-oxide compounds in radiotherapy of cancer. Experiments were carried out using cyclic voltammetry with HMDE as working electrode. The electrode reduction of the investigated compounds is quite irreversible and strongly dependent on pH.     

The stereocontrolled total synthesis of altohyrtin A/spongistatin 1: fragment couplings, completion of the synthesis, analogue generation and biological evaluation

The antimitotic marine macrolide altohyrtin A/spongistatin 1 has been synthesised in a highly convergent and stereocontrolled manner, thus contributing to the replenishment of the largely exhausted material from the initial isolation work. Coupling of the AB- and CD-spiroacetal subunits by a stereoselective aldol reaction was achieved by using either a lithium (67 : 33 dr) or boron enolate (90 : 10 dr). A highly (Z)-selective Wittig coupling was used to unite the northern hemisphere aldehyde with the southern hemisphere phosphonium salt . Deprotection and subsequent regioselective macrolactonisation on a triol seco-acid completed the synthesis of altohyrtin A. Two structural analogues were also prepared and evaluated as growth inhibitory agents against a range of human tumour cell lines, including Taxol-resistant strains, alongside altohyrtin A and paclitaxel (Taxol), revealing that dehydration in the E-ring is tolerated and results in enhanced cytotoxicity (at the low picomolar level), whereas the presence of the full C44-C51 side-chain appears to be crucial for biological activity.     

Identification of smoked Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) using PCR-restriction fragment length polymorphism of the p53 gene

Raw and smoked samples of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) were identified using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) of the p53 gene. DNA from S. salar and 0. mykiss was amplified by using primers flanking exons 5 to 6 of the p53 nuclear gene. PCR products of different length were obtained for each species (532 and 518 base pairs, respectively). Sequences of PCR products obtained from S. salar and O. mykiss were compared in the search for polymorphic restriction sites. The restriction fragments obtained with Eco RV, Hinf I, and Taq I endonucleases showed interspecific polymorphism, making it a useful method for identification of Atlantic salmon and rainbow trout.     

Construction of Heterometallic Cubanes

Incorporation of M(CO)(3) fragments by trinuclear Ti complexes [{Ti(3)Cp(μ(3)-CR)}(μ-O)(3)] and [{Ti(3)Cp(μ(3)-N)}(μ-NH)(3)] (Cp*=eta(5)-C(5)Me(5)) leads to the formation of an unprecedented class of heterometallic clusters with cubane structure [e.g., Eq. (a)]. Density functional calculations on these complexes indicate the existence of electron delocalization in the Ti(3)M cores (M=Cr, Mo, W).     

Direct measurement of homovanillic, vanillylmandelic and 5-hydroxyindoleacetic acids in urine by capillary electrophoresis

Separation conditions in CE, with a neutral coated capillary and reversed polarity, have been optimised to make direct measurement of vanillylmandelic acid, homovanillic acid and 5-hydroxyindoleacetic acid possible in urine samples without pre-treatment. The method developed has been validated, presenting adequate parameters for linearity, accuracy and precision. Detection limits range from 0.03 to 2.5 microM. Finally the method has been applied to urine samples taken from patients, both adults and children, in hospital. Some of them were also measured by immunoassay and HPLC-electrochemical detection and results have been compared.     

Method development for the determination of anthocyanins in red wines by high-performance liquid chromatography and classification of German red wines by means of multivariate statistical methods

A novel, positively charged, copolymer of allylamine and 2-hydroxy-3-methacryloyl-beta-cyclodextrin was synthesized to be used as a chiral selector in capillary electrophoresis. In the copolymer, cyclodextrin molecules are spaced from the backbone though a spacer arm which prevents sterical hindrance of the CD cavity. The self-mobility of the CD polymer in its charged form, opposite to the analytes, is the cause for the enhanced separation factor provided by this selector. Moreover, the positive charged polymer induces a reversal of electroosmotic flow which is beneficial in enantioseparations of acidic compounds as it reduces analysis time and increases peak efficiency. The ability of this copolymer to act as a CE chiral selector in the separation of 2,4-dinitrophenylamino acid enantiomers was investigated in coated and uncoated capillaries and its performance was much better then that of native beta-cyclodextrin.     

Dynamics of the Electric Double Layer: Analysis in the Frequency and Time Domains

No rigorous theory of electrokinetic phenomena is conceivable without properly accounting for double layer polarization under the action of external fields. Since processes leading to such polarization need a finite time to develop, an analysis of the behavior of the quantities of interest (potential and ion concentration profiles, particle or fluid velocity, and so on) as a function of time should be extremely illustrative. In this work, we analyze how those quantities evolve in the nanosecond to microsecond time range after the application of an electric field. The network method is proposed (in which, essentially, an electric circuit simulator program is used to solve the differential equations involved, after their proper interpretation in terms of fluxes and forces) to gain information about the evolution with time of the potential, counterion, and co-ion perturbations, the particle velocity, and the fluid velocity profile. The performance of the method is first ckecked in the frequency domain, for which rigorous solutions exist, and then the procedure is used in the time domain. Reasons are discussed for the observed time dependencies of the analyzed quantities. Copyright 2000 Academic Press.