The Doi−Edwards (DE) theory for the rheological properties of entangled polymer melts and solutions successfully predicts the response to large step-shear strains but fails to predict other nonlinear shear properties, such as the steady-state viscosity or the relaxation of stress after cessation of steady shearing. Many of these failures remain even in the extension of the theory by Marrucci and Grizzuti (Gazz. Chim. Ital. 1988, 118, 179) to allow deformation-induced “tube stretch”. Here, we find that a much more successful theory can be obtained by also accounting for “convective constraint release”, i.e., the loss of entanglement constraints caused by the retraction of surrounding chains in their tubes. (Marrucci, G. J. Non-Newtonian Fluid Mech. 1996, 62, 279 and Ianniruberto, G.; Marrucci, G. J. Non-Newtonian Fluid Mech. 1996, 65, 241). , In the molecular model developed here, convective constraint release can both shorten the reptation tube and allow reorientation of interior tube segments. The revised model predicts many of the features of steady and transient shearing flows. These include a region of nearly constant steady-state shear stress at shear rates between the inverse zero-shear reptation time and the inverse Rouse time, similar to that seen in the experiments of Bercea et al. (Macromolecules 1993, 26, 7095) and also predicted by Marrucci and Ianniruberto (Macromol. Symp. 1997, 117, 233). The predictions of transient stresses after startup and cessation of shear are also in good agreement with experiments, as are predictions of nonmonotonicity in the extinction angle after stepup or stepdown in shear rate.
We present a "slip-link" model for relaxation of entangled star polymers that accounts for chain-end fluctuations and constraint release and that explains deviations from the "dynamic dilution" equation observed in recent dielectric and stress relaxation data. In the terminal regime where tube expansion fails to keep up with chain relaxation, relaxation is controlled by rare events in which newly created entanglements near the branch point draw the chain end towards the last remaining old entanglement, where a shallow fluctuation releases it.
The chromosomal DNAs of eight medically important Candida species, C. albicans, C. stellatoidea, C. tropicalis, C. parapsilosis, C. krusei, C. guillierrnondii, C. kefyr and C. glabrata, were analysed by pulsed-field gel electrophoresis under various conditions. The corresponding bands in the gels were assigned by three kinds of DNA probe which hybridized to DNA of all the species: rDNA, TUB2 and PEP4. The best conditions for separating the chromosomal DNAs were investigated and the numbers and molecular sizes of the chromosome bands were determined for each species. The chromosomal DNAs of the species were separated into 5-14 bands ranging in size from 0-5 to 4-5 Mb. Based on the quantification of the chromosome band intensities using a laser fluorescent gel scanner, the chromosome numbers were estimated. The apparent average total number of chromosomes per cell was 16 for C. albicans, 16 for C. stellatoidea, 12 for C. tropicalis, 14 for C. parapsilosis, 8 for C. krusei, 8 for C. guilliermondii, 18 for C. kefyr, and 14 for C. glabrata; the total chromosomal DNA size of each species per cell was calculated at about 31 Mb, 33 Mb, 31 Mb, 26 Mb, 20 Mb, 12 Mb, 29 Mb and 14 Mb, respectively.
The chitin synthase 3 gene (CACHS3) has been cloned from Candida albicans. The yeast CAL1 gene encoding the chitin synthase 3 of Saccharomyces cerevisiae was used as a probe for the isolation of the gene from C. albicans. The CAL1 homolog was identified in Southern blots of C. albicans genomic DNA and cloned from a C. albicans genomic DNA library. The nucleotide sequences of two partial clones were determined and combined giving a total length of 4610 bp. A continuous open reading frame of 3525 bp encoding a predicted protein of 1175 amino acids and molecular mass of 131 850 daltons was identified. A comparison of the deduced amino acid sequences of CAL1 and the Candida chitin synthase 3 protein showed 59.3% identity. Southern blot analysis indicates that the CACHS3 gene is present in a single copy in the genome and maps to chromosome I. Northern blot analysis shows that expression of chitin synthase 3 gene is dramatically increased during the transition from the yeast to the hyphal form of C. albicans. This change in transcription level strongly suggests that CACHS3 may play a role in Candida morphogenesis.
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