Crystalline architecture

An alternative route for the generation of enantiopure oligopeptides has been elucidated recently by our group. The method comprises the self-assembly of racemic or non-racemic thio-esters or N-carboxyanhydrides of a-amino acids into either 2-D or 3-D crystalline architectures followed by lattice-controlled reactions. 

Two-dimensional (2-D), two-component systems composed of Pcs and porphyrins self-organized by UHV techniques on Au(lll) have also been reported [188-192], It is interesting to note that, in some cases, depending on the substituent groups on the Pc and the porphyrin systems, the intermolecular interactions between these macrocycles give rise to a 2-D crystalline architecture that presents a higher stability than that of the films resulting from either parent compound [188],... 

Rapaport, H., Kuzmenko, I., Kjaer, K., Als-Nielsen, J., Weissbuch, I., Lahav, M., Leiserowitz, L. Crystalline architectures at the air-Uquid interface from nucleation to engineering, Synchrotron Radiation News. 12 (1999) 25-33. 

To date, PTFE and PET (Dacron ) have been the two most commonly used polymeric biomaterials for large blood vessel substitution [61,143], PTFE has a crystalline architecture with tensile strength of 14 MPa and Young s modulus of around 0,5 GPa [60], PTFE was first introduced into clinical practice in 1970s for lower extremity bypass... 

Absolute Asymmetric Polymerization within Crystalline Architectures Relevance to the Origin of Homochirality... 

I S "Absolute" Asynmietric Polymerization within Crystalline Architectures... 

The asymmetric polymerization in crystalline architectures provides an excellent environment to conduct the absolute asymmetric synthesis of polymers, and also provides an effident route for the ampHfication of chirality. Mirror-symmetry breaking might occur either through total asymmetric transformations, either in enantiomorphous crystals that have self-assembled from achiral molecules, or within racemic crystalline architectures which are delineated by chiral rims or surfaces when one of the chiral faces is blocked by an interface. The self-assembly of nonracemic mixtures into a mixture comprising eutectic compositions of a racemic compound and an enantiomorphous assembly, followed by asymmetric transformation, provides a series of thermodynamically controlled, alternative routes for the effident ampHfication of homochirality. 

Point defects are imperfections in the actual crystalline architecture as compared to the ideal lattice in which each atom site is filled with the appropriate element. They can influence properties at extremely low levels (ppb or ppm). Typical point defects include crystal sites with missing atoms (vacancies), atoms positioned in sites that are not filled by the crystal structure in question (interstitials), crystal sites containing impurity atoms (dopants), and cations with different oxidation states. Because mass. 

Concerning the thermal properties, the melting temperature reduced with the peroxide content for both the propylene homopolymers and the copolymers. Fiowever, the crystalline degree remained constant with the peroxide addition for the propylene homopolymers but reduced slightly for the copolymers. The crosslinks produced in the elastomeric phase may disturb the crystallinity architecture of the copolymers. 

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