Single-center macromolecules

Galimberti, M., Piemontesi, F., Fusco, O., Camurati, L, and Destro, M., Ethene/ Propene Copolymerization with High Product of Reactivity Ratios from a Single Center, Metallocene-Based Catalytic System, Macromolecules, 31, 3409 (1998). 

Fait, A. Resconi, L. Guerra, G Corradini, P. A possible interpretation of the nonlinear propagation rate laws for insertion polymerizations A kinetic model based on a single-center, two-state catalyst. Macromolecules 1999,32, 2104-2109. 

Another approach, that we think may be useful in this field, is that of the single-center polymeric catalysts. In this case, observation is restricted to a localized region of the macromolecule, and if the catalytic site has good chromophoric properties, different from... 

Chirality is a pervasive property of an object, which means that in theory, a single remote asymmetric center in a macromolecule is enough to make the entire molecule chiral and, in principle, even the more distant residue could sense the asymmetry induced by the stereogenic center. On the contrary, experiences maturated by synthetic chemists in the construction of molecular species for enantioselective recognition speak for the necessity of placing the asymmetric units in close contact to allow chiral sensing and discrimination. The latter, in fact, arises from attractive forces and steric interactions that require close contact between the counterparts. On the contrary, magnetic asymmetry is not a direct consequence of weak interactions, but is more a property of the space which surrounds a chiral object. 

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

The presence of a single type 2 center in ascorbate oxidase is not consistent with the proposed concept of a quaternary structure composed of two identical subunits afi (25). On the other hand, all the multicopper oxidases described in the literature (5-8) have only one type 2 center per active molecule. Additional copper with type 2 characteristics can be bound by the macromolecule during isolation and purification (19). A close examination of the EPR spectra presented by Lee and Dawson (9) indicates the presence of so-called nonspecific copper with large hyperfine splittings at g. As expected, the ratio A330/A610 is approximately 1.5-2 for these preparations (estimated from Figure 2 in Ref. 9). 

In principle, the crystallization of a protein, nucleic acid, or virus (as exemplified in Figure 2.2) is little different than the crystallization of conventional small molecules. Crystallization requires the gradual creation of a supersaturated solution of the macromolecule followed by spontaneous formation of crystal growth centers or nuclei. Once growth has commenced, emphasis shifts to maintenance of virtually invariant conditions so as to sustain continued ordered addition of single molecules, or perhaps ordered aggregates, to surfaces of the developing crystal. 

The semiclassical Marcus equation can be applied to electron transfer between spatially fixed and oriented redox centers (26). Values obtained by employing this theoretical framework are in good agreement for both wild type and single-site mutants of Pae azurin (15), thus supporting the applicability of this analysis of the LRET process. The mechanism of intramolecular electron transfer through matrices of biological macromolecules, mainly proteins, has attracted considerable current interest (25). Moreover, the question of whether... 

Up to now, it has been assumed that the solvent in the sedimentation experiment consists of a single component. If, however, the solvent system consists of a mixture of two substances of widely different densities (e.g., CsCl in water or mixtures of benzene and CBr4), the solvent components will sediment to different extents. At equilibrium, the solvent system possesses a density gradient. One density p , applies at the bottom of the cell, and another, pm, at the meniscus. The density of the solute, pi, should lie between these two densities (pm < pi < pt). The macromolecules will then sediment from the meniscus toward the base of the cell and float from the base toward the meniscus (Figure 9-14). At equilibrium, the macromolecules will take up a position (designated by ) at which the density pg exactly corresponds to the density of the macromolecule in solution (pg = pi 1 / V2). This position is at a distance r from the center of rotation. 

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