Effects of electrostatic and hydrophobic interactions on the chromatographic behavior of biopolymers in HPLC columns

The effect of electrostatic and hydrophobic interactions on the chromatographic behavior of biopolymers with the use of chemically bonded silica-based HPLC columns and aqueous buffered mobile phases containing neutral salts in a wide range of concentration is discussed. Two columns packed with stationary phases appositely designed for biopolymer HPLC in size exclusion and anion exchange mode, respectively, are examined. Experimental data are evaluated by plotting the measured isocratic elution volumes of several standard proteins of different isoelectric point against the salt concentration in the mobile phase. Depending on the concentration and nature of salt, both columns exhibit different domains where either sieving effect or electrostatic or hydrophobic interactions are predominant. At sufficiently low salt concentrations electrostatic interactions are predominant leading to either increasing or decreasing elution volumes depending on the sign of the charges on the stationary phase and the protein, respectively. On the other hand, at high salt concentrations of a salt with sufficiently high molal surface tension increment proteins may be retained by hydrophobic interactions.     

The impact of the discovery of stereoregular polymers in macromolecular science

The paper recalls some aspects of the fascinating history of the discovery by Giulio Natta and his coworkers of the stereospecific polymerization of α-olefins and of the determination of the structure of synthetic stereoregular polymers. The most relevant recent advances in the comprehension of the mechanisms regulating their synthesis with heterogeneous supported catalysts and with the newly discovered homogeneous metallocene-based catalysts are briefly outlined.     

Molecular mechanics and mechanisms of regulation of the stereospecificity in Ziegler-Natta catalysis

The model catalytic sites, proposed in our group for the homogeneous and heterogeneous stereospecific Ziegler- Natta polymerizations of olefins, are reviewed. For all the homogeneous metallocene-based as well as for the heterogeneous catalytic models, a common mechanism of enantioselectivity is indicated by the study of the non-bonded interactions. This mechanism of enantioselectivity, which involves a chiral orientation of the growing chain, is in agreement with a large number of experimental results for these catalytic systems. The model sites for the homogeneous isospecific polymerization of propene are also able to account for the observed enantioselectivities in the regioirregular placements. Recent calculations relative to a peculiar catalytic model site, which should present a polymerization mechanism involving a regular back-skip of the chain, to the starting position after each monomer insertion, are shortly reviewed. The relevance of this polymerization mechanism to the comprehension of the stereospecificity of some homogeneous and heterogeneous catalytic systems is also briefly discussed.     

Dansylated Polyamines as Fluorescent Sensors for Metal Ions: Photophysical Properties and Stability of Copper(II) Complexes in Solution

Protonation and the Cu^II complexation constants of the dansylated polyamines N‐dansylethylenediamine ( 1 ), N‐dansyldiethylenetriamine ( 2 ), N‐dansyltriethylenetetramine ( 3 ), N′‐[2‐(dansylamino)ethyl]diethylenetriamine ( 4 ), and tris(2‐dansylaminoethyl)amine ( 5 ) were determined by glass‐electrode potentiometry in MeOH/H₂O 9 : 1 (v/v) solution. For ligands 3 and 4 , the determinations were also performed in aqueous solution. The complexes formed by these ligands in neutral form correspond to those observed for the analogous unsubstituted monoprotonated amines, whereas, when the ligands are deprotonated at the sulfonamide moiety, the species parallel those of the corresponding amines. The molecular structures of the complexes were deduced from the VIS absorption spectra. The crystal structure of the [CuL₂H₋₂] complex 6 of ligand 1 (L) was determined by X‐ray diffraction. The study of the photophysical properties of the ligands 3 – 5 showed that they are good fluorescent sensors for copper(II), which induced fluorescence quenching. Time‐resolved fluorescence measurements allowed us to clarify the sensing mechanism. The pH dependence of the quenching effect demonstrated that it occurs for all Cu²⁺ complexes, even for species in which the sulfonamide moiety is not deprotonated. Sensing of Cu²⁺ was compared with that of other metal ions (Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺), and selectivity was studied as a function of pH. Ligands 3 and 4 were found to be selective chemosensors for Cu²⁺ in weakly acidic solution (pH ca. 4 – 5).