Mechanism and prediction of retention of oligomers in normal-phase and reversed-phase HPLC

Summary The simultaneous dependence of the retention in oligomeric series on the number of repeat structural units and on the mobile phase composition may be described by very similar equations for reversed-phase and for normal-phase systems.In reversed-phase systems, the separation selectivity of the individual oligomers is determined mainly by the size and by the polarity of the repeat structural unit, but the influence of a bulky and polar structural residue may also become important so that even reversed order of elution may be observed for oligomeric series with the same oligomeric units but significantly different end groups. For example, oligoethylene glycols are eluted in the order of increasing size of the oligomers, whereas ethoxylated nonylphenols are eluted in the order of decreasing size.In normal-phase systems, the separation selectivity in oligomeric series depends on the adsorption energy and on the adsorbed area of the oligomeric unit. If the oligomeric unit is small, the concentration of the polar solvent in the binary organic mobile phase has only a minor effect on retention and selectivity, which may be controlled by taking account of the nature of the adsorbent and of the polar solvent or by varying the proportion of two polar solvents in a ternary mobile phase.     

Autoxidation of giant extracellular hemoglobin of Glossoscolex paulistus: molecular mechanism and oligomeric implications

Giant extracellular hemoglobins present high redox stability due to their supramolecular architecture, high number of polypeptide chains and great compaction of protein subunits. The oligomeric assembly and the changes in the polypeptidic structure can influence the autoxidation rate of the heme proteins, being that different nucleophiles can act in this process due to pH alterations. In the present work, we have studied the autoxidation rate of whole Glossoscolex paulistus (HbGp) giant extracellular hemoglobin, as well as the autoxidation rate of the isolated d monomer of HbGp studied regarding pH variations. The kinetic decay behavior is dependent on pH, presenting mono-exponential or bi-exponential character, depending on the oligomeric state of the protein. Thus, the oligomeric dissociation in specific pH values demonstrated a bi-exponential kinetic decay. A mono-exponential kinetic behavior was verified in the pH range of 5.9–7.3, which is assigned to the native whole protein. In alkaline medium, the presence of hydroxide ions leads the autoxidation of whole hemoglobin to a complex behavior, which is described by the combination of two first-order kinetics. The slow process occurs due to the d monomer autoxidation. At pH 7.0, the kinetic is mono-exponential, indicating a highly conserved oligomeric structure. In acid medium, the proton-catalyzed autoxidation occurs both on the whole hemoglobin and in the d monomer. It has been found that proximal and distal histidines develop determinant roles regarding the autoxidation rate, being that the distal histidine controls the contact of ligands with the ferrous center through a very interesting “swinging door” mechanism. Despite the significant sensitivity of the distal histidine to the presence of protons, water molecules and anions, the influence of chemical changes around the heme, such as pH changes, is much more effective in hemoproteins without this amino acid as distal residue. This fact denotes the ability of HbGp to adapt to environmental disturbances caused by the presence of the distal histidine, which is responsible for the great redox and oligomeric stabilities encountered in HbGp.     

两亲性寡聚体Gd(Ⅲ)配合物的合成及肝选择性磁共振造影

小分子造影剂在体内存留的时间较短,被注射到体内后,在较短的时间内就会排出体外,所以在进行造影检查时,需要多次注射造影剂．一般大分子造影剂,比小分子造影剂[如二乙三胺五乙酸(DTPA)钆的配合物等]有更好的弛豫效能,而且大分子造影剂在体内停留的时间较长,能满足造