Maozi Liu

The observation of the local ordering characteristics of spermidine-condensed DNA: atomic force microscopy and polarizing microscopy studies.

Condensation of DNA by multivalent cations can provide useful insights into the physical factors governing the folding and packaging of DNA in vivo. In this work, local ordered structures of spermidine-DNA complexes prepared from different DNA concentrations have been examined by using atomic force microscopy (AFM) and polarizing microscopy (PM). Two types (I and II) of DNA condensates, significantly different in sizes, were observed. It was found that for extremely dilute solutions (DNA concentrations around 1 ng/microl or below), the DNA molecules would collapse into toroidal structures with a volume equivalent to a single lambda-DNA (type I). In relatively dilute solutions (DNA concentrations between 1 and 10 ng/microll), a significantly larger structure of multimolecular toroids (circular and elliptical, type II) were formed, which were constructed by many fine particles. Measurements show that the average diameter of these fine particles was similar to the outer diameter of the monomolecular toroids observed in extremely dilute solutions, and the thickness of the multimolecular toroids had a distribution of multi-layers with height increments of 11 nm, indicating that the multimolecular toroidal structures have lamellar characteristics. Moreover, by enriching the DNA-spermidine complexes in very diluted solution, branch-like structures constructed by subunits were observed by using AFM. The analysis of the pellets in polarizing microscopy reveals a liquid-crystal-like pattern. These observations suggest that DNA-spermidine condensation could have multiple stages, which are very sensitive to the DNA and spermidine concentrations.

Effect of humidity on the surface adhesion force of inorganic crystals by the force spectrum method

Effect of relative humidity on the surface adhesion force of several inorganic crystals of mica, CaF2 and KCl was studied by atomic force microscopy (AFM). The results showed that the magnitude of surface adhesion force is mainly dependent on the surface free energy of the adsorbed liquid film, but almost independent of the thickness of the film. Furthermore, the deliquescence on the crystal surface was investigated, which demonstrated the capability of the force spectrum method to monitor changes in ionic concentrations of adsorbed liquid film in real-time.

Formation of domain structure of erythrocyte membrane in Wistar rat fed with CeCl3 per os

To explore the possibility of absorption of lanthanides via digestive duct and their effects on the membrane structure and permeability of erythrocytes, the fine structure of erythrocyte membrane from Wistar rats, fed for 70 days of daily administrationper os with 20 mg CeCl3/kg weight, was imaged by means of atomic force microscopy and FT-IR deconvolution spectra. The results show that, although the erythrocytes maintain the intact shape, the change of secondary structure, aggregation and crosslinking of the protein particles of membrane surface and the enlarged lipid regions lead to the domain structure formation. This structure might be responsible for the increasing permeability of erythrocyte membrane.

Influence of loop sequence on relative stability of bimolecular triplex DNA

The structures of 20 bimolecular triplexes have been built and simuleted by molecular mechanics. The sequence of pyrimidine strand is 5’-dTTCTTTC-L1 TTTI-3CTTTTcTT-3, where the five nucleotides underlined cornpaw Ioop sequences. L1 and L5 represent varied residues. The sequences of purine strands are 5′-dGAAAAGAA-3′ and the reversed orientation 5′-dAAGAAAG-3′. The influence of different loop sequences and compositicm on the relative stability of twenty triplexes have bem energetically analyzed. The results indicate that 5′-loop triplexes are more stable than 3′-loop ones and the stacking interaction of purines with their adjacent basw are stronger than that of pyrimidines. The stability of triplexes is mainly determined by the first and last nucleotides in the loop.