The diphenylalanine peptide, the core recognition motif of the beta-amyloid polypeptide, efficiently self-assembles into discrete, well-ordered nanotubes. Here, we describe the notable thermal and chemical stability of these tubular structures both in aqueous solution and under dry conditions. Scanning and transmission electron microscopy (SEM and TEM) as well as atomic force microscopy (AFM) revealed the stability of the nanotubes in aqueous solution at temperatures above the boiling point of water upon autoclave treatment. The nanotubes preserved their secondary structure at temperatures up to 90 degrees C, as shown by circular dichroism (CD) spectra. Cold field emission gun (CFEG) high-resolution scanning electron microscope (HRSEM) and thermogravimetric analysis (TGA) of the peptide nanotubes after dry heat revealed durability at higher temperature. It was shown that the thermal stability of diphenylalanine peptide nanotubes is significantly higher than that of a nonassembling dipeptide, dialanine. In addition to thermal stability, the peptide nanotubes were chemically stable in organic solvents such as ethanol, methanol, 2-propanol, acetone, and acetonitrile, as shown by SEM analysis. Moreover, the acetone environment enabled AFM imaging of the nanotubes in solution. The significant thermal and chemical stability of the peptide nanotubes demonstrated here points toward their possible use in conventional microelectronic and microelectromechanics processes and fabrication into functional nanotechnological devices.

An atomic force microscope (AFM) has been used to directly monitor specific interactions between antibodies and antigens employed in an immunoassay system. Results were achieved using AFM probes functionalized with ferritin, and monitoring the adhesive forces between the probe and anti-ferritin antibody-coated substrates. Analysis of the force distribution data suggests a quantization of the forces, with a period of 49 +/- 10 pN. This periodic force may be attributed to single unbinding events between individual antigen and antibody molecules. These results demonstrate that the AFM could be employed as an analytical tool to study the interactions between the molecules involved in biosensor systems. The potential of the technique to provide information relating to the manner in which the antibody molecule binds to its specific antigen is also discussed.

Primosomal protein cascades load the replicative helicase onto DNA. In Bacillus subtilis a putative primosomal cascade involving the DnaD-DnaB-DnaI proteins has been suggested to participate in both the DnaA and PriA-dependent loading of the replicative helicase DnaC onto the DNA. Recently we discovered that DnaD has a global remodelling DNA activity suggesting a more widespread role in bacterial nucleoid architecture. Here, we show that DnaB forms a "square-like" tetramer with a hole in the centre and suggest a model for its interaction with DNA. It has a global DNA remodelling activity that is different from that of DnaD. Whereas DnaD opens up supercoiled DNA, DnaB acts as a lateral compaction protein. The two competing activities can act together on a supercoiled plasmid forming two topologically distinct poles; one compacted with DnaB and the other open with DnaD. We propose that the primary roles of DnaB and DnaD are in bacterial nucleoid architecture control and modulation, and their effects on the initiation of DNA replication are a secondary role resulting from architectural perturbations of chromosomal DNA.