We report evidence that gene complexes, consisting of polycations and plasmid DNA enter cells via binding to membrane-associated proteoglycans.

We have imaged native rat tail and reconstituted bovine dermal type I collagen by atomic force microscopy, obtaining a level of detail comparable to that obtained on the same samples by transmission electron microscopy. The characteristic 60-70 nm D periodicity consists of ridges exhibiting high tip-sample adhesion alternating with 5-15-nm-deep grooves having low adhesion. We also observe an intraperiod or "minor" band consisting of 1-nm-deep grooves, and "microfibrils" arranged parallel to or inclined approximately 5 degrees to the fibril axis. In air collagen fibrils exhibit negligible compression under the forces exerted by the tip. When immersed in water the subfibrillar features disappear and the fibrils become softer, compressing by 5% of their height under an 11-nN force. Material on the surface of the sample sometimes accumulates on the atomic force microscope tip; contrary to expectation such tip contamination can improve as well as reduce resolution.

The challenge of the Human Genome Project is to increase the rate of DNA sequence acquisition by two orders of magnitude to complete sequencing of the human genome by the year 2000. The present work describes a rapid detection method using a two-dimensional optical wave guide that allows measurement of real-time binding or melting of a light-scattering label on a DNA array. A particulate label on the target DNA acts as a light-scattering source when illuminated by the evanescent wave of the wave guide and only the label bound to the surface generates a signal. Imaging/visual examination of the scattered light permits interrogation of the entire array simultaneously. Hybridization specificity is equivalent to that obtained with a conventional system using autoradiography. Wave guide melting curves are consistent with those obtained in the liquid phase and single-base discrimination is facile. Dilution experiments showed an apparent lower limit of detection at 0.4 nM oligonucleotide. This performance is comparable to the best currently known fluorescence-based systems. In addition, wave guide detection allows manipulation of hybridization stringency during detection and thereby reduces DNA chip complexity. It is anticipated that this methodology will provide a powerful tool for diagnostic applications that require rapid cost-effective detection of variations from known sequences.

The scanning tunnelling microscope (STM) has been used to visualize DNA under water, under oil and in air. Images of single-stranded DNA have shown that submolecular resolution is possible. Here we describe atomic-resolution imaging of duplex DNA. Topographic STM images of uncoated duplex DNA on a graphite substrate obtained in ultra-high vacuum are presented that show double-helical structure, base pairs, and atomic-scale substructure. Experimental STM profiles show excellent correlation with atomic contours of the van der Waals surface of A-form DNA derived from X-ray crystallography. A comparison of variations in the barrier to quantum mechanical tunnelling (barrier-height) with atomic-scale topography shows correlation over the phosphate-sugar backbone but anticorrelation over the base pairs. This relationship may be due to the different chemical characteristics of parts of the molecule. Further investigation of this phenomenon should lead to a better understanding of the physics of imaging adsorbates with the STM and may prove useful in sequencing DNA. The improved resolution compared with previously published STM images of DNA may be attributable to ultra-high vacuum, high data-pixel density, slow scan rate, a fortuitously clean and sharp tip and/or a relatively dilute and extremely clean sample solution. This work demonstrates the potential of the STM for characterization of large biomolecular structures, but additional development will be required to make such high resolution imaging of DNA and other large molecules routine.

The reticuloendothelial system of mice bearing EMT6 tumors was effectively blocked by intravenous injections of small unilamellar vesicles that incorporated a 6-aminomannose derivative of cholesterol in the lipid bilayer. Neutral liposomes loaded with indium-111-nitrilotriacetic acid were then injected. Fifty percent more radioactivity was deposited in tumors of the animals with blocked reticuloendothelial systems than in controls. Twenty-four hours after the injection of radioactive vesicles, well-defined tumor images were observed in whole-body gamma camera scintigraphs. Biodistribution studies showed that tumors from animals with blocked reticuloendothelial systems had more than twice the radioactivity per gram than any other tissue analyzed.