Species-specific long range interactions between receptor/ligand pairs.

Total internal reflection microscopy (TIRM) monitors Brownian fluctuations in elevation as small as 1 nm by measuring the scattering of a single sphere illuminated by an evanescent wave when the sphere is levitated by colloidal forces such as electrostatic double-layer repulsion. From the Boltzmann distribution of elevations sampled by the sphere over time, the potential energy profile can be determined with a resolution of approximately 0.1 of the thermal energy kT. Thus, the interaction between a receptor-coated (goat, horse, or rabbit immunoglobulin G (IgG)) latex sphere and a protein A (SpA)-coated glass microscope slide was studied. A typical TIRM potential energy profile measured between a bare sphere and a bare glass plate, where the sphere fluctuates around the secondary potential energy minimum formed between double-layer repulsion and gravitational attraction, agrees well with DLVO theory. The interactions measured between IgG-coated spheres and SpA-coated slides, on the other hand, displayed a weaker repulsion compared with that observed between bare surfaces under the same conditions. Analysis of the results obtained between the coated surfaces suggests an additional attractive force. The decay length of this attraction correlates with the known dissociation constants for the binding of IgG with SpA in free solution.

Radiocarbon dating using electrostatic accelerators: negative ions provide the key.

Mass spectrometric methods have long been suggested as ways of measuring (14)C/(12)C ratios for carbon dating. One problem has been to distinguish between (14)N and (14)C. With negative ions and a tandem electrostatic accelerator, the (14)N background is virtually absent and fewer than three (14)C atoms in 10(16) atoms of (12)C have been easily measured.