Figure 7. (A) BSI shows the difference in binding affinities for free-solution and surface-immobilized DNA hybridization. 25 (B) BSI detects the kinetic interaction of calmodulin and trifluoperazine. From ref 106. Reprinted with permission from American Association for the Advancement of Science, Copyright 2007. (C) Shows that the magnitude of binding as detected by BSI for α-crystallin to different T4L lysozyme mutants. Reprinted from ref 109.

Backscattering interferometry enables the detection of syphilis antibody–antigen interactions in the presence of human serum, showing promise as a diagnostic tool for the serological diagnosis of infectious disease with potentially quantitative capabilities.

Back-scattering interferometry (BSI) is a label-free, free-solution, small-volume technique used for characterizing binding interactions, which is also relevant to a growing number of biosensing applications including drug discovery. Here we use BSI to characterize the interaction of carbonic anhydrase (CAII) with five well known CAII inhibitors (±sulpiride, sulfanilamide, benzene sulfonamide, dansylamide, and acetazolamide) in the presence of dimethyl sulfoxide (DMSO). Dissociation constants (KD) calculated for each interaction were consistent with literature values previously obtained using surface plasmon resonance (SPR) and fluorescence-based competition assays. Results demonstrate the potential of BSI as a drug-screening tool which is fully-compatible with DMSO and does not require immobilization or labeling, therefore allowing binding interactions to be characterized in the native state. BSI has the potential for reducing labor costs, sample consumption and assay time while providing enhanced reliability over existing techniques.

B ackscattering interferometry (BSI), which uses a simple optical train comprising a HeeNe laser, a microfluidic channel, and a position sensor, has now enabled the measurement of both tethered and freesolution, label-free, molecular interactions within just nanoliters of sample. The simple macro-to-micro interface allows for a highly efficient assay work flow, which has been used to interrogate molecular binding interactions between proteins, ions and protein, and small molecules and proteins, with a high dynamic range of dissociation constants (K D ) and unmatched sensitivity. With this technique, the equilibrium K D for several different binding partners was determined, typically using just picomoleemicromole quantities of the binding pair at physiologically relevant concentrations.

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