Measurements of biomolecule interactions are increasingly important for proteomics and protein-protein interactions (the Interactome). With the exception of surface plasmon resonance (SPR) most of the alternative methods rely on the use of extrinsic fluorophores or probes which are covalently linked to one or more of the interacting species. These labeling steps become increasingly costly and complex as the number of biomolecules is increased. We propose a new approach to label-free measurements using one-dimensional photonic crystals (1DPCs) and intrinsic protein fluorescence. Since most proteins contain tryptophan these residues provide a type of mass sensor for proteins, analogous to SPR which also senses mass by changes in the refractive index. We will use Bloch surface waves (BSWs) on the 1DPCs. BSWs provide opportunities for highly localized excitation and observation of surface-bound proteins. The BSW substrates will be composed of multiple layers of UV-transmitting dielectrics. These structures will not contain metals so there is no quenching at short distances.
Specific Aim 1. Design BSW structures for use at UV wavelengths. Not all 1DPCs display BSW. We will use the transfer matrix method to simulate the optical modes and select the dielectrics and dimensions. More complex numerical simulations will be used to predict the efficiency of trp coupling and directionality of the emission.
Specific Aim 2. Fabricae and test 1DPCs for use with UV wavelengths. Suitable structures must display good UV transmission, a BSW and low autofluorescence. We will test combinations of three dielectrics, silica (SiO2), silicon nitride (Si3N4) and tantalum pentoxide (Ta2O5). Substrates will be tested wih model systems of albumin, streptavidin and antibodies.
Specific Aim 3. Biological testing of the BSW structures. The structures will be tested with two important biological systems. The first is binding between the p53 tumor suppressor protein and its negative effectors. The second is binding of the HIV gp120 protein with its CD4 receptor and with potential HIV antibodies. Impact. This project will result in a new generic method to measure protein-protein interactions. The substrates will be simple and inexpensive. The method can be used with existing or modified plate readers. The simplicity of the optics will permit the extension to high-throughput measurements.
Bioaffinity assays are a central feature of biomedical research. Many of these assays are performed using fluorescence detection. However, this requires attachment of fluorescent molecules called probes to one or more of the interacting species. These labeling steps increase the complexity and costs of the measurements. This project will develop a method to bypass the labeling steps. This will be accomplished using the intrinsic fluorescence from proteins which is due to the tryptophan residues. Selective observation of the target proteins will be accomplished by the design and fabrication of low-cost photonic structures which collect the emission from surface-bound proteins. This is a generic approach that can be extended to high-throughput measurements which are needed to proteomics and protein arrays.
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