Phycobiliproteins

Phycobiliproteins are photosynthetic antenna pigments found in cyanobacteria (blue-green algae), red algae, and the cryptomonads. Within the algae, the phycobiliproteins are found on the surface of thylakoid membranes; the thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. Thylakoids are the site of the light-dependent reactions of photosynthesis. These include light-driven water oxidation and oxygen evolution, the pumping of protons across the thylakoid membranes coupled with the electron transport chain of the photosystems and cytochrome b6f complex, and ATP synthesis by ATP synthase utilizing the generated proton gradient.¹ Phycobiliproteins harvest solar energy in regions of the visible spectrum and then transfer this excitation energy to chlorophyll in the thylakoid membranes of algae.² The principal method of this energy migration is a type of resonance transfer. Resonance transfer is very efficient type of energy transfer. As a result of this resonance transfer, phycobiliproteins are the most efficient energy transfer mediums in nature.

The strong optical properties of phycobiliproteins make them ideal for use in medical diagnostics and life science research. Over the years a large number of life science reagents have been developed using phycobiliproteins as a label. For example, R-Phycoerythrin (PE) is a preferred label for cytometry, Luminex® systems, tetramers and Affymetrix GeneChip®. Allophycocyanin (APC) is a commonly used label in Time-Resolved Fluorescence Resonant Energy Transfer (TR-FRET) assays for High Throughput Screening and Flow Cytometry.

Advantages of Phycobiliproteins

• The emission is free of absorption and fluorescent interference by other biological
  materials, including blood, sera and cell culture components.

• Their fluorescence properties are protected from quenching unlike synthetic
  organic fluorophores.

• Phycobiliproteins are stable at a wide range of biological pH values (4.5-8.0)

• Conjugation procedures to many biological compounds such as antibodies have been very
  well characterized.

• Large Stokes shifts provide great signal to noise ratios. (5-20x greater than small molecule fluorophores)

• Water soluble, thereby decreasing the possibility of non-specific interactions.

• High quantum yields (0.65 to 0.98)

• Compatible with a large number of instrument platforms.

1 http://en.wikipedia.org/wiki/Thylakoid

2 Phycobiliproteins,1987, MacColl Guard-Friar