Spectroscopy of biological systems

Terahertz spectroscopy of biological systems
Roughly since 2003 a number of researchers, both form the terahertz spectroscopy and biological areas, have been especulating about the potential of performing terahertz spectroscopy on biological systems (particularely proteins). Proteins are rather large molecules. They perform an enormous number of functions organisms. These functions are possible owing to their structure (or structures) and the motions they can perform. Of particular importante are the "collective" motions which involve large sections of the molecule, all motions of a physical system have to be linear combination of its "normal" modes (vibrational modes). Collective vibrations tend to be slow when compared to "atom-atom sretches" because the mass involved is greater, and they happen to fall in the terahertz region.

Animation illustrating the difference between localised (right) and collective (left) vibrations.

Photoactive yellow protein
Photoactive yellow protein (PYP) happens to be a relatively small protein (only 125 amino acid residues) that is well known for presenting a photoinduced conformational change when illuminated with blue light. Recently we performed terahertz tranmission spectrosccopy on this system [1] finding a slight but mesurable increase of absorption when the sample was illuminated. Normal mode analisys and molecular dynamics simulations suggest that the increase is caused by a red-shift of the normal mode distribution and an inclrease of the flexibility of the structure when PYP is in the photoexcited state.

Three-dimensional structure of PYP, the residues 42, 46, 50 and 69 are shown in cyan, yellow, magenta and green respectively and the chromophore p-coumaric-acid is shoy in gray. These sections of the molecule play a fundamental role in its photocyle.

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