TLS Online TPP Program

#Id: 4707

#Unit 13. Methods in Biology

From the four possible transitions (n!p*, p!p*, n!s*, s!s*), only two can be elicited with light from the UV/Vis spectrum for some biological molecules: n!p* and p!p*. The n!s* and s!s* transitions are

energetically not within the range of UV/Vis spectroscopy and require higher energies.

TLS Online TPP Program

#Id: 4708

#Unit 13. Methods in Biology

Molecular (sub-)structures responsible for interaction with electromagnetic radiation are called chromophores. 

TLS Online TPP Program

#Id: 4709

#Unit 13. Methods in Biology

In proteins, there are three types of chromophores relevant for UV/Vis spectroscopy:

• peptide bonds (amide bond);

• certain amino acid side chains (mainly tryptophan and tyrosine); and

• certain prosthetic groups and coenzymes (e.g. porphyrine groups such as in haem).

TLS Online TPP Program

#Id: 4710

#Unit 13. Methods in Biology

The presence of several conjugated double bonds in organic molecules results in an extended p-system of electrons which lowers the energy of the p* orbital through electron delocalisation.

TLS Online TPP Program

#Id: 4711

#Unit 13. Methods in Biology

Common colorimetric and UV absorption assays

TLS Online TPP Program

#Id: 4712

#Unit 13. Methods in Biology

The electronic transitions of the peptide bond occur in the far UV. The intense peak at 190 nm, and the weaker one at 210–220 nm is due to the p!p* and n!p* transitions.