On the right side of the embryo, activin is expressed, along with activin receptor IIa. This activates Fgf8, a protein that blocks expression of the gene for Cerberus.

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.

TLS Online TPP Program

#Id: 4713

#Unit 13. Methods in Biology

A number of amino acids (Asp, Glu, Asn, Gln, Arg and His) have weak electronic transitions at around 210 nm. Usually, these cannot be observed in proteins because

they are masked by the more intense peptide bond absorption.

TLS Online TPP Program

#Id: 4714

#Unit 13. Methods in Biology

The most useful range for proteins is above 230 nm, where there are absorptions from aromatic side chains While a very weak absorption maximum of phenylalanine occurs at 257 nm, tyrosineand tryptophan dominate the typical protein spectrum with their absorption maxima at 274 nm and 280 nm, respectively