Photodesmosine,an isomer of desmosine obtained by photolysis of this amino acid in ultraviolet light

Desmosine and isodesmosine are two isomers representing the main cross-links of elastin. We describe a new isomer, photodesmosine, which is produced by the photolysis of desmosine at 254 nm. The mechanism of this photolysis is described and is shown to consist of two competing paths. After opening of the pyridium ring to give a tetrasubstituted aminoketone, this compound can either be hydrolysed to give lysine and a trisubstituted analogue of glutaconic aldehyde or undergo a recyclisation and rearomatisation to give a pyridium compound substituted in positions 1, 2, 3 and 4. An understanding of this mechanism is important in order to use photolysis as a specific method to break elastin cross-links. Although only desmosine and isodesmosine have been reported in purified elastin, the chromatographic properties of photodesmosine suggests that if other natural isomers exist in this protein they could be eluted from an ion-exchange resin at much earlier times than those observed in the case of the two already described cross-links.