Why is collagen fibre autofluorescent?

Why is collagen fibre [autofluorescent](https://en.wikipedia.org/wiki/Autofluorescence)? Proteins with increased amount of trp, tyr, phe tend to fluoresce but I don't think collagen fibre has increased percentage of any of them. Some say collagen autofluoresces because of the cross links between the fibrils but how?




Two proteins, collagen and elastin, are giving autofluorescence and both have a Pyridinoline compound as a cross linker, deoxypyridinoline in collagen and desmosine in elastin:


[![Crosslinking compounds in proteins](https://i.stack.imgur.com/8LNQk.jpg)](https://i.stack.imgur.com/8LNQk.jpg)


Thus, these two compounds should be responsible for the autofluorescence shown by these two proteins. Ref.1 has studied deoxypyridinoline, which is one of the two pyridinium cross-links that provide structural rigidity to type I collagen in bone. Ref.2 has given a structural model for desmosine cross-linked peptides. Likewise to collagen, this lysine-derived cross-links in elastin in large measure, contribute to the protein's elastic mechanical properties. The simplified crosslinking patten (Ref.2) is given in [Wikipedia](https://en.wikipedia.org/wiki/Desmosine):


[![crosslinking patten  of desmosine](https://i.stack.imgur.com/odJu8.png)](https://i.stack.imgur.com/odJu8.png)


I have fallen in finding the crosslinking patten in collegen by deoxypyridinoline, but can attributed to the similar pattern displayed by desmosine because closely related structures of two. The crosslinking of collagen increases with age, and it is interesting to mention that the blue autofluorescence intensity of collagen was increased after the fifth decade of life (Ref.3).


In addition, hydroxyproline, which is derived from the post-translational hydroxylation of proline, is a major component of fibrillar collagen of all types, comprising ~14% of the total amino and imino acid content. Hydroxyproline increases collagen elasticity whereas proline reduces it. The 4-hydroxy group in hydroxyproline may have a role in crosslinking as well.


**References:**


1. Alessandro Rubinacci, Raffaella Melzi, Maria Zampino, Armando Soldarini, Isabella Villa, "Total and Free Deoxypyridinoline after Acute Osteoclast Activity Inhibition," *Clinical Chemistry* **1999**, *45(9)*, 1510–1516 (DOI: <https://doi.org/10.1093/clinchem/45.9.1510>).
2. Robert P. Mecham and Judith A. Foster, "Total and Free Deoxypyridinoline after Acute Osteoclast Activity Inhibition," *Biochem. J.* **1978**, *173(2)*, 617–625 (DOI: <https://doi.org/10.1042/bj1730617>).
3. D. Dayan, M. Wolman, and l. Hamme1, "Histochemical study of the blue autofluorescence of collagen in oral irritation fibroma: Effects of age of patients and of the duration of lesions," *Histol. Histopathol.* **1994**, *9(1)*, 11–13 ([PDF](https://digitum.um.es/digitum/bitstream/10201/18462/1/Histochemical%20study%20of%20the%20blue%20autofluorescence%20of.pdf)).


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