Purple proteins

[M.J. Collins]

Cold acid hydrolysis (HCl 33% wt vol] of a 5% solution of
ovalbumin produces a deep purple colour.  Does anyone know 
why?  

The ovalbumin is from a reputable supplier but has been 
hanging around in the lab as a dry powder for some time (5+ years?) 
in a dark brown bottle.

benjamin.stern at newcastle.ac.uk
Newcastle Research Group in Fossil Fuels and Environmental
Geochemistry
University of Newcastle upon Tyne