Digestion of Core Protein

[nc26 at wwa.com]

Had anyone had any experiences with enzymatic and/or chemical cleavage of core protein? I've been attempting to digest HBV 
core protein with tryspin in 1M urea with little success.  My digestion have resulted in fragments that are only partial cleave.  I've 
gone as high as a 1:3 protein:enzyme ratio without any effect.  I would appreciate any help, especially from anyone who had 
previous experiences with digestion of core protein.  Thanks in advance.