histidine metal chelating

[Kathleen Blackwell]

Given a complex nitrogen source with various peptides in solution, would 
digesting to various levels of free amino acids lead to a solutions with better 
metal chelating properties as the levels of free amino acids rises? My thinking 
is that higher levels of free histidine would chelate metals better than 
histidine bound in peptides, mostly for steric reasons. Any thought? TIA