Structure of His-Tag complex

[Thorsten Schmidt]

Dear reader!

The His-Tag-System is widely used for protein purification.
(Novagene, Quiagen ...)

Using a special expression vector, a "His-Tag", a repeat of six Histidines,

is added to the target protein.
A special resin is then loaded with nickel-ions.
When one loads the column with a protein mixture only
the target protein forms with its histidines a complex with the nickel and
the resin.
After washing steps the protein can be eluted with a buffer containing
Imidazole.

I want to know the exact structure of this complex and the reason, why
it is possible to elute the protein with Imidazole.

Can you help me, perhaps tell me a reference? 

Thank you very much in advance for your answer

Thorsten