Protonation of a nickel-cysteine active site in bacterial [NiFe] hydrogenases is postulated in hydrogen production by these enzymes.  To model some of this chemistry, the pH-dependent stability and redox activity of phosphine-nickel-cysteine systems (dppeNiCys)* has been investigated.  Reversible protonation of the nickel-cysteine group of dppeNiCys in phosphate-buffered aqueous solutions was documented by NMR and electronic spectroscopies.  A promising result of this chemistry is the moderately acidic aqueous solutions in which it is observed.  While tridentate phosphines like TRIPHOS* give nickel-cysteine centers amenable to forming nickel(I), the three phosphorus atoms have variable coordination to nickel.  Nickel reduction and cysteine protonation are two key steps in functional hydrogenase activity.

* dppe = 1,2-bis(diphenylphosphino)ethane                      TRIPHOS = 1,1,1-tris(diphenylphosphinomethyl)ethane