Question

Uranyl ion (UO₂²⁺ ) is commonly used to form heavy-atom derivatives for X-ray diffraction studies. Given that it is a relatively hard ion, what sorts of protein functional groups would you expect to be derived? How does this behavior compare to that expected for another heavy-metal derivatizing ion, Hg²⁺ ?

Answer 1

Uranyl ion being a hard acid can bind with hard base according to hard acid-base theory (HSAB). Carboxyl and amine groups being soft can easily bind to uranyl ion. So proteins having functional groups such as RCOOH and RNH₂ can be derived using uranyl ion. But Hg²⁺ being a soft acid react with soft bases mainly thiol groups in the amino acids. So RSH groups are derived using Hg²⁺ ion. This is different from uranyl ion because Hg²⁺ is hard and also thiol group which cannot be derived using uranyl ion is derived using this.