Lower salt concentration or higher pH was previously reported to favour position A of His119
[63,64].
During pressurization concentration of ammonium sulphate decreases in pressure medium due to formation of salt crystals, thus it promotes the situation where His119 assumes positionA.
Furthermore, the analysis of the molecular volume of wild-type RNase A molecule in which His119 in A and B position revealed that molecule with conformer A of His119 exhibits approximately 20 Å3 smaller molecular volume than those with conformer B.
At high pressure smaller volume states are favoured and that explains why only A conformation is present. Additionally, possible ionic interactions between the carbonyl oxygen of the Phe120, as well as between OD1 of Asp121 and imidazole ring of His119 make the A position of His119 preferable.
All those factors caused the change of His119 conformation from B to A in comparison to ambient pressure structures.
The subtle shift of Phe120 phenyl ring is probably induced by change in His119 position.
In hp structures atoms of Phe120 are closer to His119.