Imidazole and high ionic strength can have adverse effects such as salting out, denaturation, activity loss, etc. I recommend removing excess salts as soon as possible (e.g., using dialysis, gel filtration columns).

Afterwards, if you require long-term storage, use liquid nitrogen or dry ice to quickly freeze your sample before placing it at -20°C, with or without lyophilization. Otherwise, keep at 4°C in "standard" buffer conditions (say 20mM PB, 100mM NaCl) or in other known and tried buffers.

Roy Cohen Thank you so much!