There is no method to "cleave" a dimer into a monomer.  You can prevent dimerization by altering the solution conditions, but this can alter the function of the protein as well.  Why don't you just mutate the leu to an ala or a ile?  This can prevent dimerization while preserving the activity of the protein.   

Thank you for your response, Vicente.

Actually, when we mutate the leu to any other amino acids, the activity of our protein decreased.

Anyway, we solved that problem through resin screening.

Sorry for late thanks, have a good day.