[ccp4bb] use of beta-mercaptoethanol in crystallization

[sreetama das]

Dear all,
            I have a 20 KDa protein that has a single cys residue , about 42 
residues from the C-terminal . The protein aggregates during crystallization 
setup ( hanging drop vapour diffusion) . What ratio of protein and 
mercaptoethanol is to be used to avoid aggregation and help in crystallization ?
           Thanking in advance,
                                                   Sreetama