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Dear Careina, First: make sure your solution is completely free of competing primary amines, e.g. tris buffer. Phosphate buffers or HEPES should be used; if you need buffer exchange, do it by gel filtration or extensive dialysis (2-3 changes), not concentration-dilution. Second: Try to use BS3, which is a water-soluble analogue of DSS; this gives generally excellent results for non-membrane proteins. Third: test a few other crosslinkers. Dimethyl pimelimidate and dimethyl Suberimidate are amine-directed crosslinkers with different lengths which have worked in some cases. A very dirty crosslinker is glutaraldehye which, with time, will crosslink everything into large aggregates, but you may be able to fiddle with the conditions (low concentrations of glutaraldehyde and protein).