Testing against cancer cells, I couldn't seem to dissolve my sample, even in DMSO. A suspension was formed. My preparation method: initially 1ug sample was dissolved in 10ul DMSO and further mixed with 990ul PBS to make up 1ml of total working sample, 1%.
Here are some tricks:
Can you still see the compound (crystals) in DMSO.
Try to heat to solve the compound if it is thermostable.
Use an ultrasound bath for minutes and follow up.
DMSO is quite hygroscopic. The little water that accumulated in bottle over time can hinder solving the compound. Try fresh bottle of DMSO.
Can you see precipitation happening when adding pbs?
Try adding just water in steps 1/1 then 1/4 etc...
Follow up on a stereoscope if/when compound precipitates.
I have solved these kind of troubleshooting by calling technical service of the company who supplied the chemical or by reading the technical sheet showing the details of chemical.
I am with Johan and Surya. But still some tricks. We work with human liver cancer cell lines (and some others). In our assay we get good results with DMSO but also with ethanol, methanol and even aceton. The latter is for obvious reasons not the first choice. Another solvent which I used in the past (although on plant cells, so you would have to test) is dimethylformamide.
A completely different approach is passive dosing where you deliver your compound to the cells through a saturated silicon ring co-incubated with the cells. At has the advantage of constant exposure concentration. The fraction of compound escaping to the plastic walls of the microtiterplateis constantly replaced by the saturated silicon ring at equilibrium conditions. Additionally you cannot get to calculated concentrations above the solubility in water of your compound.
But look at the physicochemistry of your molecule at first.
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