Hi Thomas, Thiol itself doesn't have any reaction with MES buffer although MES contains sulfonic acid. Thiol can produce sulfonic acid with the present of a very strong oxidizer. However, if you see your thiol-HA  precipitate in MES buffer, it is possible that the thiol form disulfide bond and crosslink HA together. Another thing is you should check pka of HA, if it is soluble at what pH. 

Hi Thomas. Indeed, the oxidation of your thiolated-HA might be a problem, especially during long dialysis at neutral pH. To decrease the efficiency of oxidation reaction you may decrease pH of your dialysis buffer, unless it is fine for your product. Rate of thiol oxidation decreases significantly when pH is slightly decreased. In general, pH buffer should be around one unit below pKa of the thiol. Another possibility to avoid the thiol oxidation is the incubation of your thiol-HA together with TCEP (2-(cerboxythyl)phosphine). It is great reducing agent, which in generally reduces all disulfide bonds. It can be use for most buffers including MES. Good luck!

Hi Artur. Actually, to get free thiol groups on the hyluronic acid I am cleaving a disulfide bond of an hydrazine that is coupled to the acid in the presence of TCEP. The cleaving is performed at room temperature for 3 h followed by dialysing in M-water (100mM NaCl, ph 5). Unfortunately this procedure does not result into the expected functionalization degree published in literature. Do you have any suggestions how one could improve the deprotection / dialysis.

Thanks a lot for your help!