All zwitterions are least soluble when the pH equals the pI.  The pI of cysteine is 5.1.  So, pH's far above or below that value would give you the highest solubility. Also, the the ionic form of cysteine that you are trying to dissolve can make a big difference in the "rate" that it takes to dissolve. BUT, and this is a big but, I have seen literally hundreds of people make a cysteine ( or ANY sulfhydryl compound) solution and use it for several days if not weeks and wonder why things aren't working right. A 1 mM solution of cysteine at pH 7 will be completely oxidized to cystine within only a day or two at room temperature, if exposed to the atmosphere! There are two factors to consider: pH of the solution and concentrations.  It is well established that the RS- anion is needed for the oxidation reaction, so low pH (1-2) will retard the rate of oxidation to cystine significantly, as well as increase the solubility of cysteine. Trace metals also can catalyze the oxidation, so make your solution with the best water you can get your hands on, AND buy the purest cysteine you can afford (of course, a bit of EDTA would go a long way to help here...) Now, what about concentration?  It is best to make the most concentrated stock solution you can because the rate limiting step of the oxidation to cysteine at low pH's is dependent only on the rate that oxygen enters the solution and independent of the cysteine concentration. So make your stock solutions concentrated, and throw away diluted solutions after a day.  Use Ellman's reagent (DTNB) to quantify the sulfhydryl group, so you know what concentration your cysteine truly is. Hope this helps.

hello Sibasish what do you mean by denaturing? I assume you intend racemisation, do you ? if that is the case i would be quite relaxed as the the rate of racemisation of aminoacids it is quite little even in harsh conditions see link below

http://www.aslo.org/lomethods/free/2005/0318.pdf

thus if it is not a problem for you modifying the pH just acidify your distilled water untill sold is dissolved (stirring at room temperature)...

Have a nice day

(PS I would be more concerned with oxidation to cystine than anything else really )

Hi Sibasish,

I think you are talking about the oxidation of L-cysteine in water, showed as a white flake precipitate, or about racemisation, as explained by Giuseppe. However, to prevent the oxidation, I suggest you to dissolve the L-cysteine in double distilled water with pH slightly acid (6.5) and/or degassing the water with N2 to reduce the oxygen concentration. Normally I used L-cysteine as redox agent to make protein extraction buffer or during the Chromatography purification instead of DTT or -Mercapto ethanol.

Have a nice day!

Luca

Hi Sibasish, Cys should be fairly soluble in pH 7 buffers.  If you have to dissolve Cys in diH2O, you could try putting a tube of your insoluble Cys solution into a sonicating waterbath to disperse any undissolved aggregates. 

Thank you very much for the useful information

All zwitterions are least soluble when the pH equals the pI.  The pI of cysteine is 5.1.  So, pH's far above or below that value would give you the highest solubility. Also, the the ionic form of cysteine that you are trying to dissolve can make a big difference in the "rate" that it takes to dissolve. BUT, and this is a big but, I have seen literally hundreds of people make a cysteine ( or ANY sulfhydryl compound) solution and use it for several days if not weeks and wonder why things aren't working right. A 1 mM solution of cysteine at pH 7 will be completely oxidized to cystine within only a day or two at room temperature, if exposed to the atmosphere! There are two factors to consider: pH of the solution and concentrations.  It is well established that the RS- anion is needed for the oxidation reaction, so low pH (1-2) will retard the rate of oxidation to cystine significantly, as well as increase the solubility of cysteine. Trace metals also can catalyze the oxidation, so make your solution with the best water you can get your hands on, AND buy the purest cysteine you can afford (of course, a bit of EDTA would go a long way to help here...) Now, what about concentration?  It is best to make the most concentrated stock solution you can because the rate limiting step of the oxidation to cysteine at low pH's is dependent only on the rate that oxygen enters the solution and independent of the cysteine concentration. So make your stock solutions concentrated, and throw away diluted solutions after a day.  Use Ellman's reagent (DTNB) to quantify the sulfhydryl group, so you know what concentration your cysteine truly is. Hope this helps.

very detailed answer Dean ... no much more to add to that

I had that problem. You can dissolve l cysteine hydrochloride monohydrate in water.

If you have pure cysteine you have to dissolve in 1N HCl. Thats the only solution that worked for me. When I tried it with pH6, it didn't dissolve at all. 

Dissolve in the HCl and then dilute out in buffer. It works out well.

Take care.

Hi,

I want to prepare 5 M solution of L cysteine in 10 ml distilled water. Can any one suggest me the procedure so that I could be able to form this solution.

i have HCl and L cysteine individually. Now i need to synthesis to both to get L-cysteine hydrochloride monohydrate solution. 

after i google it, valueless information i found. can you pls give the procedure, how to do it.

you can heat diH2O read link below, please

https://www.jbc.org/content/126/1/147.full.pdf