What will be the structure of cysteine at pH=4? [closed]








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As per my understanding since pKa of RSH and the amino group is >4 hence they would be in cationic form while the acid should be in anionic form. So charge on cysteine would be +1. But while looking on internet i found that nowhere the RSH group is protonated.
Please someone clarify





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> ...hence they would be in cationic form
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> 
> 


Protonated does not imply cationic form. Some conjugated acid/base pairs have a neutral and an anionic form (like the carboxylic acid/carboxylate pair).


The acid/base equilibrium of the thiol group is the following:


$\ce{R-SH <=> R-S- + H+}$


The left-hand side is called protonated, and the right-hand side is called deprotonated.


$\ce{R-SH2+}$ is not observed in aqueous solution.



> 
> since pKa of RSH and the amino group is > 4
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The $\mathrm{p}K\_\mathrm{a}$ of the ammonium group is > 4. The amino group does not act as an acid in aqueous solution, and its $\mathrm{p}K\_\mathrm{a}$ is too high to be able to deprotonate an amino group in aqueous solution.


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