Protonation?

In a PI (isoelectric point) problem,when the pk is higher than the given ph,we protonate the group right ? For example - the amino group becomes from NH2 to NH3+ .So my question is this.Why dont we protonate the R group in serine ? The pk is around 13 and if we have a ph lower than that,should we not protonate it to H2O+ connected to the main chain ? Also,what happens to it above 13 ?

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  • 6 months ago
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    The R group of serine is a hydroxyl. At high pH it will ionize losing a proton:

    R-OH --> R-O^- + H+

    At any pH below its pKa, it is simply in the form of the hydroxyl group. You can think of this sort of like a carboxyl group. Below its pKa, it is protonated and uncharged. Above its pKa, it loses a proton and becomes negatively charged.

    Exactly the same thing happens with the -SH of cysteine and the -OH of tyrosine. They are uncharged below their pKa's and negatively charged above them.

    Hope that helps...

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