Protonation states of individual amino acids?
Hey, I have a question about amino acids and their different states in different pH ranges. Since I have to draw a peptide at pH 7, I would be interested in which protonation states the amino acids can assume.
For example, in the case of aspartic acid, it is completely protonated in acidic conditions and then gradually deprotonated until it is completely deprotonated in a strongly alkaline environment.
Now my question is: In which amino acids are the side chains also protonated/deprotonated?
So I am aware that with glycine, alanine… this will not be the case
But what about tyrosine, theronine, serine, for example?
Can the OH group be deprotonated at high pH
And what about arginine and histidine?
Thank you very much in advance!
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Yes, but not under physiological conditions . The OH group at this point has a pK value of 17 (possibly 18). You hardly have anything in the environment that could spread the negative charge and the oxygen itself is a bad charge carrier. You need a very strong base (alcoholate or hydride, etc.) to abstract a proton here.
These are both AS, which are quite able to accept a proton and will, if necessary, also! Histidine is often part of the catalytic triad, where it particularly absorbs the proton of cys to increase the nuc.
However, the two bases are unclearly split off a proton at the end. These have a basic character. However, here the pKs value is at 12, so that a strong base (for example a strongly concentrated NaOH solution) could possibly be sufficient to abstract a proton there. Under physiological conditions, however, this would not happen.
Thank you for your answer.
So Tyrosine/Theronine would have only three retention states
and arginine/histidine 4, since here the side chains can still be protonated?
What do you understand under “protonation states”?
For example, this (I just take an example from Wikipedia):