Positive amino acids

Amino acids that are usually positive (i.e. protonated) at physiological pH:

An amino acid that is sometimes positive (i.e. protonated) at physiological pH:

It is false to presume that Histidine is always protonated at typical pHs. The side chain has a pKa of approximately 6.5, which means that only about 10% of of the species will be protonated. Of course, the precise pKa of an amino acid depends on the local environment.

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Please cite: M.J. Betts, R.B. Russell. Amino acid properties and consequences of subsitutions.
In Bioinformatics for Geneticists, M.R. Barnes, I.C. Gray eds, Wiley, 2003.