Larger amino acids side chains can wiggle quite a bit, and their dipole moment is difficult to define. The dipole moment is the usual measure of polarity, but this depends on the molecule's geometry, and consequently is only well-defined for small molecules that cannot "wiggle" around, such as water. Polarity is rather difficult to define for amino acids. I assume you are interested in the latter. First, the polarity and hydrophobicity of the free amino acid is very different from that of the amino acid side chain in the context of a peptide (protein). The reason for the confusion is that are there several ways to define and measure polarity and hydrophobicity for amino acids. There is no broad consensus on whether cysteine and tyrosine should be considered hydrophobic or polar. TL/DR: these are borderline, complicated cases. Please help me to clear out the confusion, why the polarity and hydrophobicity of these amino acid are not agreed with each other? To summarize the result, I'll put them in the following table The article Cysteine states that Cysteine is polar and hydrophobic, AS WELL AS Tyrosine. The article Tyrosine supports Tyrosine is polar and hydrophilic. The article Proline supports Proline is nonpolar. I will list here several sources that state differently on the polarity and hydrophobicity of Proline and Tyrosine:įrom the article Amino Acid on Wikipedia, it states that Proline is a special case, Tyrosine is hydrophobic and Cysteine is also a special case.įrom Khan academy, it also states that Proline is polar, Tyrosine is nonpolar and Cysteine is polar.įrom Pearson Prentice Hall's Biological Science, it states that Proline is nonpolar, Tyrosine is polar and Cystein is polar.įrom BioNinja, it states that Proline is nonpolar, Tyrosine is polar and Cysteine is nonpolar.
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