Polar or non-polar: Conflicting answers

[biology_06er]

Hey there

Just wondering while going over the amino acids if Tyrosine is polar or non-polar..i would say polar right? because of the OH group on it's side chain...however on one of the lecture slides we were given about amino acids it says its non-polar and looking on the net I found a table of non-polar/polar amino acids and it says Tyr is polar...can someone set me straight

Thanks
biology_06er

[Revelark]

I found the following information at http://www.curesnaturally.com:

Tyrosine: This water soluble polar molecule with a hydrophilic side chain has no electrical charge. A Heterocyclic and Aromatic amino acid, similar to other organic compounds, which combine other elements into the carbon ring. Essentially hydrophobic, but due to the possession of a polar (OH) group, is less strictly segregated from water, and can participate in hydrogen bonding. Direct precursor of adrenaline and thyroid hormones, it helps control depressions and anxiety, and acts as a growth hormone stimulant. Plays an intermediary role in the synthesis of the neurotransmitter Norepinephrine from Phenylalanine. Considered an antioxidant and appetite suppressant.

Hope this helps.

[kiekyon]

http://www.biology.duke.edu/class/bio118/spring03/questions/jan13.htm

[xand_3r]

I think the confusion is between the charge of an aminoacid and the polarity of the molecule. The charge of a molecule (and therefore of an aminoacid) depends on its ionized atoms or groups of atoms. For example the carboxyl and amino groups are ionized at the normal body pH (around 7.38), which is not true for the tio (-SH) or hydroxil group. If the total charge of an aminoacid is 0, then the AA is neutral (but it can be still polar like thyrosine), otherwise the AA has either a negative charge (like aspartic acid or glutamic acid) or a positive charge (like lysine). Charged AA are polar.

[MrMistery]

Yes tyrosine is a polar aminoacid. The differentiation that biology books make between polar aminoacids and charged aminoacids can only be done at the pH of body, as was said. Depending on the pH of the exterior solution and the pK of the aminoacid side chain, any polar aminoacid can have a charge of 0 or different of 0.

[samandeep]

Tyrosine is not polar its non-polar!!
i know this might come as a shock to some of u but just because a compound as a polar group doesnt mean its OVERALL polar.
tyrosine's polar OH group is overcome by the majority of non-polarity found in the C-C and C-H bonds in the ring portion of the R group.

[blcr11]

Dredging up old threads, I see. The side chain of Tyrosine is polar. But polarity is a relative thing. Certainly, compared to an ionized Aspartate or Glutamate, it is relatively non-polar, but compared to an Alanine or Leucine, it is polar. The phenolic OH can be fully ionized (though it’s rare to find one in a protein), and Tyrosines often hydrogen-bonds with ligands or other amino acid residues—but Tyrosine can also form aromatic stacking- or edge-on-type interactions as well, so Tyrosine polarity seems to lie somewhere between the charged, polar residues (Arg, Lys, Asp, Glu) and the merely aromatic and non-polar Phenylalanine, though probably closer to the latter than the former.

[victor]

I can say that tyrosine is a polar amino acid. Even it has a phenolic side-ring, it doesn't affect the solubility of this amino acid in polar solvent (ex: water). It's all because of the strong charge of the hydroxyl group in the aromatic ring. Phenol resembles the side chain of tyrosine and thus it's water soluble compound.
Well, polar and non-polar is indeed relative, so we can see the polarity of tyrosine and we can compare it with another non-polar compound by dipole moment comparison. If I'm not mistaken, the dipole moment of ether (a common non-polar compound) is about 1.4, so if tyrosine's dipole moment is a lot bigger than 1.4 then we can consider that tyrosine is a polar compound.