Cysteine is Polar or Nonpolar?

[betterfuture]

Kaplan considers Cysteine to be polar while I have read in my Cell Bio book that cysteine is nonpolar. Does anybody know what AAMC considers Cysteine to be classified under?

Which brings me to my next question. When 2 cysteines come together to form a disulfide linkage, why do they form a hydrophobic core, like for example in insulin? And is this due to the fact that the 2 cysteines each come from different polypeptide chains and after they form the disulfide linkage, the 2 separate chains come together and aggregate spontaneously resulting in an inner core being hydrophobic and outer as being hydrophilic? Or is it because the CYSTINE that is formed is hydrophobic so it attracts only other nonpolar amino acids?

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[betterfuture]

Yeah. S is not as electronegative as O because the general trend is electronegativity increases from bottom to top in groups so makes sense. And plus, I believe H bonding occurs mostly with F, O, and N. It's kind of complicated when you are taught different things about the same topics. I was taught in biochemistry that tyrosine was polar due to the OH group, but I guess the benzene ring discounts that making it more nonpolar than polar. So that is why I wasn't sure cause it's not as clear cut. Is it safe to say cysteine is polar then?

The reason I bring up the protein folding is because there was a question on one of the FL's. The question asked which amino acids were likely to be found around the disulfide linkage regarding insulin's structure. The answer was all nonpolar groups. I wasn't sure what made it so since I didn't know the nature of disulfide bonds. Cysteine is polar but after two cysteines come together, they are fairly hydrophobic.

 

[aldol16]

Cysteine is an oddball. It's polar, but not completely polar. Think about the electronegativity of S compared to O. S-H should be very similar to O-H, but not quite enough to make hydrogen bonds. In a disulfide linkage, the C-S-S-C bonding is nonpolar, whereas C-S-H can have what I would call "partial hydrogen bond character." (https://en.wikipedia.org/wiki/Thiol#Boiling_points_and_solubility) Instead of memorizing this fact, it would probably be higher yield to understand group trends on the periodic table.

The S-H bond may be polarized slightly, but the molecule itself is non-polar. In fact, a S-H bond is essentially equivalent to a C-H bond in terms of polarity if you measure it by electronegativity values and one would not consider a normal C-H bond to be polar by any means. Cys is nonpolar by many measures of hydropathy - in almost all cases more nonpolar than alanine (the "default" amino acid). One should know that Cys is a nonpolar amino acid for the MCAT.

 

[aldol16]

Yeah. S is not as electronegative as O because the general trend is electronegativity increases from bottom to top in groups so makes sense. And plus, I believe H bonding occurs mostly with F, O, and N. It's kind of complicated when you are taught different things about the same topics. I was taught in biochemistry that tyrosine was polar due to the OH group, but I guess the benzene ring discounts that making it more nonpolar than polar. So that is why I wasn't sure. Is it safe to say cysteine is polar then?

Many molecules have polar parts but are overall nonpolar due to contrasting effects of other groups. Fatty acids are an excellent example. Carboxylic acids are very polar. However, the long acyl chains make the fatty acids nonpolar overall. See above about Cys polarity.

 

[El-Rami]

For the MCAT, just consider Cysteine to be polar without charge.

 

[betterfuture]

@El-Rami What do you mean without charge? Cysteine does not have a charged R side chain, if that's what you meant.