Polar vs Nonpolar

[rainashley]

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How can you tell if a molecule is polar or nonpolar?
I understand dipoles and being charged and everything... but I dont understand why some amino acids are polar and others are nonpolar. For example, why is proline nonpolar but threonine is polar?
What are the exact rules for distinguishing between polar and nonpolar?
Thanks

 

[Eye Cue]

Just compare the R groups. In your example, threonine's R group has an OH and can hydrogen bond, while proline has an alkyl R group and can't hydrogen bond. This would be the most basic way to look at it.

 

[novawildcat]

Proline and threonine by themselves are certainly both polar (you have an amine and carboxylic acid), however, amino acids are grouped by the polarity of the substituent on alpha carbon. Amide bond formation in proteins takes away a lot of the polarity of the carboxylic acid and amine moieties, thus the polarity of each amino acid residue in a protein becomes dominated by its substituent on the alpha carbon.

 

[seep]

Most nonpolar is phenylalanine, right? And most polar serine or aspartic acid?

 

[loveoforganic]

Most polar is probably arginine