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Nicole_5521
#1 Posted : Thursday, June 03, 2021 9:47:33 PM
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Hello,

I'm wondering when we classify amino acids as polar, non-polar, acidic, basic, is that according to just the R-group alone?

For example, a molecule like glycine is considered a non-polar amino acid, but glycine as a whole is considered a polar molecule?

Could you explain why that is?

Thanks!
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INSTR_Katrina_128
#2 Posted : Friday, June 04, 2021 10:28:01 PM
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I agree, this can get very confusing!

When you are asked to classify amino acids based on polarity, YES, you focus only on the R-group. That is because all amino acids have the same basic structure, and differ only in this group! https://s3-us-west-2.ama...36/figure-03-04-01.jpeg

Here is a great summary of the polarity of amino acids: https://s3-us-west-2.ama...938/figure-03-04-02.png

Like you've said with glycine, the R group is an H atom, which is non-polar. So, the unique part that makes the amino acid glycine (the glycine residue) is non-polar.

But, when we consider glycine as an entire molecule (taking into account the rest of the protein structure), we can see that it consists of a C-NH2 group (polar), a CH2 group (non-polar), and a COOH group (polar). Both of these groups (C-NH2 and COOH) are capable of gaining formal charges under the right conditions.

https://lh3.googleuserco...SLT1i65pxxAWH-r1RwqT0cds
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