TLDR
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Khan Academy video by Tracy Kovach explains how the 20 amino acids are grouped by R-group properties: polarity, charge, and acid/base behavior.
Key Takeaways
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20 amino acids split into two broad groups: nonpolar/hydrophobic (alkyl and aromatic side chains) and polar/hydrophilic.
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Nonpolar subgroups: alkyl side chains (glycine, alanine, valine, leucine, isoleucine, methionine, proline) and aromatic (phenylalanine, tryptophan).
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Polar subgroups: neutral (serine, threonine, asparagine, glutamine, cysteine, tyrosine), acidic (aspartic acid, glutamic acid), and basic (histidine, lysine, arginine).
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Polarity in neutral amino acids arises from oxygen or sulfur atoms hogging electrons; acidic ones carry carboxylic acid side chains; basic ones carry nitrogen-rich side chains.
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Proline is a structural exception: its side chain cyclizes back onto the backbone amino group.
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