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How do you figure out whether an amino acid is hydrophobic or hydrophilic? (other than memorization) I know that it has something to do with the R groups of the amino acids, and that polar molecules are hydrophilic, while nonpolar molecules are hydrophobic, but that's about it. Please give a simple answer, I don't need too much information other than an easy way to tell.

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closed as off-topic by MattDMo, AliceD, James, WYSIWYG Sep 16 '16 at 5:09

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Homework questions are off-topic on Biology unless you have shown your attempt at an answer. For more information see our homework policy." – AliceD, James, WYSIWYG
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ This question isn't about biology but about learning it, so no doubt it will be closed very soon. However the general answer is that you need to learn chemistry in order to know other than by memorisation. Like it or not, biochemistry/structural biology/molecular biology involves basic chemical knowledge. Without that you are going to have to memorize things without understanding them. I would start learning chemistry as soon as possible. $\endgroup$ – David Sep 10 '16 at 21:38
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    $\begingroup$ Checkout: "What is the difference between polar and charged amino acids?" on Biology StackExchange $\endgroup$ – mdperry Sep 10 '16 at 23:23
  • $\begingroup$ @mdperry if you're going to reference a different question, please include a link. $\endgroup$ – MattDMo Sep 11 '16 at 16:10
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Does the side chain contain any Nitrogen (N), Oxygen (O), or Sulfur (S) atoms? If it does, then chances are high that this particular R-group is polar (although it may not be charged).

The converse is also true, if the R-group lacks any of those three atoms then chances are high that the side chain is polar. There are exceptions; Methionine has a Sulfur, but is quite hydrophobic (non-polar). Tryptophan has a nitrogen but is non-polar.

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    $\begingroup$ ...and lysine and arginine can be considered both hydrophobic and hydrophilic, as can tyrosine (the latter being particularly important in certain enzyme mechanisms). Encouraging this sort of rote learning of dangerous half-truths is anti-science. The only decent advice is to learn some very basic chemistry, for which there must be dozens of reasonable on-line sources. $\endgroup$ – David Sep 11 '16 at 13:19
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    $\begingroup$ I know this isn't how you are "supposed" to do it, but it isn't easy to memorize all 20 amino acids, and this helps a lot with identifying whether they are hydrophobic or hydrophilic. I'll just have to remember that methionine and tryptophan are hydrophobic. $\endgroup$ – suomynonA Sep 11 '16 at 15:51
  • $\begingroup$ @Anonymous you need to learn basic chemistry first. Going along with what david said, it's not just a matter of memorizing the 20 AAs and saying "W is hydrophobic, S is hydrophilic". You need to be able to understand why, and how some can be classified as both, depending on context. $\endgroup$ – MattDMo Sep 11 '16 at 16:12
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    $\begingroup$ @Anonymous Thank you for selecting my anti-scientific "dangerous half-truths" as the answer to your question; I am very glad that you found it helpful. I realized I did not provide you with a reference of any sort, so here is a biochemistry textbook of mine: pearsonhighered.com/program/…. I fear I am growing old and may not be up-to-date on the newest methods for learning these chemical principles. We did indeed memorize all 20 a.a. structures as biochemistry undergraduates in the 1970s (in our Organic chemistry course). $\endgroup$ – mdperry Sep 13 '16 at 8:09
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    $\begingroup$ @mdperry Your answer helped a lot with my question, and that's what an answer is supposed to do. Btw, studying biology really hasn't really changed much besides extra information to know $\endgroup$ – suomynonA Sep 14 '16 at 0:58

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