Which would you expect to be more soluble in water, a peptide ...

Question

Which would you expect to be more soluble in water, a peptide containing mostly alanine and leucine or a peptide containing mostly lysine and aspartic acid? Explain. (Hint: Consider side-chain interactions with water.)

Accepted Answer

Welcome back, everybody. Let's take a look at our next question are glutamic acid and histidine containing peptides, more water soluble than vine and phenylalanine containing peptides. Why? There's a hint, consider the intermolecular side chain interactions with water. And then we have four answer choices A through D giving different possibilities for which are more water soluble and why? So we noticed that in choice A it says glutamic acid and histidine are the more water soluble because they can exhibit hydrogen bonding. Choice B has valine and Phelan phenyl alanine as the more water soluble because they can exhibit hydrogen bonding. Choice C has glutamic acid and histidine as more water soluble because they can exhibit dipole dipole bonding or D valine and phenylalanine more water soluble because they can exhibit dipole dipole bonding. So let's recall that in amino acids, they all have the same backbone, but there are groups are different, their side chains are different. So we want to think about what are these side chains for each of these amino acids which will be more soluble in water? And why? So we can see that our answer choices give us the choice between hydrogen bonding or dipole dipole bonding. But when we think about solubility, we think about the rule that like dissolves like. And of course, when we think about water, its major characteristic that gives it so many unique properties. But H2O is that it exhibits hydrogen bonding that gives it that very strong intermolecular forces. We recall, of course, the strongest are the ones that bind compounds, ion dipole interactions, covalent bonds. But then after that hydrogen bonding is the strongest interaction. That's not an actual bond, hydrogen bonding occurs when you have a hydrogen. So an H atom bonded to an electron atom. So of course, in water, that would be oxygen. And that's of course, due to that uneven electrons sharing the electrons hang out most of the time with the electron atom creating that partial positive partial negative charge that allows the molecules to be attracted to each other. So our other possibility is dile dippo interactions. So that's we have a polar molecule. So again, we have difference in electron negativity. You have partial positive partial negative charges. Hydrogen bonding is actually a form of dile dile. But it's especially strong due to that small size of hydrogen. And the great difference in electron negativity dile dile interactions are weaker then hydrogen bonds. So if we're going to talk about solubility, we can eliminate CND because dile dile bonding wouldn't be an explanation for greater solubility if we have hydrogen bonding as an option. Now, of course, we would want to make sure that we can have hydrogen bonding as a possibility. So let's check that by looking at our possible R groups. So we have to work through each one and we start with glutamic acid. Well, we've got our clue right there. And the name glutamic acid. Indeed, when we look at the R group and note this is just the R group, not the backbone of the amino acid. it's a carboxylic acid. So ch two ch two carbonyl C double bond to an O and then bonded to an oh So this is definitely capable of hydrogen bonding with that oh group there. So now let's look at histamine. Well, histadine has in its R group, a structure called an amita zole ring. So it's CH two and then this five member ring, that's a carbon with a single bond to a nitrogen that has a hydrogen, single bonded to a carbon with a double bond to a nitrogen, single bond to another carbon and then double bond to a carbon. But you do have this hydrogen atom bonded to a nitrogen. So again, this is capable of hydrogen bonding. So we have glutamine and histadine both capable of hydrogen bonding. Now, let's compare that to veiling and pine. Well, for the R group of bale, we have essentially an is soral group here. Chch three and then also ach three bonded to that verse ch this has not only no hydrogen bonding it doesn't even have a dipole dipole interaction because it is nonpolar. This is just a hydrocarbon only sees in HS. And then finally, if we look at phenylalanine, well, we get that clue right from the word fenny, petty alanine is ach two group connected to a benzene ring. So again, we only have carbons and hydrogen, we have a nonpolar our group. So there's really no competition here. We have the two glutamine and histamine capable of hydrogen bonding are the strongest of dile dial interactions. And then vine and phenylalanine being completely nonpolar. So they definitely will not help in water solubility. So our answer here is choice. A glutamic acid and histidine are containing peptides are more water soluble than valine and phenylalanine containing peptides because they can exhibit hydrogen bonding. And of course, the choice that has valine and phenylalanine being more water soluble is incorrect due to their nonpolar R groups. See you in the next video.

Which would you expect to be more soluble in water, a peptide containing mostly alanine and leucine or a peptide containing mostly lysine and aspartic acid? Explain. (Hint: Consider side-chain interactions with water.)

Key Concepts

Polarity of Amino Acids

Hydrophilicity vs. Hydrophobicity

Ionic and Hydrogen Bonding

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