A mixture of seven amino acids (glycine, glutamate, leucine, l...

Question

A mixture of seven amino acids (glycine, glutamate, leucine, lysine, alanine, isoleucine, and aspartate) is separated by chromatography. Explain why only six spots show up when the chromatographic plate is coated with ninhydrin and heated.

Accepted Answer

Hey everyone. Let's do this problem. It says thin layer chromatography was used to separate a mixture of Elaine in phenylalanine, tyrosine and serene rank. The order of amino acid spots formed from top 12 bottom four when the chroma to graphic plate was coated with nin hydrant solution and heated. So we're using thin layer chromatography to separate in mixture here. So let's review what is thin layer chromatography or TLC for short. So in T L C, we have a plate that is coated with silica gel and that silica gel is polar and that's going to be our stationary phase, stationary phase. So then this plate is in maybe a little beaker or something and there's a liquid inside and that is going to be our mobile phase and that will be less polar. So in order to separate, we need one phase that is polar, one phase that is non polar or less polar. Alright. So our mixture is going to go into the mobile phase right. And then slowly it will move up the liquid will move up the plate and based on the polarity of the compound that determines where the spots are formed on the plate. So at the bottom, do you think those compounds are really interacting with that silica gel or they really like the mobile phase, they'll really like the silica gel which will make them more or less polar, more polar, right? Because like attracts like, so if that compound gets onto the plate and it's like, oh I'm really polar. I really like that silica gel. It's going to stick there and form its spot. If it is less polar, if the compound is less polar, it's going to like staying in the mobile phase and it's not going to gel with the silica gel very well. So the less polar compounds will continue being carried up staying in the mobile phase and their spots will be higher on the plate. So the spots, the spot placement on the plate depends on polarity. So how do we know the polarity of a compound? Well, we want to look at the difference of electro negativity in the atoms of a compound because if one atom is highly electro negative, one atom is less electro negative, then the bond between them will create a die pole. So increase in the difference of electro negativity increase in polarity. So with that in mind, we know hydrocarbons are going to be less polar right there, just carbon and hydrogen bonds versus say a nitrogen, hydrogen bond or an oxygen hydrogen bond, those will be more polar. And the problem also mentioned a nin hydrant solution. So what is that all about? Well, amino, acids are actually colorless. So when we use TLC for amino acids, We use nine hydrant to give it color so that we can see the spots on the chromatograph. Okay. So that's all that that is doing. So let's look at the structures of our amino acids determine their polarity and that will help us determine the placement of the spots on the plate. So we have Allen een which will look like. So we have um ethel a carbon, carbon alcohol group. So metal, then next to that metal, we have an amino group, NH two and our carbonell group next to our alcohol group. So that is the amino acid structure. And then we have Fenno alone in. So that's basically going to be the same compound. But instead of that methyl group, we have a fennel group. So fennel group, NH two carbonell alcohol. All right. Now, we have tyrosine, tyrosine is going to be similar to fennel Elhanan, except on the ring in the Ortho position to the amino acid group, we're going to have an alcohol group. So 1234 carbons away, we have an alcohol group on the ring. And lastly, we have serene and where that fennel group is instead, we'll have an alcohol group. Alright. So let's compare these compounds and their polarity. So I noticed that Fennel Elhanan and tyrosine have these big fennel groups. So those large hydrophobic benzene rings. Do you think that will make these compounds more polar or less polar? That will make these the least polar compounds? Right? Because of all those carbon hydrogen bonds not going to be very polar. Okay. So these will be our two least polar compounds, which one is going to be the very least polar phenylalanine, right? Because our tyrosine at least has this alcohol group so that O H bond is polar. So that will make it a little bit more polar. So we'll have our least polar is fennel allen een. Next, a little bit more polar we have tyrosine. Okay. Now comparing alunan and serene, looking at the groups that are different, right? All these groups highlighted in yellow are the ones that are different. The rest of the compound is the same between all of all four of these. So in a linen, we have a methyl group in serene, we have an alcohol group. So which one is more polar? Definitely that O H group, right? So that's going to make serene the most polar compound and that'll make Allen in less polar than serene. So Allen in will be third in our list since we're doing increasing polarity. So allen in And 4th are most polar compound is Serene. So again, looking at our T L C plate, as the problem mentioned, compound, four will be at the bottom. So serene will interact the most with the silica gel and be the lowest on the plate and compound one, our least polar will be more happy in the mobile phase and travel all the way to the top of the plate. So this is the correct order for this problem which makes d the correct answer for this problem. That's it for this one. I hope this video is helpful and thanks for watching.

A mixture of seven amino acids (glycine, glutamate, leucine, lysine, alanine, isoleucine, and aspartate) is separated by chromatography. Explain why only six spots show up when the chromatographic plate is coated with ninhydrin and heated.

Key Concepts

Amino Acid Structure and Properties

Chromatography Principles

Ninhydrin Reaction with Amino Acids

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