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Lecture Comments (4)

1 answer

Last reply by: Professor Hovasapian
Mon Oct 13, 2014 5:52 PM

Post by Jenika Javier on October 12, 2014

How can we identify the reducing end?

0 answers

Post by Professor Hovasapian on February 10, 2014

Hi Alan.

The Nitrogen has an understood hydrogen. My sincerest apologies: It's been one of my bad habits to ignore hydrogens that do not play a direct role in a reaction.

Hope all is well.

Take care.


0 answers

Post by Alan Delez on February 10, 2014

Hello Dr. Hovasapian,

For N-Acetyl-B-Glucosamine does that nitrogen contain an understood Hydrogen or a negative charge? Regards!

Hexose Derivatives & Reducing Sugars

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Hexose Derivatives 0:15
    • Point of Clarification: Forming a Cyclic Sugar From a Linear Sugar
    • Let's Recall the α and β Anomers of Glucose
    • α-Glucose
  • Hexose Derivatives that Play Key Roles in Physiology Progression 17:38
    • β-Glucose
    • β-Glucosamine
    • N-Acetyl-β-Glucosamine
    • β-Glucose-6-Phosphate
    • D-Gluconate
    • Glucono-δ-Lactone
  • Reducing Sugars 29:50
    • Reducing Sugars Overview
    • Reducing Sugars Example: β-Galactose

Transcription: Hexose Derivatives & Reducing Sugars

Hello and welcome back to, and welcome back to Biochemistry.0000

Today we are going to continue our discussion of monosaccharides.0004

I'll finish that off a little bit, and then we are going to start talking about disaccharides, 2 sugar units attached together.0006

OK, let's go ahead and get started.0013

Before I begin, I wanted to make sure there was one thing, point of clarification from the previous lesson.0018

It concerns the formation of the actual ring form from the linear form of the sugar.0026

I just wanted to make sure that everything was OK there.0033

So, let's go ahead and discuss that before we actually go on.0036

Point of clarification forming a cyclic sugar from a linear sugar.0043

OK, let's go ahead and do a linear sugar.0067

Let me go through the entire process.0069

Remember when I said rotate it to the right, and then grab it and bring it around.0071

I just wanted to make sure that we have everything OK.0075

There is lots of hydroxy groups that can actually react with the carbonyl carbon, with either the ketone or the aldehyde; and I just wanted to make sure which hydroxide is going to be reacting.0077

You know, most of the time, we are going to be dealing with the hexoses, so glucose.0089

I just wanted you to see exactly how we arrange things, how we rotate it, how we pull.0094

I just want to make sure that that's clear.0099

Once that that is clear, again, once the basics are clear, everything after that is absolutely perfect.0102

Let's go ahead and draw out this: 1, 2, 3, 4, 5 and 6.0108

Let's go ahead and draw out our glucose here.0116

We have right, left, right, right- that is the pattern, so CH2OH.0120

Now, what we are going to do here is we are going to go ahead and rotate this 90° to the right from the top.0130

OK, when we do that, we end up with the following.0140

I'm going to draw out the 6 linearly this way, horizontally: 1, 2, 3, 4, 5, 6.0143

Now, the aldehyde group is over here; and now, I have this OH down at the bottom, this OH up at the top, this OH at the bottom, this hydroxy at the bottom, and over here at this end, I have my H2OH.0150

OK, so far so good.0164

Now that I have it arranged like this, I grab the left end.0166

I grab this end and I pull it around to the back to have it come around in a circle and set itself up for attack.0171

Let me go ahead and do that.0179

From your perspective, what you are looking at is now, the carbonyl side is here; the CH2OH is here.0181

I'm going to pull it around to the back and just arrange it.0186

OK, I'm going to draw that now.0190

You know what, I'll stay with black; why not?0195

Let's go that, that, that, that.0200

Nope, we have, of course, this is the carbon; there is this.0204

I'm going to go ahead and put the H there.0207

Now, this hydroxy, this is this carbon.0209

This is down; this hydroxy is up.0212

This hydroxy is down.0216

Now, I've just pulled it straight back, so what I've got here is the following.0219

Let me see.0224

This is down, up, down, down.0226

On the no. 5 carbon...wait...1, 2, 3, 4, 5...yes.0228

And now, I have 1 more down; this hydroxy is down.0235

And, of course, this is the CH2OH.0240

Now, this is the no. 1 carbon; this is 2.0244

This is 3, 4, 5 and 6; 1, 2, 3, 4, 5, 6.0246

It is the hydroxy on the no. 5 that is actually reacting with the carbonyl.0255

Here, when I pull it around, when I'm doing this systematically to form the ring, when I pull it around, I still have to make an adjustment.0260

At this carbon right here, let me do this in red, I have to actually rotate 90° up.0270

Now, the CH2 group which is here and the hydroxy which is here, I need to do this.0276

So now, it is the hydroxy on the no.5 carbon and it is the CH2OH group, that is actually pointing up.0282

That is where we get our glucose structure.0289

I'm going to rotate this 90° that way, at the no. 5 carbon.0291

Let me redraw this ring structure here.0298

Let me go ahead and draw it down here.0300

OK, I'll write "Rotate the no. 5 carbon, so the hydroxy group is now horizontal, and the CH2OH group, which is this thing right here, this thing, the no. 6 carbon is pointing up, is vertical".0305

OK, I just want to make sure that this is absolutely clear.0342

It is not the hydroxy on the no. 6 carbon that attacks; it is the hydroxy on the no. 5 carbon that attacks.0346

Once we rotate it, now, we get this structure.0353

We get this, this, this, this.0356

Let me go ahead and write the C.0361

It's always a good idea to write the Cs.0363

Actually, you know what, I'm going to do this in black still.0367

Let me go back to black.0373

1, 2, 3, 4- this is our aldehyde.0375

Now, our OH is here and now, our CH2OH is up.0381

See here, it is that way.0386

That is just from turning it, pulling it back.0388

Now, we still have to rotate this one in such a way that the hydroxy is now ready for attack.0390

It is the no. 5 carbon, it is the hydroxy on the no. 5 that attacks the no. 1 carbon to form the 6-membered ring, so now, it is ready.0396

It can attack on the top; it can attack from the bottom, and, of course, what you end up getting is our glucose ring.0407

That and I'm going to do a little wavy line here because again, I'm not specifying the stereo of chemistry here.0418

It can be either alpha or beta.0422

If the hydroxy ends up below, in other words, if it attacks from above, it ends up being an alpha-glucose.0424

If it attacks from below and pushes the oxygen up, so that the hydroxy is above the ring, then it is going to be the beta-glucose.0430

So, this wavy line means I'm not specifying the stereo chemistry, but this CH2 group is up there; and of course everything else stays the same.0437

This is down; this is up, and this is down.0445

Now, we have our glucose.0449

I'm not going to be calling it D-glucose anymore, where just the assumption is, the sugars that we deal with are going to be the D-isomer.0451

If they happen to be an L-isomer, we will specifically say L, so I'm just going to write glucose.0460

There we go; that is what you are doing: rotate, pull around, make a little rotation.0466

So, it's the no. 5, the hydroxy on the no. 5 carbon that gives you this arrangement.0471

This is the conventional arrangement- oxygen on the back right.0478

And, let me go ahead and draw the actual projection here- the 3-dimensional projection.0481

We have that like that, and there we go.0487

This is our nice, basic glucose structure.0491

OK, I hope that helped.0495

Now, let's go ahead and move on to a discussion of conformation.0500

Let's recall the alpha and beta anomers of glucose.0505

And again, you can never get enough practice in actually drawing out these structures.0508

I hope you are not getting sick of actually, be drawing them out, drawing the out- it is really, really important.0513

Repetition is what keeps these things solid in your mind- repetition with your hand, in fact.0517

OK, let's recall - excuse me - the alpha and beta anomers.0523

And remember, we call them anomers because the carbonyl carbon is the anomeric carbon- once it has been converted to a hydroxide, once you form the ring, the anomers of glucose.0535

OK, we have 1, 2, 3.0546

OK, let's go ahead and draw the alpha.0550

This is that; this is that.0555

This is that, and CH2OH.0559

Let's go ahead and give a little bit of projection here.0563

OK, this is our alpha-glucose.0567

Hydroxy is down below the ring, and of course, we have our beta-glucose, where the hydroxy is above the ring, but everything else is the same, down and CH2OH.0572

Let's go ahead and put a little perspective on it.0587

There we go, good solid pictures of alpha and beta-glucose.0591

OK, now, these projections give you stereo chemistry, but they don't give you conformation.0599

These tell you exactly where the hydroxys are.0606

Here, it is below the ring; here, it is below the ring, above, below.0610

CH2 is above.0614

They give you stereo chemistry, but you remember from organic chemistry that a 6-membered ring is not a flat molecule.0616

It is not benzene.0621

OK, It achieves, these are the 1 Ns.0624

These Ns are fluked like this, so what you end up getting is 2 chair conformations.0627

When we work with them, we'd like to keep these forms simply because they are easy to see.0631

They are easy to see relationships, but how it really looks is a little bit different.0638

I'm going to go ahead and draw out - just so you see it - the chair conformations of these glucose molecules.0641

Let's see.0652

I'm going to go ahead and just do the alpha-glucose.0654

I said we are not going to be writing the D, so for alpha-glucose.0661

I'm having a hard time spelling today.0668

OK, let's try this again.0669

For alpha-glucose, OK, let's see if we can do this here.0673

Let me go ahead and just do...let me do it over this way.0676

This is that; that is that.0681

That is that; that is O.0684

Let me go a little bit further down here.0687

It is going to be there; this is going to be there.0689

This is going to be there.0693

And now, I'll draw in my axial position.0696

Axial is vertically down, 1, 2, 3; and, of course, boom, boom.0699

And now, I'll do my equatorial positions.0705

There is 1; there is another.0707

There is another; there is another, and this is always an interesting one.0711

It goes that way, and I have the oxygen.0718

OK, let me go ahead and do, here OH.0723

That means OH is down; OH is down.0727

This OH is up, so it is up here, and this OH, this is the 1, 2, 3, 4.0732

Let me number these so you can see them.0740

Oops, I wanted to do that in red, make sure we press it.0743

This is the no. 1 carbon, so 1, 2, 3, 4, and the 4 carbon is down; so the OH is here.0749

And now, over here, we have, of course, our CH2OH; and the others are just hydrogens.0757

That is OK, I can go ahead and put them in; it is not a problem.0765

Here, OK, that is one chair conformation.0770

Now, let me go ahead and put a little perspective on it.0772

I'm going to bold that out with a wedge.0777

I'm going to bold this out, and I'm going to bring this out and make it bold.0781

There you go.0788

This alpha-glucose, this is one of the conformations; this is the left side flipped up and the right side flipped down.0790

Now, we are going to be the left side flipped down, the right side flipped up.0797

Let me go ahead and draw that one out.0800

We have got this, that, that.0804

Again, oxygen is back there.0812

This is there; this is there, and this is there.0815

OK, now, our axials are here, here, here, here, here, and nothing over there; and now, I'll do our equatorials.0819

We have one there; we have got probably one like that, or it is probably not the best representation.0830

Actually, let me make it a little bit more angled.0837

These are always interesting to draw, aren't they?0841

OK, this one goes that way; this one goes that way, and this one goes that way.0844

OK, we said, we are still dealing with alpha-glucose.0851

OK, this is for alpha-glucose.0855

This is one conformation of alpha-glucose; this is the other chair conformation.0857

Alpha-glucose, this has no. 1 hydroxy, so let me go ahead and number.0861

This is 1, 2, 3, 4, 5, 6.0865

I'll go ahead and put the CH2 group up here; I'll just go ahead and write it in.0874

Now, let me go back to black.0878

It is still below the ring.0880

OK, this is below; this is above.0884

This is below.0889

OK, let me go ahead and put the Hs in just so we see them.0891

OK, so, we have something that looks like this; and now, this is the other.0896

Now, let me put a little perspective on it.0903

I just wanted you to see what this looks like.0912

These are the actual forms that they take.0916

Now, notice, in this particular case, in this one over here on the left, this CH2OH group, that is in equatorial position, not axial position.0917

It is actually pointing away from it; it's not vertical.0927

It is away from the ring.0929

Here, with a little bit of a flip, this flipping down, this flipping up, it takes on an axial position; it is vertical.0931

You remember from organic chemistry, the largest substituent on a 6-membered ring, because of steric reagent, it will actually arrange itself such that the largest substituent is in the equatorial position.0939

This particular conformation will probably be more abundant simply because the CH2OH in this conformation achieves an equatorial position.0955

Now, these 2, alpha and beta, are configurations.0966

The only way to go from 1 configuration to another configuration, bonds have to break.0971

In other words, this bond has to break and has to reform up here as a hydroxide.0975

These are conformations; bonds don't break.0981

The only thing that happens is the molecule flips around a little bit or bonds rotate.0984

That is the difference between conformation and configuration.0990

Alpha-glucose has 2 conformations, 2 chair conformations- this and this.0992

We don't give names to them; they are just the 2 conformations.0997

Beta-glucose also has 2 conformations- this and this.1000

They look exactly the same except for beta, the hydroxy would be up here, and the hydroxy would be up here.1004

That is the difference.1012

Let's go ahead and mark in blue just so we know the no. 1 carbon.1014

This is the anomeric carbon.1018

OK, and again, we still kept our conventional.1020

The oxygen is in the back right, oxygen is in the back right, but now, it is in actual position.1025

Now, this is what the molecule looks like in space, and here is the anomeric carbon for that one- there and there.1029

OK, now, let's talk about some hexose derivatives.1040

Derivatives just mean we have reacted them with something, and we have attached new groups to it.1046

That is all a derivative means; we start with something basic, and we derive something from it.1050

Let me go ahead and stick with blue; I like blue.1055

Hexose derivatives that play key roles in physiological processes.1061

I'm not sure about the extent to which your teachers are going to have you necessarily memorize these.1077

It is good to be introduced to them.1081

We will tell you a little bit about them; tell you their names just so if you run across them, you will have a good idea what it is that you are dealing with.1084

You are going to see these again, anyway.1090

Again, it is going to be up to your particular professor about the extent to which they want you to know the structure, the name, what it is, things like that; but it is good to see some, anyway.1093

OK, let's start off with our beta-D-glucose.1100

Again, it is always great to start with your basic structure, so you always know where you are coming from.1105

Beta, the hydroxy is up on the top; this one is down.1111

This one is up; this one is down, and we have CH2OH.1115

This is our beta-D-glucose.1120

OK, now, let's go ahead and make a little bit of a change here.1127

Let's go ahead and draw this same thing.1131

O, except, instead of a hydroxy - let's go ahead and do this one in red - let's go ahead and put an NH2 group there.1136

Let's go back to blue, and let's finish off the hydroxys and CH2OH.1147

OK, so, what I have done is I have taken this no. 1, no. 2.1155

The hydroxy on the no. 2 carbon, I've gone ahead and replaced that with an amino group and NH2.1158

This turns it into beta-glucosamine.1163

Beta-glucosamine or glucosamine, however you want to pronounce it- not a problem.1168

That is it; I've just replaced this with this.1177

The hydroxy on the no. 2 carbon is replaced with an amino group- nice and easy, OK, in other words an NH2.1187

OK, now, let's do another derivative.1208

Let's go ahead and draw it again; this time I'm going to do it.1210

You know what, let me stick with the blue, and let me go draw out my hexose ring.1214

And again, this is beta, except now, what we are going to do is we are going to put an N, and what we are going to put is C, and we are going to put a CH3, and we are going to put that there; and everything else is going to stay the same.1224

Oops, let me go back to blue.1239

Hydroxy, no, that is not right.1245

Hydroxy goes up; this is glucose and CH2OH.1249

And again, I hope that you are checking these structures with me because again, once you just sort of start writing them, mistakes are made.1258

We are all human.1265

OK, here, what we have is, notice, we have this amine part, which is the same.1266

So, we have the nitrogen, but we have stuck this acetyl group on it.1272

This is called N-acetyl-beta-glucosamine.1276

That is it; the acetyl is attached to the end.1289

I haven't attached it anywhere else; I could have.1293

So, this is N-acetyl-glucosamine.1296

That is it.1301

This is my new group.1304

This is a derivative.1306

That is it.1308

An acetyl group is attached to the N of the amino group.1310

OK, alright.1333

Now, let's do another derivative here.1342

Let's go ahead and go back to blue.1345

Let's draw our structure.1348

Actually, you know what, this one, I'm going to need a little bit more room up on top; so, let me try this again, draw it a little bit lower.1354

Let me draw it down here.1362

O and this is OH, and I'm going to go CH2O.1364

Let me actually finish the glucose part first.1375

This is up, and this is down.1377

There we go; and now, I have P, double bond O, single bond O-, single bond O-.1382

We have this phosphate group - OK - attached to the no. 6 carbon.1389

This is the no. 6 carbon, no. 1 carbon- anomeric.1401

This is, well, beta-glucose-6-phosphate.1405

This is beta-glucose-6-phosphate.1411

It just is that I have a beta-glucose, and on the no. 6 carbon, I have attached a phosphate.1418

OK, this is the first step of glycolysis, where glucose is converted to glucose-6-phosphate.1424

OK, now, let's draw another derivative here.1431

Let's see how these turn out.1435

I wonder if I should do...that's OK, I guess I can do it in this page, not a problem.1437

This one I'm going to do in black.1440

Let me go ahead and go this way.1444

Well, that is fine; I'll go ahead and just do, I'm going to draw this particular one out.1449

No, what am I doing?1456

Let me go OH, CH2OH, and this is going to be O-.1463

OK, that is up; that is up.1472

That is down; that is up, and this is down.1476

OK, now, what we have done here is the aldehyde, which is originally this thing, is now, the aldehyde has been oxidized to a carboxylic acid or a carboxylate.1479

In this case, it is carboxylate because the hydrogen is deprotonated, so it is a -1 charge.1508

If the hydrogen were attached, it would be the actual carboxylic acid.1513

OK, this is called, in this form, D-gluconate or just gluconate.1517

Again, we are dealing with the D configuration.1526

So, this is gluconate.1530

Now, this particular one, gluconic acid - that is the protonated form, OK - is called gluconic acid.1532

The deprotonated form is just gluconate.1545

Carboxylic acid, carboxylate, OK, the general term for this class- an aldonic acid.1550

Whenever you take the linear form of the sugar, and then when you oxidize the aldehyde end to a carboxylic acid but without oxidizing anything else, just this one, just the aldehyde end, you turn it into something called an aldonic acid or an aldonate.1560

In this particular case, since we used glucose, we turned it into gluconic acid; but it is deprotonated, so we call it a gluconate.1575

Again, it is just a question of protonation and deprotonation.1583

This is the general term; it is called an aldonic acid when you oxidize just the aldehyde, not anything else.1586

OK, now, here is what's interesting about this.1594

We have this carboxylate group, and we still have this hydrogen over here, and it still has these nucleophilic electrons.1600

So, the hydroxy on the no. 5 carbon - and again, this is the no. 5 carbon there - can still react with the no. 1 carbon - this is the no. 1 carbon, the no. 1 carbon hasn't changed - to form something called a lactone.1609

And, a lactone is just a fancy word for a cyclic ester.1647

It is an ester that is a part of a cycle.1651

We put the parentheses down here.1658

A lactone is nothing more than a cyclic ester.1662

OK, let's go ahead and draw this out.1666

What I have got is this, this, this, this, this, that.1671

I have that; I have OH.1678

This is down; this is up.1680

This is down, and this is CH2OH.1682

Nothing else has been oxidized, and this is called glucono-delta-lactone.1687

Remember what we have done here, that hydroxy?1698

It actually reacts with the carbonyl; it kicks off that other OH, that other O-.1703

Actually, let me go ahead; and let me draw it here, again, just off to this side.1711

So, we have something like this: COO, O-, this, and CH2OH, down, up, down.1715

Remember we had something that looked like this on the previous page?1728

This is our gluconic acid.1731

Well, this thing is actually going to form a bond with this thing, and this thing is going to end up going away.1737

We won't worry about the mechanism, but you end up with something like this.1741

This is a lactone; it is a cyclic ester.1744

An ester is a carbonyl with an oxygen attached to another carbon, but this carbon happens to be part of the ring.1747

So, a cyclic ester is called a lactone.1756

Now, it is called a delta-lactone because, remember what we said, the carbonyl is the no. 1 carbon, but the carbon next to that is called the alpha carbon.1758

This is alpha, well, this is the beta carbon.1766

This is the gamma carbon; this is the delta carbon.1770

Because the oxygen next to the carbonyl is attached to delta carbon, it is called a delta-lactone.1773

It will be a gamma-lactone, an epsilon-lactone.1780

That is all that's going on here.1785

OK, alright.1787

Let's go ahead and close this lesson off by talking a little bit about the idea of a reducing sugar.1790

Let me draw a little bit of a line here.1795

Now, monosaccharides - I'll just write monosacchs - can be oxidized by very mild reagents such as, in particular, I should say, Fe3+ and Cu2+.1798

So, the sugar is oxidized.1830

OK, they are reducing agents.1832

Reducing agents are the things that are oxidized.1843

They are reducing agents because they reduce the iron, and they reduce the copper; but they themselves are oxidized- very, very important.1847

OK, as such, they are called reducing sugars.1856

They are called reducing sugars.1862

And, the carbonyl carbon, the carbonyl C, is oxidized to a carboxyl group.1874

The general reaction is as follows.1897

You have RC.1900

This is the aldehyde.1904

OK, let's just go ahead and use 2Cu2+ to 2Cu1+, and what you end up with is RC.1907

What you end up with is that.1922

This aldehyde is actually converted to a carboxyl, OK, because that is available, the free end of the linear form of the sugar.1924

So, we call them reducing sugars because they are capable of being oxidized by iron or copper, iron ionic, copper ion; as such, we call them reducing sugars because they are reducing agents.1935

That whole oxidation-reduction thing, calling them reducing agent, it still confuses me.1949

I just think in terms of oxidation, but they are called reducing sugars.1953

OK, let's go ahead and draw out the reaction in full form.1956

Let's go ahead and start off with a nice hexose structure like this.1962

Let's go ahead and use our beta form.1970

OH, OH, OH, and then we have CH2OH there, so this is...actually you know what, I'm going to use...I'll make a little bit of a change.1974

Just for a little, slight variation, I'm going to use galactose.1993

Instead of glucose, I'm going to use beta-galactose just for a little bit of a variation.1998

This is beta-galactose.2007

Notice, in the glucose so, 1, 2, 3, 4.2008

Galactose is a c-4 epimer of glucose.2012

Everything is the same except at the fourth carbon, the hydroxy is on the other side.2016

It is above the ring instead of below the ring.2021

So, it is a different, actual molecule.2023

Now, the ring form is going to be in equilibrium with the linear form.2028

There is going to be some linear form of this sugar that is going to be available.2034

1, 2, 3, 4, 5, I have 6 carbons.2038

I open it up; I have the aldehyde on top, and of course, I have OH here.2041

This time, I have 2 OHs on the left.2047

I have this one - oops - and, of course, I have my non-chiral carbon.2050

CH2OH, there we go.2057

Now, once it actually opens, and this end is free, it can react with 2Cu2+ ions to release 2Cu1+ ions, and, of course, now, 1, 2, 3, 4, 5, 6.2061

This is H2OH; I have my carbonyl which has been ox by aldehyde, has been oxidized now, to a carboxylic acid.2086

This hydroxy is on the right; this hydroxy is on the left.2097

This hydroxy is on the left, and this hydroxy is on the right.2100

So, what I have here is beta-galactose ring form.2104

This is also beta-galactose linear form.2110

This one, galactonate, remember gluconate, galactonate.2115

This O-N, just drop the O-S-E from the galactose, glucose, mannose.2125

Add the O-N, and either put galactonic acid, if it is protonated, or galactone if it is deprotonated; and, that is it.2129

I'll go ahead and write that, in other words, if this is protonated right up here, if that is protonated.2139

OK, now, of course, the aldehyde group must be free in order for this reaction to take place.2157

In order to be oxidized, the aldehyde has to be free.2175

These are in equilibrium.2184

In solution, yes, it is going to exist mostly in this form, but there is always going to be some of this available; and once this is actually used up and converted to this, then more of this is going to open up.2185

This is just an application of Le Chatelier’s principle.2195

A reducing sugar is precisely this.2199

A reducing sugar is a sugar that can actually be oxidized by Cu2+ or Fe3+ to form the corresponding aldonic acid, which itself can actually form a ring again to form that delta-lactone that we just mentioned a little bit earlier.2201

OK, that should just about cover it as far as derivatives is concerned.2220

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