monosaccharides, sugar

To whoever first posted this meme
on fb, kudos!
You even almost got it right...
almost.
Yes, the car is a Cube.
Yes, carbohydrates are considered sugars by scientists.
So, yes, that makes that chemical formula that of a 'sugar'.
However, the chemical formula
on that license plate is not that of
the 'sugar' kept in bowls on counters and tables for sweetening beverages and cereals.
That 'sugar', aka sucrose, is a disaccharide, with the chemical formula C 12 H 22 O 11.

The license plate is for the class of 'sugars' called monosaccharides, specifically that subset known as the hexoses.
The person who crafted the meme would have been more correct to have called the car a "glucose Cube", if an aldohexose was to be specified for that car tag.
However, if a ketohexose was to have the limelight, then the meme-creator would have called the vehicle a "fructose Cube".
Not that those are the only hexoses which naturally occur, but they are the ones most people know, at least by name.
Why do I care to make this fuss?
Well, I am a teacher of biochemistry and I prefer to have correct science out there. While I am elated that someone saw that license plate and thought "chemical formula", their linking of it to table sugar galls me.
(Not that I was going for the joke, but sweet, isn't it?)
(smile)

As I've mentioned before, at least a time or two, the CHEM1152 class I teach every Spring semester covers organic chemistry and biochemistry.
Well, we have completed the organic portion and are heading straight into the other right now.
Tonight, before lecture, I had meant to take a quick look at the monosaccharides to refresh my memory. I try to select a few different ones from one year to the next, to keep it interesting for me.
That didn't happen.
Somehow, it was suddenly time for class and I had no time to peruse the new textbook.
Into the classroom I strode, ready to 'fly without a net' as I have been doing.
No notes, no list of topics, just me and my experience up there at the board...
again.
And guess what?
My brain led me, and quite logically, down a path I had not trod before...
but one I will try to remember for the future.
Usually, I spend this first biochem lecture on a range of monosaccharides, drawing a variety of molecular structures in the Fischer projection style, assigning names and classifications to them.
That means a lot of the words coming out of my mouth sound very similar.
Aldohexoses, ketohexoses, aldopentoses, aldotetroses, ketopentoses...
you get the drift.
In addition, I present the concept of chirality to account for the stereoisomers for each of the enantiomers for a monosaccharide of a given chemical formula. I even draw some of the D and L forms, to emphasize the point.
That is not how I presented the material tonight.

Instead, after a few definitions and general discussion about carbohydrates, I drew a structure, leaving it incomplete at first.
Only the six-carbon skeleton, with its aldehyde crown, was shown.
I spoke of it being the Fischer projection of glucose.
I spoke of the ways to classify it.
Then, I added the alcohol groups onto carbons 2, 3, and 4, defining that as the 'fingerprint' for which was specific to glucose.

Finally, I talked about that penultimate carbon, explaining briefly about plane-polarized light and the ability of chiral carbons to rotate that light to one side or another.
I gave credit to the Latin language, of course, for the "dextro" and the "levo" prefixes for that right or left direction of light rotation.
After all, how better for the students to remember the meaning of "D" and "L" in the nomenclature?

Everyone seemed to have a good grip on that!
So I moved on, showing the
Fischer for fructose, giving this molecule the same slow-motion treatment as I had for the aldo-enantiomer, pointing to the fingerprint and its resemblance
to that of glucose.
(Note: D-glucose and D-fructose
are joined together to form
the sucrose molecule,
the one which began this post.)

The next topic was Haworth structures, the cyclic depictions of the saccharides.
I took full advantage of having different colors to illustrate the method I use for drawing the rings.
They can be a bit tricky for the novice, but I am sure everyone was quite comfortable with the three-step method employed.
Next, I added the alcohol groups
to the fingerprint carbons and
we were almost done.

Their placement was below the bond ("down") for right-hand -OH's and above the bond ("up") for the left-hand hydroxyls.
What to do about placing an alcohol on that first carbon?
Well, that was our choice!
In the 'up' position, the molecule is marked as 'beta'; the 'down' position denotes the 'alpha' version.
That means each disaccharide molecule will have four possible ring structures, two for each "D" and two for each "L".


On Monday, I'll talk about galactose, then launch into the disaccharides and their structures.
Who knows?
Maybe I'll even deign to share them here.
(smile)
Now, I'm going to wind down for an early night.
Sweet dreams, y'all!
May visions of sugarplums dance in your head!
(smile)