CO 11 · Polymers 1 — Answer Key

(a) · Monosaccharide · Fischer → chair

Draw the β-pyranose form as a chair.

Starting Fischer projection is D-glucose. Answer: β-D-glucopyranose (all substituents equatorial — the flattest, most thermodynamically stable hexose).

β-D-GLC-p

Given · Fischer projection

Answer · β-D-Glucopyranose chair (⁴C₁)

↻ Loading β-D-glucopyranose · CID 64689 …

β-D-Glucopyranose · 3D

Derivation — how to turn a Fischer into a chair

Identify the sugar first. The Fischer has CHO at top, CH₂OH at bottom, six carbons, one stereocenter flipped at C3 (HO on left), others on right. Pattern R, S, R, R → D-glucose.

Rule: bottom-most stereocenter (C5) has OH on the right = D.

Hemiacetal ring closure. The C5 hydroxyl attacks the C1 aldehyde. The ring O sits between C1 and C5. This makes C1 the anomeric carbon — the only stereocenter whose configuration is set by ring closure (α or β).

β anomer: new C1–OH is cis to CH₂OH at C5 (reference substituent).

Apply the Fischer→chair rule. For any D-sugar in the ⁴C₁ chair with ring O at upper-right:
· right of Fischer = down on chair (axial/equatorial determined by position)
· left of Fischer = up on chair
· C5 CH₂OH points up (equatorial) for D-sugars.

Place each substituent. Using the rule on D-glucose:
C2 OH (right in Fischer) → down-equatorial
C3 OH (left) → up-equatorial
C4 OH (right) → down-equatorial
C5 CH₂OH → up-equatorial
C1 OH β → up-equatorial

β-D-glucopyranose is the only common aldohexose with every substituent equatorial — that is why glucose is biology's default monosaccharide.

Verify by comparing to the α anomer. α-D-Glcp has the C1–OH axial (down); all other positions identical. The β→α energy difference in water is about +0.5 kcal/mol for β (slightly more stable due to all-equatorial), but anomeric effect narrows the gap — equilibrium in water ≈ 36% α : 64% β.

Sanity check. Count the stereodescriptors. β-D-glucopyranose = (2R,3S,4R,5R)-pyranose with β (1R) anomeric. Five stereocenters total, all derivable directly from the Fischer pattern you started with.

(b) · Monosaccharide · chair → open-chain Fischer

Draw the open form as a Fischer projection.

Chair shown is β-D-glucopyranose. Open the hemiacetal at C1 → aldehyde carbonyl back. Result: the same linear D-glucose Fischer projection.

OPEN-CHAIN

Given · β-D-glucopyranose chair

Answer · D-glucose Fischer (open chain)

↻ Loading D-glucose (linear) · CID 107526 …

D-glucose · open chain · 3D

Derivation — chair → Fischer, step by step

Verify the chair is a D-aldohexose. Ring O at upper-right, CH₂OH at C5 pointing up (equatorial) → D-sugar. Six ring atoms (5 C + 1 O) → pyranose. Anomeric carbon is C1, right of ring O.

Note which substituents are up vs down. In this chair: β-OH at C1 (up-eq), OH at C2 (down-eq), OH at C3 (up-eq), OH at C4 (down-eq), CH₂OH at C5 (up-eq). Anomeric β means C1–OH is on the same face as C5 CH₂OH.

Break the C1–O(ring) bond. Hemiacetal → aldehyde + free OH. The ring oxygen stays on C5 as the C5–OH. C1 now has C=O (the original aldehyde comes back).

Translate up/down to right/left on Fischer. Invert the up-down rule from Part A:
up on chair → left on Fischer
down on chair → right on Fischer
So: C2 down → OH right; C3 up → OH left; C4 down → OH right; C5 up (CH₂OH) → CH₂OH at bottom.

Draw vertically with CHO on top, CH₂OH on bottom. Horizontal bonds = toward viewer, vertical = away (Fischer convention). Final pattern: OH at C2 right, C3 left, C4 right, C5 right. That's D-glucose.

The anomeric information (α vs β) is lost when you ring-open — in the Fischer, C1 is an sp² aldehyde with no stereochemistry.

Why this matters. In solution D-glucose is a four-way equilibrium: α-pyranose ⇌ open-chain ⇌ β-pyranose ⇌ (trace) α/β-furanose. The open chain is <0.03% but it's the reactive form — this is why glucose reduces Tollens reagent (silver mirror) and Fehling's (red Cu₂O) even though almost none of it is linear at any moment. This dynamic is called mutarotation.

(c) · Disaccharide · sugar unit 1

Classify Sugar 1.

Carbon chain · ring size · carbonyl type · configuration at the anomeric position.

α-D-GLC-p

Carbon chain

Hexose6 carbons (C1–C6), CH₂OH at C6

Ring size

Pyranose6-membered ring (5 C + 1 O), 4C1 chair

Carbonyl type

AldoseC1 is the anomeric carbon — latent aldehyde (hemiacetal)

Anomeric config.

α · DC1–O(glycosidic) is axial (down) — trans to C5 CH₂OH

How we reached "α-D-glucopyranose" for sugar 1

Count carbons → hexose. Sugar 1 has a pendant CH₂OH (external CH₂-OH off C5) plus four ring-carbons bearing OH/H — six total. Hexose.

Ring size → pyranose. Six-atom ring, one ring O between C1 and C5 (upper-right of ring in the drawing). 5 C + 1 O = pyranose.

Carbonyl type → aldose. In the cyclic form the original carbonyl sits at the anomeric C1. For an aldose the anomeric C is at the end (position 1) adjacent to ring O, with a free OH and an H. Sugar 1 matches. A ketose would have its anomeric C at C2 (internal, two C-substituents).

Configuration at C1 → α. The glycosidic O at C1 points axially down in the drawing (below the ring plane) while the reference CH₂OH at C5 points up. "C1 substituent trans to the CH₂OH reference on a D-sugar" = α. For β it would be cis (both up).

Rule of thumb: α D-sugar → anomeric OR group axial; β D-sugar → equatorial.

D vs L → D. C5 CH₂OH points up (equatorial) on a standard 4C1 chair with ring O upper-right — D-configuration.

↻ Loading α-D-glucopyranose · CID 79025 …

α-D-Glucopyranose · 3D

Why α vs β matters. An α(1→4) glycosidic bond (like in starch / glycogen) gives a helical polymer. A β(1→4) bond (like in cellulose) gives straight extended chains with H-bond networks. Same sugars — different single-atom geometry at C1 — produces fundamentally different materials. Starch is digestible; cellulose is not (mammals lack β-glucosidase).

(d) · Disaccharide · sugar unit 2

Classify Sugar 2.

A 3-(N,N-dimethylamino)-3-deoxy-D-glucose. Same glucose skeleton as Sugar 1, but the C3-OH has been replaced with a dimethylamino group. CH₂OH at C6 is retained. This is the canonical cationic aminosugar motif seen in nucleoside-sugar conjugates, aminoglycoside and macrolide antibiotics.

β-D-3-NMe₂-GLC

Carbon chain

Hexose6 carbons — even though 3 of them lack hydroxyl (see below)

Ring size

Pyranose6-membered ring (5 C + 1 O)

Carbonyl type

Aldoseanomeric C at C1 — derived from an aldohexose (D-glucose biosynthetically)

Anomeric config.

β · Dglycosidic O at C1 equatorial, cis to C5 methyl (reference)

Why this isn't just "glucose with an amine"

Start from D-glucose and swap one hydroxyl. D-glucose has OH at C2, C3, C4, and CH₂OH at C6. Sugar 2 retains the CH₂OH at C6, OH at C2, OH at C4 — the only modification is C3: OH → N(CH₃)₂.

3-(N,N-dimethylamino)-3-deoxy-D-glucose. Formal name. The 3-deoxy part reflects that the original C3 oxygen is gone; the 3-(dimethylamino) part reflects what replaced it. Nitrogen has two methyls attached (tertiary amine). pKa of the ammonium conjugate acid is ≈ 9.2, so at physiological pH this group is protonated, cationic.

Biological precedent: desosamine (on erythromycin) is the 3,4,6-trideoxy version of this same aminosugar. If your worksheet lacks the C6 CH₂OH, you're looking at desosamine; if it has CH₂OH, it's the "glucose-like" 3-aminodideoxy.

Aldose vs ketose → aldose. Anomeric C is C1 (terminal, adjacent to ring O). Parent carbonyl was an aldehyde. Same as Sugar 1.

Ring size → pyranose. Six-atom ring: 5 C + 1 O. Ring O sits between C1 and C5 as always.

Anomeric configuration → β. In the drawing, the anomeric OH at C1 of Sugar 2 sits equatorial, on the same face as the C5 CH₂OH. Equatorial = cis to reference = β on a D-sugar. (Sugar 2's anomeric C is free — the glycosidic bond attaches at Sugar 2's C4, not C1.)

D vs L → D. C5 CH₂OH points up/equatorial, matching D-glucose.

↻ Loading 3-(dimethylamino)-3-deoxy-D-glucose · CID 23615671 …

Desosamine · 3D

Position	D-glucose	Desosamine
C1 (anomeric)	OH	OH / O-glycosidic
C2	OH	OH
C3	OH	N(CH₃)₂
C4	OH	OH (or glycosidic O if it's the linkage point)
C5	H (sp³ C)	H (sp³ C)
C6	CH₂OH	CH₂OH (retained in this sugar)

Why this motif matters. The protonated tertiary amine NMe₂H⁺ (pKa ≈ 9) is a cationic pharmacophore. It lets sugars H-bond AND ion-pair simultaneously — exactly the combo that drives ribosomal binding of macrolide antibiotics (erythromycin, clarithromycin) via 23S rRNA, and that drives aminoglycoside / glycopeptide drug binding more broadly. Removing the NMe₂ typically abolishes activity.

(e) · Entire structure · polymer class

Classify the entire structure by number of units.

Two covalently linked monosaccharide residues. One glycosidic linkage. No repeat structure.

DISACCHARIDE · n=2

Count

2 unitsSugar 1 (α-D-glucopyranose) + Sugar 2 (β-D-desosamine)

Class

Disacchariden = 2 · joined by a single glycosidic bond

Heterogeneity

Hetero-two different sugar residues (not two glucoses) → heterodisaccharide

Reducing?

ReducingSugar 2 anomeric C is in glycosidic bond, but Sugar 1 anomeric C is free (hemiacetal) → still reducing

Spectrum of "polymer size" terms. monosaccharide (n=1) → disaccharide (n=2) → oligosaccharide (n=3–10, roughly) → polysaccharide (n≫10). This molecule sits at n=2 → disaccharide. Specifically a heterodisaccharide (two different units) and, because one anomeric C is free, it is also a reducing disaccharide (unlike sucrose, where both anomeric centers are locked in the glycosidic bond).

↻ Building disaccharide · glucose-α(1→4)-desosamine approximation …

Glucose-α(1→4)-Desosamine disaccharide · 3D

(f) · Connectivity · glycosidic linkage

Circle the glycosidic bond(s) and classify the linkage(s).

One linkage. α(1→4) O-glycosidic bond from the C1 of Sugar 1 (glucose) to the C4 of Sugar 2 (desosamine). Circle highlighted in orange.

α(1 → 4)

Feature	Value	How we know
Bond type	`O-glycosidic`	Linker atom between C1 of sugar 1 and C4 of sugar 2 is an oxygen. If it were N it would be N-glycosidic (as in nucleosides).
Anomeric configuration	`α`	At Sugar 1's C1, the glycosidic O points axially (down in the ⁴C₁ chair) — trans to the C5 CH₂OH. α.
Linkage carbons	`1 → 4`	From C1 (anomeric) of Sugar 1 to C4 of Sugar 2. C4 of desosamine is a plain sp³ carbon bearing H in the free sugar, becomes substituted here.
Full descriptor	`α-D-Glcp-(1→4)-β-D-desosamine`	Donor sugar (anomeric end) written first, then linkage in parentheses, then acceptor.
Number of glycosidic bonds	`1`	Disaccharides have exactly one. (Branched oligos have more.)

Caveat on which anomer is which sugar. The worksheet's chair depictions can be read with minor ambiguity depending on orientation. If your instructor's key has Sugar 1 drawn with β at C1 instead of α, the entire logic above swaps α↔β. The identity of each residue (glucose / desosamine) and the (1→4) position of the bond are not in doubt — only the axial-vs-equatorial read at the anomeric C.

Circling the bond — what exactly to draw around

Draw a loop around three atoms: C1 – O – C4. The oxygen is the glycosidic oxygen; the two C–O bonds flanking it are the glycosidic bond system.

Label outside the circle with the full classification: α(1→4) O-glycosidic.

If asked "how many": one. There is a single glycosidic bond in any linear disaccharide.

bonus · cheat sheet

Classification vocabulary — the quick version.

Everything a question like this can ask, in one place.

REFERENCE

Axis	Buckets	How to tell
Carbon chain length	`triose(3)` · `tetrose(4)` · `pentose(5)` · `hexose(6)` · `heptose(7)`	Count carbons including the CH₂OH and carbonyl carbons.
Carbonyl type	`aldose` (CHO at C1) · `ketose` (C=O at C2, usually)	Look at Fischer: terminal CHO = aldose; internal C=O with CH₂OH above = ketose.
Ring size	`furanose` (5-mem: 4C+1O) · `pyranose` (6-mem: 5C+1O)	Count ring atoms. Name derives from furan / pyran.
Anomeric configuration	`α` · `β`	Relative to the reference substituent (the one that sets D/L, i.e., the CH₂OH at the last chiral C). trans=α, cis=β for D-sugars.
D vs L	`D` · `L`	OH on right at the bottom-most stereocenter in Fischer = D. (In nature, almost all monosaccharides are D.)
Polymer size	`mono` · `di` · `oligo (3–10)` · `poly (>10)`	Count residues.
Glycosidic bond	`α/β` + `(x→y)`	Anomeric config of donor + carbon numbers linked. `α(1→4)`, `β(1→4)`, `α(1→6)`, `α,β(1↔2)` (sucrose-style), etc.
Reducing vs non-reducing	`reducing` (free anomeric C) · `non-reducing` (both anomeric C's locked in glycosidic bonds)	If at least one anomeric C is still a hemiacetal (free OH), it can ring-open → reducing.