What is the name of the molecule below? Source: Knox et al 1994

Question

BlackTom AI · Accepted Answer

The structure shows glucose residues joined by α-1,4 linkages along a chain with an α-1,6 branch point — the defining feature of amylopectin.

1. Relevant knowledge points:
- Glucose carbons in the pyranose ring are numbered 1–6; glycosidic bonds are described by the carbon numbers involved (e.g., 1→4, 1→6).
- "α" vs "β" refers to the configuration at the anomeric carbon (C1): in α-glucose the C1–OH is on the opposite side of the ring from C6 (in Haworth projection it appears "down").
- Starch consists of two polysaccharides: amylose (mostly linear α-1,4 chains) and amylopectin (branched, with α-1,4 chains and α-1,6 branch points). Cellulose is a linear polymer of β-1,4-linked glucose.

2. Step-by-step reasoning:
- Inspect the linkages: the diagram is annotated with an "α-1,6 linkage" at the branching point (a bond from C1 of one glucose to C6 of another). That explicitly indicates a branch via C6.
- The main chain linkages between residues are α-1,4 (typical for starch components).
- The stereochemistry at the glycosidic bonds corresponds to the α configuration (anomeric oxygen orientation consistent with α).
- A branched polymer having α-1,4 chains and α-1,6 branch points matches the structure of amylopectin.

3. Intermediate steps (identification):
- Find a branch: bond labeled 1→6 = α-1,6.
- Confirm chain linkages: remaining inter-residue bonds are 1→4.
- Confirm anomeric configuration: drawn as α (not the alternating orientation of β links).

4. Why the correct answer is correct:
- Only amylopectin among the choices is a branched α-1,4/α-1,6 glucose polymer. The annotated α-1,6 branch point directly identifies amylopectin.

5. Why other options are incorrect:
- Amylose: linear, no α-1,6 branch points.
- α-glucose / β-glucose: single monosaccharide units, not a branched polymer.
- Cellulose: linear polymer of β-1,4-linked glucose (different linkage and stereochemistry), no α-1,6 branches.

Conclusion: The structure is an amylopectin component of starch, confirming answer a.

What is the name of the molecule below? Source: Knox et al 1994单项选择题

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