Structure of vitamin K1 (phylloquinone) Vitamin K1 was first isolated from alfalfa and contains a phytyl side chain composed of how many isoprene units?

Introduction / Context:
Vitamin K1 (phylloquinone) is a naphthoquinone with a characteristic phytyl side chain. Recognizing its structural features helps in understanding its lipid solubility and biological handling in membranes.

Given Data / Assumptions:

• The phytyl group is derived from isoprenoid units (C5 each).
• Phylloquinone has a C20 phytyl side chain.
• Counting isoprene units connects structure to biosynthetic origin (mevalonate/isoprenoid pathway).

Concept / Approach:
Convert chain length to isoprene count: C20 corresponds to 4 × C5 units. Therefore, the phytyl side chain contains four isoprene units.

Step-by-Step Solution:
Note: phytyl length ≈ C20.Isoprene unit = C5.Calculate units: C20 / C5 = 4 units.Select “Four isoprene units.”

Verification / Alternative check:
Chemical depictions of phylloquinone show a long saturated isoprenoid tail consistent with four isoprenoid repeats, explaining its hydrophobicity and membrane localization.

Why Other Options Are Wrong:
Three or five or six or nine units do not match the C20 phytyl chain of vitamin K1.

Common Pitfalls:
Confusing vitamin K1 (phylloquinone) with vitamin K2 (menaquinones), which have multiple isoprenoid units that vary by species and length.

Final Answer:
Four isoprene units.