Organic Chemistry

agent used in margarine. and other colored compounds are found in the

leaves of trees, but their characteristic colors are usually obscured by the green color of

chlorophyll. In the fall, when chlorophyll degrades, the colors become apparent. The

many conjugated double bonds in lycopene and cause the compounds to be

colored (Section 8.13).

PROBLEM 11 SOLVED

Mark off the isoprene units in menthol, zingiberene, and squalene.

SOLUTION For zingiberene, we have

PROBLEM 12

One of the linkages in squalene is tail-to-tail, not head-to-tail. What does this suggest

about how squalene is synthesized in nature? (Hint: Locate the position of the tail-to-tail

linkage.)

PROBLEM 13

Mark off the isoprene units in lycopene and Can you detect a similarity in the

way in which squalene, lycopene, and are biosynthesized?

26.7 Vitamin A

Vitamins A, D, E, and K are lipids (Sections 25.9 and 29.6). Vitamin A is the only

water-insoluble vitamin we have not already discussed. which is cleaved

to form two molecules of vitamin A, is the major dietary source of the vitamin. Vita-

min A, also called retinol, plays an important role in vision.

The retina of the eye contains cone cells and rod cells. The cone cells are responsi-

ble for color vision and for vision in bright light. The rod cells are responsible for vi-

sion in dim light. In rod cells, vitamin A is oxidized to an aldehyde and the trans

double bond at C-11 is isomerized to a cis double bond. The mechanism for the en-

zyme-catalyzed interconversion of cis and trans double bonds is discussed in

Section 18.15. The protein opsinuses a lysine side chain (Lys 216) to form an imine

with (11Z)-retinal, resulting in a complex known as rhodopsin.When rhodopsin

b-Carotene,

b-carotene

b-carotene.

b-selinene,

-carotene

lycopene

b-carotene

b-Carotene

016 CHAPTER 26 Lipids

Tutorial:

Isoprene units in terpenes