Organic Chemistry

compound responsible for the burning sensation felt in muscles during anaerobic exer-
cise, and it is also found in sour milk. Spinach and other leafy green vegetables are rich
in oxalic acid. Succinic acid and citric acid are important intermediates in the citric acid
cycle (Section 25.1), a series of reactions that oxidize acetyl-CoA to in biological
systems. Citrus fruits are rich in citric acid; the concentration is greatest in lemons, less
in grapefruits, and still less in oranges. -Malic acid is responsible for the sharp
taste of unripe apples and pears. As the fruit ripens, the amount of malic acid in the fruit
decreases and the amount of sugar increases. The inverse relationship between the levels
of malic acid and sugar is important for the propagation of the plant: Animals will not eat
the fruit until it becomes ripe—at which time its seeds are mature enough to germinate
when they are scattered about. Prostaglandins are locally acting hormones that have
several different physiological functions (Sections 17.10 and 26.5), such as stimulating
inflammation, causing hypertension, and producing pain and swelling.

Esters are also commonly found in nature. Many of the fragrances of flowers and fruits
are due to esters. (See Problem 27.)

Carboxylic acids with an amino group on the are commonly called amino
acids. Amino acids are linked together by amide bonds to form peptides and proteins
(Section 23.7). Caffeine, another naturally occurring amide, is found in cocoa and cof-
fee beans. Penicillin G, a compound with two amide bonds (one of which is in a
ring), was first isolated from a mold in 1928 by Sir Alexander Fleming.

H 3 NCHCO

+

O

R

NHCHC

O

R

NHCHC

O

R′

NHCHC

O

R′′

an amino acid general structure for a peptide or a protein

−

amide bonds

b-lactam

a-carbon

benzyl acetate

jasmine

isopentyl acetate

banana

methyl butyrate

apple

CH 3 COCH 2

O

CH 3 COCH 2 CH 2 CHCH 3

OCH 3

CH 3 CH 2 CH 2 COCH 3

O

(S)-(-)

CO 2

Section 17.4 Naturally Occurring Carboxylic Acids and Carboxylic Acid Derivatives 679

pyruvic acid oxalic acid succinic acid

C

C

O

O

C

C

O

O

(S)-(+)-lactic acid citric acid

OH

O

O

HOOCCH 2 CH 2 COOH

OH

OH HO OH HO

CH 3

CH 3

H

COOH

OH

COOH

COH

O

O

(S)-(−)-malic acid prostaglandin A 2

H OH

COH

O

HO

prostaglandin F 2 α

HO H OH

HOOCCH H

(^2) OH
COOH
CH 3
CH 3
N N
N N O
caffeine piperine
the major component of black pepper
penicillin G
N
O
CH 3
O
O
O
S
N
HH
O
COO−K+
CH 2 CNH CH 3
CH 3
O