section

2.2

Classification

19

Threonine

(a) Nonpolar

H — CH — COO

H C —CH— coo

CH—CH— coo

. a .

(hydrophobic)

Glyone

Alanine

valine

H £— H £

r u — r w ---

CH,— CH— COO

CH— CH— COO

CH— CH, — CH—

COO

Phenylalanine

Leucine

isoteucine

CH, — CH—

COO

H £ —

s —

CH— CH,:— CH—

coo

-- - N

co o

Pro

line

Methionine

Tryptophan

(b) Polar, negatively

OOC — CH — CHy— CH—

co o

o o c—

c h ,:— c h —

co o

charged (hydrophilic,

acidic m the protonated

state)

Glutamate

Aspartate

CH,— CH — COO

nh!

(c) Polar.

positively charged

(hydrophilic, basic

m the unprotonated

state)

Histidine

Lysine

rLN — C— NH — CH,— CH,— CH*.—

CH — COO

Arginine

(d)

Polar, neutral

(hydrophilic)

Asparagine

Giutamme

'

' X — rw »— r w

CH,— CH—

COO“

HS — CH,— CH—

COO

Cysteine

Tyrosine

FIGURE 2-3

Classification of commonly occurring L-a-amino acids based on the polarity of side chains (R-groups) at pH 7.0.