Food Chemistry

16 1 Amino Acids, Peptides, Proteins

Fig. 1.5.Ultraviolet absorption spectrum of tyrosine as
affected by pH. (according toLuebke, Schroederand
Kloss, 1975)---- 0.1mol/l HCl, --- 0.1mol/lNaOH

Absorption readings at 280 nm are used for the
determination of proteins and peptides. Histidine,
cysteine and methionine absorb between 200
and 210 nm.

1.2.4 ChemicalReactions

Amino acids show the usual reactions of both
carboxylic acids and amines. Reaction specificity
is due to the presence of both carboxyl and amino
groups and, occasionally, of other functional
groups. Reactions occurring at 100–220◦C, such

as in cooking, frying and baking, are particularly
relevant to food chemistry.

1.2.4.1 Esterification of Carboxyl Groups

Amino acids are readily esterified by acid-
catalyzed reactions. An ethyl ester hydrochloride
is obtained in ethanol in the presence of HCl:

(1.13)

The free ester is released from its salt by the ac-

tion of alkali. A mixture of free esters can then

be separated by distillation without decomposi-

tion. Fractional distillation of esters is the basis

of a method introduced byEmil Fischerfor the

separation of amino acids:

(1.14)

Free amino acid esters have a tendency to form

cyclic dipeptides or open-chain polypeptides:

(1.15)

(1.16)

tert-butyl esters, which are readily split by acids,

or benzyl esters, which are readily cleaved by

HBr/glacial acetic acid or catalytic hydrogena-

tion, are used as protective groups in peptide syn-

thesis.

1.2.4.2 Reactions of Amino Groups.

1.2.4.2.1 Acylation

Activated acid derivatives, e. g., acid halogenides

or anhydrides, are used as acylating agents:

(1.17)