2009_Book_FoodChemistry

16 1 Amino Acids, Peptides, Proteins
The free ester is released from its salt by the action
of alkali. A mixture of free esters can then
be separated by distillation without decomposition.
Fractional distillation of esters is the basis
of a method introduced by Emil Fischer for the
separation of amino acids:
(1.14)
Free amino acid esters have a tendency to form
cyclic dipeptides or open-chain polypeptides:
Fig. 1.5. Ultraviolet absorption spectrum of tyrosine as
affected by pH. (according to Luebke, Schroeder and
Kloss, 1975) ---- 0.1mol/l HCl, --- 0.1mol/l NaOH
(1.15)
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 Chemical Reactions
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 acidcatalyzed
reactions. An ethyl ester hydrochloride
is obtained in ethanol in the presence of HCl:
(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 hydrogenation,
are used as protective groups in peptide synthesis.
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:
(1.13)
(1.17)