2009_Book_FoodChemistry

22 1 Amino Acids, Peptides, Proteins
amino acids using spectrophotometry (cf. Reaction
1.42). The detection limit lies at 1–0.5nmol.
The resultant blue-violet color has an absorption
maximum at 570 nm. Proline yields
a yellow-colored compound with λ max = 440 nm
(Reaction 1.43):
(1.44b)
The derivatives are used for amino acid analysis
via HPLC separation. Instead of mercaptoethanol,
a chiral thiol, e. g., N-isobutyryl-Lcysteine,
is used for the detection of D-amino
acids. The detection limit lies at 1 pmol. The very
fast racemizing aspartic acid is an especially suitable
marker. One disadvantage of the method is
that proline and hydroxyproline are not detected.
This method is applied, e. g., in the analysis
of fruit juices, in which high concentrations of
D-amino acids indicate bacterial contamination
or the use of highly concentrated juices. Conversely,
too low concentrations of D-amino acids
in fermented foods (cheese, soy and fish sauces,
wine vinegar) indicate unfermented imitations.
Fluorescamine reacts with primary amines and
amino acids – at room temperature under alkaline
conditions – to form fluorescent pyrrolidones
(λ ex = 390 nm, λ em = 474 nm). The detection
limit lies at 50–100 pmol:
(1.43)
The reaction of amino acids with o-
phthaldialdehyde (OPA) and mercaptoethanol
leads to fluorescent isoindole derivatives
(λ ex = 330 nm, λ em = 455 nm) (Reaction
1.44a).
(1.45)
The excess reagent is very quickly hydrolyzed
into water-soluble and non-fluorescent compounds.
1.2.4.3 Reactions Involving Other Functional
Groups
(1.44a)
The most interesting of these reactions are those
in which α-amino and α-carboxyl groups are