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Figure 4.2
Reaction scheme for bioconjugation between primary amines and
an isothiocyanate (A), a NHS ester (B), and a carboxylate in combination with the
coupling reagents EDC and NHS (C). Figure provided by courtesy of Vu Hong (TSRI,
La Jolla, CA, USA).
carboxylate that is not engaged in the formation of a peptide bond) can also
be utilized; its p
3.0-4.0. That means
at physiological pH the carboxylates are ionized (unprotonated). However,
even an unprotonated carboxylic acid, known as a carboxylate, is not a
good nucleophile. In order to make use of carboxylates for bioconjugation,
coupling reagents such as EDC and NHS are used (Aslam & Dent, 1999;
Hermanson, 1996). The reaction is as outlined in Fig. 4.2 (panel C); however,
in this case, the carboxylate is derived from the VNP and the amine from the
molecule to be coupled.
Carboxylates derived from Asp and Glu have been exploited on a few
VNPs (see Section 4.3.2); however, carboxylate-derivatization techniques are
by far less popular compared with amine- and thiol-selective techniques.
K
is typically lower and lies at around
≈
a
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