Page 56 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002G-51 May 25, 2001 13:44

94 Bioconjugate Chemistry

FIGURE 3 PEG. R and R refer to attached groups.
reactive site in a protein containing many other, nitrogen-
FIGURE 1 D-Biotin conjugated to another molecule, represented
by R. or oxygen-based, nucleophiles. Because of their affinity
for metals, thiol groups are also used to attach molecular
linkers to metal particles or surfaces.
substances that add useful properties (polymers, drugs, ra-
dionuclides, toxins, fluorophores, photoprobes, inhibitors,
enzymes, haptens, ligands, surfaces, metal particles, etc.). 2. Amines
The amino ( NH 2 ) group, which occurs on the side chain
I. CHEMICAL PRINCIPLES of the amino acid lysine, is also a reactive nucleophile.
Because lysine is a common amino acid (a typical an-
tibody molecule of type IgG has more than 80 lysine
A. Nucleophiles and Electrophiles
residues), it is often the target of chemical modification by
A basic feature of almost all applications of bioconjugate electrophiles.
chemistry is the attachment of one chemical species to
another. Most often this is done by forming an ordinary co-
valent chemical bond, taking advantage of naturally occur- 3. Disulfides
ring reactive groups where possible. For example, proteins
The disulfide bond ( S S ), formed by mild oxidation
commonly have an abundance of nucleophiles (electron-
of two thiols, plays a significant role because it is eas-
rich atoms or molecular groups) on their surfaces (see
ily broken by mild reducing agents. The disulfide bond is
Fig. 4), which can react with electrophiles on synthetic
not particularly reactive to nucleophiles or electrophiles,
reagents.
providing a strategy to protect a thiol from reaction. A
disulfide bond can also be used to temporarily attach one
B. Chemistry of Functional Groups component of a bioconjugate to the other; reductive cleav-
and Reagents age can easily reverse the attachment.
1. Thiols
4. Active Esters
The thiol ( SH) group, which occurs on the side chain of
the amino acid cysteine, is a particularly reactive nucle- a. Carboxylic. Of the formula R CO X, where X
ophile. When treated with an appropriate electrophile, it is is a leaving group that is easily displaced, these reactive
not unusual that a single thiol will serve as by far the most groups are commonly used to acylate amino groups to

FIGURE 2 Example of a liposome (phospholipid vesicle) conjugated to functional molecules R.