Biomedical Engineering Reference
In-Depth Information
Fig. 1 Schematic diagram of a biosensor
an ability to design and synthesize complex molecular systems [ 3 ]. From this
chemistry perspective, significant impetus exists to construct novel functional
receptors with high performance characteristics to enhance and/or supplement
conventional biomolecules (i.e., antibodies, enzymes, peptides, DNA). In particu-
lar, it is well known that transition metal ions serve many purposes in natural
biological systems, from guiding protein folding and tertiary structure stabilization
to crucial roles in information storage and retrieval through ligand binding, electron
transfer, and catalysis. It follows then that innovative transition metal-modified
receptors are beginning to emerge with similar functions that hold considerable
promise as next generation biosensing elements.
This chapter aims to highlight some recent advances in the burgeoning field of
transition metal-modified receptors. Specifically, systems that incorporate organo-
metallic, monometallic, and supramolecular coordination complexes will be
presented. The intention is to overview emerging concepts in synthetic receptor
design and function that incorporate metal complexes through relevant examples
from recent years rather than provide exhaustive and in-depth coverage of the field.
The discussion will include transition metal-modified receptor scaffolds, allosteric
supramolecular enzyme mimics, and integrated monolayer-based electroactive
reporter systems.
2 Transition Metal Complexes as Scaffolds in Receptor Design
In biological systems, information is routinely stored in the size, shape, and
electronic properties of molecules and transmitted by the way those molecules
bind and react with each other. Beyond DNA, many of these information transfer
tasks are accomplished using proteins. As distinct domains in multi-subunit protein
building blocks fold and cooperate via noncovalent and directional bonding
interactions, sophisticated nanoscale architectures are assembled that carry out
complex life processes and chemical transformations.
Roughly one-third of all proteins require specific metal ion cofactors to assist in
macromolecular folding and/or function [ 4 ]. Of the 20 naturally occurring amino
acids programmed by the genetic code, only a relatively small number are fre-
quently employed as metal ligands. These groups include the thiolate of Cys, the
phenolate of Tyr, the imidazole nitrogens of His, and the carboxylates of Asp and
Glu. Metal-binding sites in proteins are often classified into two categories: (1) pre-
organized environments, where metal binding only occurs if coordinating ligands
are in the appropriate orientation, and (2) disordered environments, where protein
folding is metal-directed. Zinc finger proteins are excellent examples of the latter as
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