Sequence-selective peptide detection by small synthetic chemosensors selected from an encoded combinatorial chemosensor library

Design of fluorescent materials for chemical sensing

There is an enormous demand for chemical sensors for many areas and disciplines. High sensitivity and ease of operation are two main issues for sensor development. Fluorescence techniques can easily fulfill these requirements and therefore fluorescent-based sensors appear as one of the most promising candidates for chemical sensing. However, the development of sensors is not trivial; material science, molecular recognition and device implementation are some of the aspects that play a role in the design of sensors. The development of fluorescent sensing materials is increasingly captivating the attention of the scientists because its implementation as a truly sensory system is straightforward. This critical review shows the use of polymers, sol-gels, mesoporous materials, surfactant aggregates, quantum dots, and glass or gold surfaces, combined with different chemical approaches for the development of fluorescent sensing materials. Representative examples have been selected and they are commented here.

One-Armed Artificial Receptors for the Binding of Polar Tetrapeptides in Water: Probing the Substrate Selectivity of a Combinatorial Receptor Library

We have recently developed a new class of one-armed artificial receptors 1 for the binding of the polar tetrapeptide N-Ac-D-Glu-L-Lys-D-Ala-D-Ala-OH (EKAA) 2 in water using a combined combinatorial and statistical approach. We have now further probed the substrate selectivity of this receptor library 1 by screening a second tetrapeptide substrate (3) with the inverse sequence N-Ac-D-Ala-D-Ala-L-Lys-D-Glu-OH (AAKE). This "inverse" substrate is also efficiently bound by our receptors, with K(ass) approximately 6000 M(-1) for the best receptors, as determined both by a quantitative on-bead binding assay and by UV and fluorescence titration studies in free solution. Hence, the inverse tetrapeptide 3 is in general bound two to three times less efficiently than the "normal" peptide 2 (K(ass) approximately 17,000 M(-1)), even though the complexation mainly involves long-range electrostatic interactions and both the receptor and substrate are rather flexible. Molecular modeling and ab initio calculations have been used to rationalize the observed substrate selectivity and to analyze the various binding interactions within the complex.

Synthesis of Peptidocalix[4]arene Libraries and Their Application to the Development of Chemical Sensors for Oligopeptides

Calix[4]arene libraries consisting of ca. 50000 members substituted with peptides at the lower rim were synthesized using encoded spilt synthesis with 15 amino acids. Screening of the library for binding with a dye-labeled oligopeptide indicated that some peptidocalix[4]arenes selectively bind the oligopeptide. The binding constant was estimated to be approximately 2 x 10(4) mol dm(-3). In an attempt to develop chemical sensors for peptides, fluorescence-labeled peptidocalix[4]arene libraries consisting of ca. 3500 members were synthesized. The fluorescence spectrum of peptidocalix[4]arene, which was found in the screening of libraries against the target peptide, was dependent on the concentration of the peptide. The libraries substituted at the upper rim were also synthesized with the aim of developing more selective and sensitive chemical sensors. The binding selectivity of the library members for the target peptides was higher and the behavior of the sensing was markedly different from the lower rim-modified peptidocalixarene sensor.

Artificial Receptor-Attached Amphiphilic Copolymer for Barbiturate Binding in Aqueous Media

A water-soluble self-associating amphiphilic copolymer was employed to provide a microenvironment for the solvation of a hydrogen-bonding barbiturate artificial receptor, to facilitate molecular recognition in water. The receptor-attached amphiphilic polymer (RP) was synthesized through random copolymerization of 3% (mol) barbiturate receptor-monomer, 70% (mol) 3-sulfopropyl methacrylate, and 27% (mol) n-dodecyl acrylate. Difference UV spectra of pH 6.5 aqueous solutions of phenobarbital and receptor-polymer (RP) gave peaks and valleys at 272 and 301 nm respectively, consistent with binding characteristics of monomeric barbiturate receptors in chloroform. Specific association between phenobarbital and the receptor-polymer was further indicated based on investigations of a receptor-free control polymer (CP) of similar polar/nonpolar monomer ratio. Micellar electrokinetic chromatography was applied for studying polymer-phenobarbital association, by capillary electrophoresis.

Combinatorial approaches to synthetic receptors

Combinatorial chemistry can be efficiently used for the synthesis and evaluation of binding properties of libraries of synthetic receptors. This approach has been applied particularly to 'tweezer' and other 'multi-armed' receptors, and has been used for the identification of receptors for peptides in aqueous media, and for the development of new sensors and sensor arrays.