Synthesis, photochemistry, and biological characterization of photolabile protecting groups for carboxylic acids and neurotransmitters

The α,5-dicarboxy-2-nitrobenzyl caging group, a tool for biophysical applications with improved hydrophilicity: synthesis, photochemical properties and biological characterization

Earlier we reported on the synthesis of α,4-dicarboxy-2-nitrobenyzl caged compounds (Schaper, K. et al. [2002] Eur. J. Org. Chem., 1037-1046). These compounds have the advantage of an increased hydrophilicity compared with the well-established α-carboxy-2-nitrobenzyl caged compounds; however, the release of the active compound becomes slower due to the introduction of the additional carboxy group. Based upon theoretical calculations we predicted that the release would become faster when the additional carboxy group is moved to the 5-position. Here we describe the synthesis and the photochemical and biological characterization of an α,5-dicarboxy-2-nitrobenyzl caged compound. The high hydrophilicity of the new caging group is maintained due to the fact that the additional carboxy moiety is preserved, while the release of the active species from the new derivative is even faster than for the reference, an α-CNB caged compound.

Rapid chemical reaction techniques developed for use in investigations of membrane-bound proteins (neurotransmitter receptors)

New techniques for investigating chemical reactions on cell surfaces in the microsecond-to-millisecond time region are described. Reactions mediated by membrane-bound neurotransmitter receptors that control signal transmission between approximately 10(12) cells of the nervous system are taken as an example. Cells with receptors on their plasma membrane are equilibrated with photolabile, biologically inactive precursors of the neurotransmitters. Photolysis of these compounds releases free neurotransmitter that interacts with the receptors, leading to the transient opening of transmembrane receptor-formed channels that are permeant to small inorganic ions. The current thus induced can be measured. The technique can be used to measure the elementary steps of the receptor-mediated reactions. To illustrate the approach it was shown that an understanding of the mechanism of inhibition of the nicotinic acetylcholine receptor by the drug cocaine was obtained and led to the first proof that compounds exist that alleviate the inhibition.

Caging proteins through unnatural amino acid mutagenesis

The caging of specific residues of proteins is a powerful tool. This discussion attempts to alert the reader to the considerations that must be made in preparing and analyzing a caged protein through nonsense suppression. Although the suppression methodology is conceptually straightforward, it not possible to provide a failsafe "cook book" method for using caged unnaturals. We have emphasized the preparation of caged receptors expressed in Xenopus oocytes, but these approaches can clearly be adapted to many other systems.

Synthesis and characterization of photolabile o-nitrobenzyl derivatives of urea

We present here the synthesis and characterization of four photolabile derivatives of urea in which alpha-substituted 2-nitrobenzyl groups are covalently attached to the urea nitrogen. These derivatives photolyze readily in aqueous solution to release free urea. The alpha-substituents of the 2-nitrobenzyl group strongly influence the rate of the photolysis reaction measured with transient absorption spectroscopy. Rates of photolysis at pH 7.5 and room temperature (approximately 22 degrees C) for N-(2-nitrobenzyl)urea, N-(alpha-methyl-2-nitrobenzyl)urea, N-(alpha-carboxymethyl-2-nitrobenzyl)urea, and N-(alpha-carboxy-2-nitrobenzyl)urea are, respectively, 1.7 x 10(4), 8.5 x 10(4), 4.0 x 10(4), and 1.1 x 10(5) s(-)(1). The quantum yields determined by measurement of free urea following irradiation by a single laser pulse at 308 nm were 0.81 for N-(2-nitrobenzyl)urea, 0.64 for N-(alpha-methyl-2-nitrobenzyl)urea, and 0.56 for N-(alpha-carboxy-2-nitrobenzyl)urea. The caged N-(alpha-carboxy-2-nitrobenzyl)urea is not a substrate of the enzyme urease, while the photolytically released urea is. Also, neither this caged urea nor its photolytic side products inhibit hydrolysis of free urea by urease. Thus, the alpha-carboxy-2-nitrobenzyl derivative of urea is suitable for mechanistic investigations of the enzyme urease.

Photoremovable protecting groups: reaction mechanisms and applications

Photolabile protecting groups enable biochemists to control the release of bioactive compounds in living tissue. 'Caged compounds' (photoactivatable bioagents) have become an important tool to study the events that follow chemical signalling in, e.g., cell biology and the neurosciences. The possibilities are by no means exhausted. Progress will depend on the development of photoremovable protecting groups that satisfy the diverse requirements of new applications--a challenging task for photochemists.

Mechanism-based discovery of ligands that counteract inhibition of the nicotinic acetylcholine receptor by cocaine and MK-801

Nicotinic acetylcholine receptors (AChR) belong to a family of proteins that form ligand-gated transmembrane ion channels. They are involved in the fast transmission of signals between cells and the control of intercellular communication in the nervous system. A variety of therapeutic agents and abused drugs, including cocaine, inhibit the AChR and monoamine transporters and interfere with nervous system function. Here we describe a mechanism-based approach to prevent this inhibition. We had previously developed presteady-state kinetic (transient kinetic) techniques, with microsecond-to-millisecond time resolutions, for investigations of reactions on cell surfaces that allow one to determine the effects of inhibitors not only on the channel-opening probability but also on the opening and closing rates of the AChR channel. The transient kinetic measurements led to two predictions. (i) Ligands that bind to a regulatory site on the closed-channel conformation of the AChR with higher affinity than to the site on the open-channel form shift the equilibrium toward the closed-channel form, thereby inhibiting the receptor. (ii) Ligands that bind to a regulatory site with an affinity for the open conformation equal to or higher than their affinity for the closed conformations are expected not to inhibit the receptor and to displace inhibitors. The identification of such ligands in a combinatorial library of RNA ligands is reported. The implication of this approach to other protein-mediated reactions in which an inhibitor changes the equilibrium between active and inactive conformations is discussed.

gamma-Aminobutyrate, alpha-carboxy-2-nitrobenzyl ester selectively blocks inhibitory synaptic transmission in rat dentate gyrus

gamma-Aminobutyrate, alpha-carboxy-2-nitrobenzyl ester (cGABA) is a stable photoactivatable probe used to study gamma-aminobutyrate (GABA) receptors. GABA is released from this compound when it is exposed to ultraviolet light, but little is known about the electrophysiological effects of the compound itself. Whole cell patch clamp recordings on rat hippocampal slices demonstrated that cGABA blocked polysynaptic inhibitory postsynaptic currents (IPSCs) evoked in dentate granule cells by antidromic stimulation of the mossy fibers. It also reduced monosynaptically evoked IPSCs with an IC(50) of 28 microM. In contrast, cGABA had no effect on excitatory postsynaptic currents (EPSCs) evoked by perforant path stimulation. The effect of cGABA was not mediated by depression of GABA release through activation of presynaptic GABA(B) receptors. cGABA inhibited muscimol-evoked currents by only 15% at a concentration of 40 microM. At this same concentration, it reduced the mean frequency of miniature inhibitory postsynaptic potentials by 71%, their mean peak amplitude by 44%, their mean decay time constant by 26% and the mean charge transfer per event by 52%. These effects may be explained by a phenothiazine-like modification of GABA(A) receptor kinetics and/or a selective block of somatic GABA synapses.