A series of para-substituted N-methyl-N-phenylnitrenium ions (N-(4-biphenylyl)-N-methylnitrenium ion, N-(4-chlorophenyl)-N-methylnitrenium ion, N-(4-methoxyphenyl)-N-methylnitrenium ion, and N-(4-methylphenyl)-N-methylnitrenium ion) were generated through photolysis of the appropriately substituted 1-aminopyridinium salt. Laser flash photolysis using UV−vis detection as well as photoproduct analysis verified that the expected nitrenium ions were formed cleanly and rapidly following photolysis. Laser flash photolysis with time-resolved infrared detection allowed for structural characterization of the nitrenium ions through observation of a symmetrical aromatic CC stretch in the region 1580−1628 cm-1. The specific frequencies reflect the degree of quinoidal character present in each phenylnitrenium ion (i.e., the degree to which the nitrenium ion resembles a 4-iminocyclohexa-2,5-dienyl cation). The 4-methoxy derivative shows the highest frequency CC stretch, indicating that this strongly π-electron-donating substituent imparts more quinoidal character, and the 4-chloro derivative shows the lowest frequency CC stretch, suggesting that it possesses the least quinoidal character. Quantum calculations using density functional theory (BPW91/cc-pVDZ) were carried out on the same nitrenium ions. The theoretically derived IR frequencies showed excellent quantitative agreement with the experiment. The computed structures show significant bond length alternation in the phenyl rings, shortened C−N bond lengths, and substantial positive charge delocalization into the phenyl rings. All of these effects are more pronounced with increasing π-donating character of the ring substituent. Arylnitrenium ions are well described as 4-iminocyclohexa-2,5-dienyl cations.

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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. T he nicotinic acetylcholine receptor (AChR) is the prototypical member of a family of structurally related membrane proteins, the ligand-gated ion channels (1). These proteins regulate intercellular communication between the approximately 10 12 cells of the mammalian nervous system, a process considered essential for brain function (2). Many therapeutic agents and abused drugs affect their function (3). For instance, the AChR is inhibited by the anticonvulsant MK-801 [(ϩ)Ϫdizocilpine] (4-6) and by several abused drugs, including cocaine (7-9). Cocaine affects more than three million people annually in the United States alone, at an estimated cost to society of more than 100 billion dollars.Understanding the mechanism of the AChR and its inhibition is a longstanding and challenging problem (10) with major implications for medicine and drug addiction (11-12). Two decades ago, single-channel current-recording (13) measurements led to the proposal of a simple and generally accepted mechanism in which inhibitors enter the open channel and block it (14-17) (the channel-blocking mechanism, Mechanism A in Fig. 1). Although several variations of this open-channelblocking mechanism have been proposed, including the conversion of an inhibitor-bound closed-channel conformation to a blocked open-channel form (18-21) (Mechanism B, Fig. 1), the open-channel-blocking mechanism, based mainly on singlechannel current or other steady-state kinetic measurements (14-21), has met the test of time during the last 20 years. In the techniques used for those measurements, the channel-activating ligand is in quasi equilibrium (steady state) with the receptor. The question we asked was: Can additional information about the receptor-mediated reactions be obtained by using presteadystate kinetic techniques? Recently, presteady-state kinetic techniques that are suitable for measuring receptor-mediated reactions on cell surfaces in the millisecond-to-microsecond time region were developed (22-31). The time resolution of the laser-pulse photolysis technique (23-26) is sufficient to investigate the reaction before the channel has opened. One can, therefore, obtain info...