Tetramethylammonium
Tetramethylammonium (TMA) or (MeN) is the simplest quaternary ammonium cation, consisting of four methyl groups attached to a central nitrogen atom, and is isoelectronic with neopentane. It is positively charged and can only be isolated in association with a counter-ion. Common salts include tetramethylammonium chloride and tetramethylammonium hydroxide. Tetramethylammonium salts are commonly used in chemical synthesis and are widely employed in pharmacological research. Common nomenclature. In the toxicological literature, "naturally occurring" tetramethylammonium (anion unspecified) is often referred to by the name "tetramine". Unfortunately, this non-systematic or "trivial" name is also used for other chemical entities, including a  toxic rodenticide (Tetramethylenedisulfotetramine). Similarly, the acronym "TMA", which is frequently used for tetramethylammonium in the pharmacological literature, may also refer to the investigational drug 3,4,5-trimethoxyamphetamine, which, being a close structural analog of mescaline, has been the subject of numerous publications. Occurrence. TMA has been detected in or isolated from a number of marine organisms, mostly amongst the Cnidaria and Mollusca, notably in some species of "Neptunea" (commonly called whelks) that are eaten by humans. It has also been found in one plant, the African "Courbonia virgata" (Cappariaceae). Preparation, reactions, solution properties. One of the most straightforward methods of preparing  a simple salt containing the tetramethylammonium ion is by the reaction between trimethylamine and a methyl halide: [C]-labeled TMA has been made by this method. Although this reaction is suitable for the common halides, tetramethylammonium salts with more complex anions may be prepared by salt metathesis reactions, e.g. tetramethylammonium borohydride has been made from tetramethylammonium hydroxide as shown: Although TMA salts do possess some of the phase-transfer catalytic properties that are characteristic of quaternary ammonium compounds, they tend to behave atypically because of the relatively high "hydrophilicity" of the TMA cation. TMA cation is hydrophilic. The octanol-water partition  coefficient of TMA iodide, "P", is (or ). In the TMA cation, the methyl groups are tetrahedrally arranged around the central N atom, as is evident from X-ray crystallographic studies of various of its salts. From measurements taken on molecular models, it has been estimated that the diameter of the TMA ion is ~0.6 nm; From more accurate physico-chemical measurements, the ionic radius for TMA is given as 0.322 nm; several thermodynamic parameters for the TMA ion are also recorded. The paper by Aue et al. gives a good discussion of the methods by which the ionic radius was  determined. Pharmacology. The pharmacological literature on tetramethylammonium is extensive. In general, TMA is a cholinomimetic whose effects mimic most of those produced by exogenous acetylcholine. Pharmacological experiments with TMA have been performed using one of its salts, typically the chloride, bromide or iodide, since these anions were not expected to interfere with the actions of the TMA cation. In the early pharmacological literature, however, there are references to the use of "tetramethylammonium hydroxide" or "tetramethylammonium hydrate", which were meant to facilitate comparison between weight-based dosages of different TMA salts, but did not involve the actual use of tetramethylammonium hydroxide , whose strong basicity would have been incompatible with physiological conditions. A thorough review of the pharmacology of TMA from a toxicological perspective, and current up to 1989, has been given by Anthoni and co-workers. Thus, the effects of TMA on nicotinic and muscarinic ACh receptors first stimulate, then block neurotransmission in sympathetic and parasympathetic ganglia, with depolarization. TMA also acts as an agonist at muscarinic receptors in post-ganglionic nerve endings in smooth muscles, cardiac muscle, and exocrine glands. In skeletal muscle, TMA initially causes fasciculations, then paralysis, as a result of the depolarization from stimulation of nicotinic ACh  receptors. Absorption; distribution; metabolism; excretion (ADME). Absorption: TMA is readily absorbed from the gastro-intestinal tract. Studies on the rat jejunum indicated that TMA absorption involved a combination of simple diffusion and carrier-mediated transport, with nearly 100% absorption occurring within 60 to 90 minutes. By comparison, tetraethylammonium and tetrapropylammonium ions were only absorbed to the extent of ~30%. Distribution: Intraperitoneal administration of radio-labeled tetramethylammonium iodide to mice showed that TMA was rapidly distributed to all parts of the body, with the highest concentrations being in the kidney and liver. Similar results were reported by Neef and co-workers  using rats. Metabolism and excretion: Parenteral administration of radio-labeled tetramethylammonium iodide to rats resulted in almost the whole dose being excreted in urine, without any evidence of metabolic transformation. Toxicology. The human toxicology of TMA (under the name "tetramine") has been studied primarily in the context of accidental poisoning after ingestion of "Neptunea" species. Symptoms include the following: nausea, vomiting, headache, vertigo/dizziness, impaired vision/temporary blindness, diplopia, photophobia, lack of balance, feeling of intoxication and urticaria. These symptoms appear within 30 minutes but recovery is usually complete after a few hours. Only one account of human death following  ingestion of TMA (from the plant "Courbonia virgata") has been recorded. Although many of these symptoms can be accounted for on the basis of impairment of neurotransmission in the autonomic nervous system, there also seem to be distinct indications of central affects. In animal studies, parenteral administration of TMA-containing extracts from "Neptunea" to mice, cats and fish mainly show effects involving skeletal muscles: there are muscular fasciculations, convulsions, loss of balance, motor paralysis and ultimately cessation of respiration. The lethal oral dose of TMA for humans has been estimated at 3–4 mg/kg. The lethal dose for rats was  estimated to be ~45–50 mg/kg, p.o., and ~15 mg/kg, i.p. Acute toxicity. LD for TMA chloride: 25 mg/kg (mouse, i.p.); 40 mg/kg (mouse, s.c.). LC for TMA chloride: 462 mg/L for 96 hrs. (Fathead minnow, "Pimephales promelas").