The rates of exchange of 2-hydrogens in 3,4-dimethyloxazolium ion, 3,4-dimethylthiazolium ion, and 1,3,4-trimethylimidazolium ion have been measured at a variety of pH's and buffer concentrations. In each case the rate constant for exchange catalyzed by OD~has been evaluated. The relative second-order rate constants are respectively 105•1 2345:103•5:1. The 18C-H coupling constants for ring hydrogens have also been measured. These data for rates are compared with knowledge of the ground states. The similarity of 18C-H coupling constants in homologous imidazolium and thiazolium ions at the 2 position indicates that these C-H bonds are similar in the ground state. The exchange rate 3000 times greater in 3,4-dimethylthiazolium ion than in 1,3,4-trimethylimidazolium ion suggests the transition state for ylide formation must be considerably stabilized in the case of the sulfur heterocycle. This indicates a special role for sulfur and provides partial understanding of the importance of a thiazolium ion in the structure of thiamine.

The interaction between the alpha 2- and beta 2-adrenergic receptors of ciliary processes has been studied by examining dose-response curves for adrenergic agonist stimulation of cyclic AMP production by intact, excised rabbit ciliary processes. Stimulation of cyclic AMP production by 1-isoproterenol is maximum from 0.1 to 1.0 microM; at higher concentrations stimulation decreases and approaches basal levels. Decreased cyclic AMP production at high concentrations of isoproterenol is blocked by the specific alpha 2-adrenergic antagonist, yohimbine, but not by the alpha 1-adrenergic antagonist, prazosin. Ciliary processes from animals after bilateral cervical ganglionectomy also show reduced cyclic AMP production at high concentrations of isoproterenol and this reduction is blocked by yohimbine, but not prazosin. This experiment suggests that the inhibition at high concentrations of isoproterenol is mediated by postsynaptic alpha 2-adrenergic receptors. Cyclic AMP production is relatively insensitive to epinephrine and norepinephrine, but their responses are potentiated by yohimbine. Catecholamines and clonidine, a specific alpha 2-adrenergic agonist, exhibit dose-dependent inhibition of forskolin-stimulated cyclic AMP production by ciliary processes. I50s from the dose-response curves are consistent with the characteristic binding affinities of these adrenergic agonists for alpha 2-adrenergic receptors: clonidine = epinephrine greater than norepinephrine greater than isoproterenol. Inhibition of forskolin-stimulated cyclic AMP production by clonidine is blocked by yohimbine but not by prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)

2213not valid for the transitions to the unoccupied orbitcm-I transition. This will be a very important conals. The neglect of configuration interaction may sideration for the description of other transition-metal account for the failure to predict accurately the 43,100-complexes.The pK,'s of the protonated forms of several oxazoles, thiazoles, and imidazoles have been determined and are given in parentheses following the name of the compound : 4-methyloxazole (1.07), ethyl 4-methyloxaeole-5-carboxylate (0.83), 4-methyloxazole-5-carboxylic acid (1.09, 2.88), 4-methylthiaeole (3.07), ethyl 4-methylthiazole-5-carboxylate (1.69), 4-methylthiazole-5-carboxylic acid (pKHA = 3.51), 4-methylthiazole-2-carboxylic acid (1.20, 3.18), 4-methylimidazole-5-carboxylic acid (2.53, 7.02), and l-methylimidazole-2-carboxylic acid (1.53, 7.25). Therefore, oxazoles are -lOB less basic than imidazoles, and thiazoles are w104 less basic than imidazoles. The pK's for azolium acids indicate that zwitterionic forms are favored for imidazole acids, but uncharged forms are favored for thiazole and oxazole acids.

No abstract

A solid phase enzyme-linked immunosorbent assay for quantitation of substance P is presented. The assay measures the capacity of soluble substance P to compete with the solid phase antigen for a limited quantity of specific substance P antibody. The solid-phase antigen consists of a synthetic substance P.poly-D-glutamic acid conjugate coated to polystyrene micro-ELISA plate wells. Soluble substance P and antibodies to substance P are first preincubated together and then added to the wells containing solid-phase antigen. Subsequently the wells are incubated with anti-antibodies conjugated to alkaline phosphatase. The wells are finally incubated with p-nitrophenyl phosphate an the absorbance is read in a spectrophotometer 16--24 hr after the start of the assay. The threshold for detection of substance P was 5--10 pg per well (0.25 ml). Substance P was extracted from rabbit eyes and the values obtained with the present method are compared with previously reported values based on radioimmunoassay.

Studies of the acid-catalyzed hydrolysis of phosphinanilides demonstrate that protonation has a profound labilizing effect on P-N bonds. In H2S04: -V-(p-nitrophenyl)diphenylphosphinamide exhibits a kinetic dependence on A0 with rate law, v = k[amide]/io° 9, and nonlinear dependence of log k on log [acid]. Examination of the dependence of the rate constants on acidity functions indicates an Ai mechanism: application of the Zucker-Hammett or Bunnett methods requires an acidity function which correctly describes the protonation behavior of the substrate-when log k is plotted vs. Ha, the acidity function for amides, a straight line with slope 1.0 results. This analysis of the dependence of rate on acidity and the large solvent isotope effect, kfk-= 2.8, provides strong evidence for an Ai mechanism. The variation in rate with TV-aryl substituents, p = -1.7, is consistent with a tran-Amer. Chem. Soc., 94,9260 (1972).

Decarboxylation of thiazolium, oxazolium, and imidazolium carboxylates has been used as a technique for generation of heterocyclic ylides in order to evaluate the effect of an adjacent sulfur atom on ylide stability. The data suggest that both 2-and 5-acids decarboxylate through their zwitterionic tautomers. Studies on Nmethylated azolium-5-carboxylates indicate that the 5-ylides are generated at approximate relative rates of 105•4: 10s 0:1 for oxazolium, thiazolium, and imidazolium rings. Potential acidities in the ground state would lead to predicted rates which are similar for the thiazolium and imidazolium rings. The large difference between the two is therefore a transition state effect and must reflect the special bonding properties of sulfur in an azolium ring.The fundamental structural reasons for the unique functions of the thiazolium ion (1) as the active part of the vitamin, thiamine, are still not clear. The role of the sulfur atom has been a subject of considerable discussion,3 and this research provides further data toward understanding that problem.Our work on the rate of ~OD-catalyzed generation of 2-ylides (2, R = CH3) indicates that the thiazolium ylide (2, X = S) is stabilized beyond the extent expected from the inductive electronic effects of the adjacent heteroatoms alone.4 One possible reason may be delocalization, possibly involving d-cr overlap,5 of the unshared pair of electrons at the 2 position toward R + 4N/A .Os+ -NxVS n^3¿x /NVX 3 sulfur; this is not possible for X = nitrogen or oxygen. Recent work supports the view that the sulfur atom can expand its valence shell of electrons6 beyond an octet, as in SF6.6'7If such stabilization is present in 2, it should be quite useful to examine the rates of formation of 3. In 3, the unshared pair of electrons at the 5 carbon is adjacent to only one heteroatom, X. Therefore, interpretation of results for generation of 3 will not be complicated by the presence of the other heteroatom, nitrogen, as in 2. However, since the 5-ylide, 3

The interaction between alpha 2-adrenergic and VIP receptors has been studied by examining inhibition of VIP-stimulated cyclic AMP production by adrenergic agonists in intact, excised rabbit ciliary processes. Epinephrine, norepinephrine, isoproterenol, dopamine, and the specific alpha 2-adrenergic agonists clonidine and p-aminoclonidine exhibit dose-dependent inhibition of VIP-stimulated cyclic AMP production. I50s, clonidine (0.05 microM) = p-aminoclonidine (0.05 microM) congruent to epinephrine (0.1 microM) less than norepinephrine (2.0 microM) less than isoproterenol (15 microM) = dopamine (15 microM), are consistent with the characteristic binding affinities of these adrenergic agonists for alpha 2-adrenergic receptors. Inhibition of VIP-stimulated cyclic AMP production by clonidine, epinephrine, isoproterenol, and dopamine is blocked by yohimbine but not by prazosin. These data establish the alpha 2-adrenergic specificity of the inhibitory effects observed. We have previously shown that beta 2-adrenergic receptor-mediated stimulation of cyclic AMP production in rabbit ciliary processes is also inhibited by postjunctional alpha 2-adrenergic receptors. These studies support the idea that the catecholamines may regulate aqueous humor formation by inhibiting stimulation of cyclic AMP production via postjunctional alpha 2-adrenergic receptors in ciliary processes.