Secondary valence force catalysis. XII. Enhanced reactivity and affinity of cyanide ion toward N-substituted 3-carbamoylpyridinium ions elicited by ionic surfactants and biological lipids

Abstract

Rate and equilibrium constants for the addition of cyanide ion to a series of N-alkyl-3-carbamoylpyridinium ions are markedly increased by dilute solutions of n-alkyltrimethylammonium bromides. For example, 0.02 M hexadecyl surfactant increases the rate constant for addition of cyanide to the N-hexadecyl substrate 950-fold and increases the corresponding association constant about 25,000-fold. More concentrated solutions of these surfactants also elicit marked increases in rate and equilibrium constants for the same reactions although the increases are not quite so pronounced as with more dilute solutions. This may, in part, reflect inhibition by the surfactant counterion since it has been established that the surfactant-dependent reactions are sensitive to inhibition by anions. The effectiveness of anions as inhibitors increases in the order F-< NO3-< CI-< Br-. The extent of surfactant facilitation of the addition of cyanide ion to the pyridinium ions increases with increasing hydrophobicity in both surfactant and substrate, suggesting that hydrophobic interactions may contribute to the activation energies. The zwitterionic surfactant dodecyldimethylammoniopropane sulfonate increases the rate and equilibrium constants for addition of cyanide ion to N-dodecyl-3-carbanioylpyridinium bromide by 40-and 5000-fold, respectively. Finally, this same reaction is subject to promotion by sonicated aqueous dispersions of lecithin, lysolecithin, and sphingomyelin. series of studies in the past several years has estab-A lished that rate and equilibrium constants for a number of organic reactions are sensitive functions of the concentration of ionic s~r f a c t a n t s .~ In most such cases the observed effects are the consequence of adsorption of one or more of the reacting species onto or into micelles formed from the surfactants. Electrostatic and hydrophobic interactions between the substrates and the components of the micelle dominate both the adsorption process itself and the rate and extent of bond-changing reactions which follow.Since those factors which influence rates and equilibria of organic reactions may not only provide additional insight into the reaction mechanisms concerned but also yield data pertinent to enzyme-catalyzed reactions, it appears profitable to pursue the study of organic reactions in the presence of surfactants more fully. Moreover, recent results suggest that the presence of ionic micelles may be important in determining the course of organic reactions, as well as their rate and extent.4 In this study, the rate and equilibrium constants for the addition of cyanide ion to a series of Nsubstituted 3-carbamoylpyridinium ions have been determined in the presence of a series of n-alkyltrimethylammonium ions, one zwitterionic surfactant, and several biological lipids.

Effect of changes in surfactant structure on micellarly catalyzed spontaneous decarboxylations and phosphate ester hydrolysis

“…Cordes and his coworkers noted that it was dif- ficult to get good rate data for the reaction of cyanide ion with Nalkylpyridinium ions using liposomes of sonicated phospholipids. 13 Because of these problems with sonicated «-lecithin we also used lysolecithin (Sigma) because it forms normal micelles rather than liposomes.21 Solutions of lysolecithin became cloudy during reaction of I, but this cloudiness was removed by centrifugation. Our first-order rate constants were determined by following the formation of the nitrophenoxide ion for approximately 1 half-life of reaction, and determining the absorbance of nitrophenoxide ion at complete reaction after centrifugation.…”

Binding Energy, Specificity, and Enzymic Catalysis: The Circe Effect

“…141), are increased by (141) R R 1000-and 25,000-fold, respectively, in the presence of hexadecyltrimethylammonium ion micelles when the N-alkyl group is a hydrophobic hexadecyl chain (230). Here the catalyst and the bound substrate have the same charges.…”

“…141) provides an example of the utilization of binding energy for the destabilization of the bound reacting group in a system of this kind (230). The 64-fold rate increase as the N-alkyl chain length is increased from octyl to octadecyl corresponds to an additional destabilization of the cationic nicotinamide group by 2.5 kcal/mole that is brought about by the larger binding energy of the longer alkyl group.…”

Bioorganic Chemistry

“…The presence of 0.02 M cetyltrimethyl ammonium [CH3-(CH2)ls-NN(CH3h] salts increases the rate constant by 950-fold for the addition of CH-to the N-hexadecyl substrate (R = C I6H 33) and increases the corresponding association constant about 25,000-fold (341). Notice that in this transformation, the charged substrate becomes neutral after reaction, so the product of the reaction will be reoriented within the micelles and will be pulled inside.…”