John W. Bunting

Merged mechanisms for hydride transfer from 1,4-dihydronicotinamides

Abstract Recent work on the reduction of heteroaromatic cations by 1,4-dihydronicotinamides and related reducing agents is reviewed. Extensive correlations are presented between the second-order rate constants ( k 2 ) for these reactions and the second-order rate constants ( k OH ) and equilibrium constants (p K R + ) for hydroxide ion attack on these cations. Close correlations of log k 2 with the electron affinities and one-electron reduction potentials of these cations are also presented. These relationships are considered in the context of a direct hydride transfer from donor to acceptor and also in terms of SET mechanisms which are also commonly discussed for such reactions. It is shown that the interpretation of these formal hydride transfer reactions in terms of an imbalanced development of electronic charge and CH bond fission within the transition state species leads to a rational merging of the single-step hydride transfer mechanism and the SET mechanisms. The structures of the transition state species are expected to be highly variable and quite dependent upon the nature of the hydride donor and acceptor species, with considerable contribution from charge-transfer interactions. Such imbalanced transition state species are analyzed in terms of two different types of reaction coordinate diagrams and also in terms of the valence bond configuration mixing theory.

pKR+ values for pyridinium cations

Abstract Literature data for nucleophilic addition at C-4 of C-3 substituted 1-methylpyridinium cations are evaluated. It is shown that sufficient data are available to allow the confident evaluation of a linear free energy relationship for the equilibration of these cations with their C-4 hydroxide adducts (pseudobases) in aqueous solution: pKR+ = -8.5 (±0.4) σ-p + 20.5 (± 0.5) This allows the prediction of pKR+ = 20.5 for pseudobase formation at C-4 of the 1-methylpyridinium cation. A less confident estimate of pKR+ = 18.2 at C-2 of this cation is also made.

Relative reactivities of heteroaromatic cations towards reduction by 1,4-dihydronicotinamides

Abstract Kinetic data are reported for the equilibration of the 1-methyl-3-nitropyridinium cation with its pseudobase (hydroxide adduct) and for the reduction of this cation by 1-benzyl-1, 4-dihydronicotinamide. The C-2 hydroxide adduct is the kinetically controlled product (pKR+ = 11.6) when this pyridinium cation is mixed with aqueous base, however, this species rearranges to the C-4 adduct as the themodynamically more stable product (pKR+ = 9.42). The pH-dependence of this equilibration may be analysed to give kOH = 1600 M-1s-1 for hydroxide ion attack at C-4 of this cation. Reduction of this pyridinium cation by 1-benzyl-1, 4-dihydronicotinamide appears to occur exclusively at C-4 with second-order rate constant k2 = 0.72 M-1s-1 and k2 H/k2D = 2.0 in 20% CH3CN - 80% H2O, ionic strength 1.0, 25°C. The reactivities of pyridinium, quinolinium, isoquinolinium, acridinium and phenanthridinium cations of pKR+ = 10.0 towards both hydroxide ion and 1-benzyl-1, 4-dihydronicotinamide are evaluated. Relative reactivities (K2/KOH) for these two processes are shown to be acridinium : quinolinium (C-4) : pyridinium (C-4) : quinolinium (C-2) : isoquinolinium : phenanthridinium = 1.6×105 : 3400 : 80 : (4 : 1.0 : 0.7 for predominantly aqueous reaction media. These data support the hypothesis that formation of 1, 2-dihydropyridine systems upon reduction of heteroaromatic cations by 1, 4-dihydronicotinamides occurs via direct one step hydride transfer, while formation of 1,4-dihydro-pyridines in such processes occurs preferentially by a mechanistica11y more complex process involving electron transfer.

Kinetics of the reduction of C-9 substituted acridinium cations by 1,4-dihydronicotinamides

Abstract Second-order rate constants ( k 2 ) are reported for the reduction of 9-R-10-methylacridinium cations ( 5 :R = H, CH 3 , CH 3 CH 2 , C 6 H 5 CH 2 , (CH 3 ) 2 CH, C 6 H 5 , 4-(CH 3 ) 2 NC 6 H 4 ) by 1-benzyl-1,4-dihydronicotinamide ( 2 :R = C 6 H 5 CH 2 ) in 20% CH 3 CN-80% H 2 O at 25°C. All 5 :R ≠ H are reduced in the range 20- to 140-fold more slowly than 5 :R = H. However, there is no simple relationship between k 2 and the nature of R, nor between k 2 and the second-order rate constant for hydroxide ion attack at C-9 of these cations in pseudobase formation. Rates of reduction of 5 by 1-benzyl-4,4-dideuterio-1,4-dihydronicotinamide allow the calculation of the following kinetic isotope effects in this solvent medium: 5 :R, k H k D : H , 1.56; C 6 H 5 CH 2 , 2.7; C 6 H 5 , 5.4. Substituent effects upon k 2 were evaluated for the reduction of 5 by 1-(X-benzyl)-1,4-dihydronicotinamides, and lead to the following Hammett ϱ parameters: 5 :R, ϱ: H, −0.68; C 6 H 5 CH 2 , −0.92; C 6 H 5 , −0.96. The latter two values require essentially complete unit positive charge generation on the nicotinamide moiety in the rate-determining transition state. It is shown that these Hammett ϱ values and the above isotope effects can only be rationalized by a two-step e − + H • mechanism for hydride transfer from 2 to 5 in this solvent system. This result contrasts with our earlier conclusion of direct, one-step hydride transfer in the reduction of isoquinolinium cations by 2 , but is consistent with our observation that acridinium cations are reduced 37500-fold faster by 2 than predicted on the basis of the relative rates of nucleophilic attack (hydroxide ion) on acridinium and isoquinolinium cations. It is suggested that the availability of both Hammett ϱ values and primary kinetic isotope effects will generally allow the establishment of the mechanism of hydride transfer in these systems. Application of these ideas to literature data suggests that 5 :R = H is reduced by direct hydride transfer in acetonitrile solution, in contrast to the above result in predominantly aqueous solution. The ready formation of acridanyl radicals by electron transfer to acridinium cations is demonstrated by the formation of Wursters Blue radical cation upon mixing solutions of acridinium cations with N,N,N′,N′ -tetramethyl- p -phenylenediamine.

Nucleophilicity towards a saturated carbon atom: rate constants for the aminolysis of methyl 4-nitrobenzenesulfonate in aqueous solution. A comparison of the n and N+ parameters for amine nucleophilicity

Second-order rate constants (kNu) have been measured in aqueous solution (I= 0.1 mol dm –3, 25 °C) for the SN2 reactions of methyl 4-nitrobenzenesulfonate with ammonia, 41 primary amines, 20 secondary amines. 29 tertiary amines and 7 anionic nucleophiles. For the aminolysis reactions, Bronsted-type correlations of nucleophilicity with basicity require the classification of all amines in terms of strictly defined structural classes with βnuc in the range 0.15–0.39. Swain–Scott plots indicate that simple amines, water and other light-atom nucleophiles (hydroxide, azide and cyanide, anions) are five times more reactive than heavy-atom nucleophiles (thiosulfate, thiocyanate, iodide and bromide ions). For amine nucleophiles there is a close linear correlation (of slope 0.44, and including both primary and secondary amines) between log kNu for the aminolysis of methyl 4-nitrobenzenesulfonate and log kNu for amine addition to the 1-methyl-4-vinylpyridinium cation. This correlation demonstrates a close linear relationship between the Swain–Scott n parameter and Ritchies N+parameter for amine nucleophiles in aqueous solution (N+= 2.1n– 4.3).

Substrate Inhibition in the Hydrolysis of Hippuric Acid Esters by Carboxypeptidase A

The dependence of initial velocity upon substrate concentration has been examined in the carboxypeptidase A catalyzed hydrolysis of the following hippuric acid esters (at pH 7.5, 25°, ionic strength O.5): C6H5CONHCH2CO2CHRCO2H: R=CH3; CH2CH3;(CH2)2CH3; (CH2)3CH3; (CH2)5CH3; CH(CH3)2; CH2CH(CH3)2; C6H5; CH2C6H5. All of these esters display marked substrate inhibition of their enzymic hydrolyses. With the exception of R=CH3, the velocity-substrate concentration profiles for each of these esters can be rationalized by the formation of an E.S2 complex which, independent of the alcohol moiety of the ester, reacts approximately 25 times more slowly than the E.S complex. For most of these esters, the formation of E.S2 approximates ordered binding of the substrate molecules at the catalytic and inhibitory sites. While binding at the catalytic site is markedly dependent on the nature of the R group, binding of a second substrate molecule to E.S is not significantly affected by the nature of the R side chain. For R...

An important enzyme-substrate binding interaction for xanthine oxidase

The following pairs of heterocyclic cations have been examined as potential substrates for bovine milk xanthine oxidase (xanthine: oxygen oxidoreductase, EC 1.2.3.2) at pH 10.6: (i) the 1-methyl-5,6,7,8-tetrahydroquinolinium cation (1) and the 1-methyl-5-oxo-5,6,7,8-tetrahydroquinolinium cation (2); (ii) the 1-methyl-3-phenylpyridinium cation (3) and the l-methyl-3-(2′-pyridyl)pyridinium cation (4); and (iiii) the 4-methyl-4,7-phenanthrolinium cation (5) and the 7-methyl-l,7-phenanthrolinium cation (6). The cations 1 and 3 are not substrates for this enzyme, whereas their respective structural analogs 2 and 4 are enzymicaHy oxadized. The cations 5 and 6 are both readily enzymically oxidized, but 6 displays k2/Km 30-fold larger than its isomer 5. All of these observations confirm the importance of an interaction between an electronegative atom (Y) in cationic substrates and an active-site enzymic functional group. The l-methylthionicotinamide cation is readily oxidized by the enzyme and has kc/Km quite similar to that of its oxygen analog; this indicates that a sulfur atom is an acceptable Y atom. An analogous enzyme-substrate interaction is postulated in the productive binding of xanthine and lumazine and their thio derivatives, with the C-6 and C-4 oxygen (or suifur) substituents, respectively, playing the role of the electronegative Y atom. Appropriate interactions of the substrate with this same enzymic functional group also allow the rationalization of the observed sequence of oxidations at C-6, C-2 and C-8 of purines and C-4, C-2 and C-7 of pteridines.

Nucleophilicity towards a vinylic carbon atom: rate constants for the addition of amines to the 1-methyl-4-vinylpyridinium cation in aqueous solution

Second-order rate constants (KNu) have been measured for the addition of 44 primary amines (including five α-effect amines), 28 secondary amines, 19 tertiary amines, ammonia and hydroxide ion to the vinyl group of the 1-methyl-4-vinylpyridinium cation (1) in aqueous solution at 25° C (ionic strength 0.1 mol dm–3). Nucleophilic attack is shown to be rate-determining for primary and secondary amines, with secondary amines being generally more reactive than primary amines of the same basicity. After classification of these species in terms of structure, they describe a number of Bronsted-type correlations having βnuc in the range 0.35–0.54 for six structural classes of primary amine, βnuc= 0.48 for α-effect amines, and βnuc in the range 0.23–0.34 for four structural classes of secondary amine. Substitution upon the α-carbon atom reduces amine nucleophilicity of both primary and secondary amines. The presence of an unsaturated carbon atom (either sp2- or sp-hybridized) as the β-carbon atom leads to an enhanced reactivity relative to the corresponding β-sp3 species in all cases. Tertiary amines are in general less reactive than other amines of the same basicity. Bronsted-type plots for tertiary amines present the appearance of random scatter which is not readily decipherable in terms of structure. β-Hydroxy and β-amino tertiary amines are unusually reactive relative to their basicity. All of these phenomena suggest that protonation of the carbanionic intermediate by a molecule of water is the rate-determining step for the addition of tertiary amines to 1.Rate constants for the attack of primary and secondary amines on 1 are shown to correlate with literature data for a variety of other reactions involving rate-determining nucleophilic attack of amines upon electrophilic carbon. These kNu for primary and secondary amines reacting with 1 are also shown to correlate with Ritchies N+ parameters for nucleophilic attack at electrophilic sp2-carbon. N+ parameters for amine nucleophiles have not been widely available previously; the parameters that have been available for selected amines are known to be sensitive to the nature of the defining electrophile. The minimal steric hindrance at the electrophilic centre in nucleophilic attack upon 1 suggests that this species is an appropriate electrophile for the definition of N+ parameters for amine nucleophiles; these parameters are evaluated for 70 primary and secondary amines and ammonia and are suggested to provide an appropriate data base for future investigations of the reactivity and selectivity of amine attack upon sp2-carbon electrophiles in aqueous solution.

Substrate inhibition in the hydrolysis of N-acylglycine esters by carboxypeptidase A.

The rates of hydrolysis of a series of 21 N-acylglycine esters (YCONHCH2CO2CH(CH2CH3)CO2H (2)) by bovine pancreatic carboxypeptidase A (peptidyl-L-amino-acid hydrolase, EC 3.4.12.2) have been studied over the substrate concentration range 10(-4)-10(-1) M at pH 7.5, 25 degrees C, ionic strength 0.5. All substrates display substrate inhibition except Y = CH3, CH3CH2 and (CH3)3C for which normal Michaelis-Menten kinetics are observed. In all cases substrate inhibition is consistent with the formation of an ES2 complex and parameters for the second-degree rate equation v/E = (kapp2 S + kapp3 S2/KappSS)/(KappS + S + S2/KappSS) have been evaluated. For a series of eight aliphatic groups varying in size between Y = CH3 and Y = cyclo-C6H11 the following linear correlations were observed: -log KappS = 0.82 pi + 1.32 and log kapp2/KappS = 0.71 pi + 5.81 (pi is Hanschs hydrophobicity parameter). Aryl and aralkyl Y moieties deviate from these correlation lines. KappSS also depends on the hydrophobicity of Y but no quantitative correlation is obvious. Thus the Y unit of 2 is involved in a hydrophobic interaction with the enzyme when 2 binds at both the catalytically productive and inhibitor sites. Parameters for the enzymic hydrolysis of the esters YCONHCH2CO2CH(CH2CH(CH3)2)CO2H (3) (Y = C6H5(CH2)n (n = 0, 1, 2)) are also presented. Pronounced nonproductive 1: 1 enzyme.substrate complex formation is observed for each of 2: Y = C6H5(CH2)n (n = 2, 3) and 3: Y = C6H5(CH2)2. Hippurate anion is shown to be an uncompetitive inhibitor (Ki = 12 mM) for the hydrolysis of 2: Y = (CH3)3C. Data are now available which can only be interpreted in terms of at least three enzymic sites being available for hydrophobic interactions with ester substrate molecules.

The mechanism of substrate inhibition of the esterase activity of carboxypeptidase A

Abstract Substrate inhibition of the hydrolysis of esters of hippuric acid by carboxypeptidase A is consistent with the formation of an E.S2 complex which, independent of the alcohol moiety of the ester, reacts approximately 26 times more slowly than E.S complex. The binding of a second substrate molecule to E.S also does not depend significantly on the structure of the alcohol portion of the ester.