Lactam isoassociation was investigated with ir spectroscopy and a special dielectric relaxation technique. Synoptic analysis of the results suggests that the associative behavior should be described in terms of a trimerization model which is more marked in cyclohexane than in tetrachloromethane, but not recognizable in benzene. The primary bimolecular rate constants decrease in that order of the solvents with values from 7 to 5 X 109 M~x sec-1. Interpretation of the amplitudes of the kinetic reaction signals leads to the conclusion that the second possible hydrogen bond of the associated complex may not be formed in benzene (offering an explanation of the rather low association constant K2 = 28 M~x) but in cyclohexane (K2 = 550 M~x). Tetrachloromethane seems to represent an intermediate case (K2 = 200 M-1). The enthalpy of about 9 kcal/mol as determined from the temperature dependence of the ir spectra is considered to be the sum of two formed hydrogen bonds. The monomer dipole moments of e-caprolactam and -butyrolactam as derived from the chemical field effect measurements are 4.30 and 4.10 D, respectively, within ±5%.

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Cytoplasmic pyruvate decarboxylase (EC 4.1.1.1, from Saccharomyces curfshevgensis) exhibits in its circular dichroic spectrum in the 250 -320-nm range a multiple two-signal band. This couplet disappears on increasing the pH up to pH 8.5. Two classes of two protons each can be quantified by these spectral changes. The first class dissociates rapidly and the apparent pK is 7.84. The thermodynamic data are AG = 87.7 kJ mol-', AH = + 56.0 kJ mol-', AS = -108 J mol-' K-', very characteristic for the deprotonation of an amino-acid side chain. The second class of the protons hasthefollowingthermodynamicdata: AG = 88.3 kJmol-', A H = -64.3 kJmol-', AS = -5205 mol-' K-' which, in conjunction with kinetic reasoning and in view of enzyme stoichiometry and symmetry, suggests a conformational equilibrium exposing the second two protons. The enzyme dissociates into two dimeric subunits. This dissociation step is considered to be rate-determining for the overall process. The data are: k, = 1.4. lop3, AH' = + 128.3 kJ mol-', AS' = + 136 J mol-' K-' . If there is a conformational equilibrium, the rate constant of product formation k , will be modified by a factor p = Kc/(l + K,) (0 < p 5 1) where Kc is the conformational equilibrium constant. The subunit dissociation appears to be controlled by the enthalpy of activation indicating that a number of interactions, i.e. ionic, hydrogen and hydrophobic bridges, are to be broken. Optimal conditions for the preparation of the apo-enzyme are derived from the data.Pyruvate decarboxylase (2-0x0-acid carboxy-lyase) is a thiamin-pyrophosphate-dependant enzyme and, in view of its function, is advantageously isolated from the cytoplasm of brewers yeast (Sucrharomyces curlshergensis) [l]. Most of our knowledge of the chemical basis of its catalytic mechanism originates from studies of its activity in reconstitution experiments with a variety of chemicaIIy modified coenzyme analogs [2]. These experiments require the preparation of the apo-enzyme [3], which is easily achieved by dissolving the enzyme in alkaline buffer and leaving it for a while at room temperature. Under these conditions, the enzyme decomposes into two subunits with the concomitant release of the coenzyme molecules and the magnesium ions. The enzyme must be composed of two subunits of two monomers each, according to ultracentrifugation [4] and gel chromatographic [5] results.

According to previously developed ideas on the nucleation process of micelle formation the kinetics of the micellization of an ionic surfactant (Aerosol OT) was studied in various apolar solvents. The applied kinetic method utilizes the dielectric field effect due to a considerable variation of the reaction moment produced by the extensive compensation of the effective dipole moment of the monomers in the micelle. Rotational relaxations of about 120, 23 and 20 MHz in dioxane, benzene and cyclohexane solutions, respectively, were assigned to the expected premicellar aggregates (nuclei) and the micelles; in the latter solutions, however, orientational and chemical relaxation processes are superimposed. The analysis of the corresponding data yields a concentration dependence of the reciprocal relaxation times of the chemical process which conforms very satisfactorily to the corresponding theoretical plot derived from a model which assumes a conformational change between two premicellar states during the formation of the micelle.