The infrared spectra of aqueous potassium ascorbyl palmitate were studied as a function of temperature using Fourier transform infrared techniques. From a light scattering experiment the Krafft point of 0.1 M potassium ascorbyl palmitate was determined to be 48°C. The temperature-induced changes in infrared spectral parameters such as frequency and bandwidth characterize this Krafft point as a phase transition from a conformationally ordered, poorly hydrated solid phase, to an isotropic micellar phase. The phase transition of this "pseudosoap" occurs over a temperature range of about 1O0C, reflecting the progressive hydration of the solid upon micellization, a behavior typical of surfactants such as soaps. Chem. 59,2543Chem. 59, (1981. En utilisant les techniques infrarouges de la transformation de Fourier, on a etudie les spectres infrarouges des solutions aqueuses de palmitateascorbyle de potassium en fonction de la temperature. A partir d'un experience de dispersion de la lumiere, on a determine que le point Krafft pour une solution 0,l M de palmitateascorbyle de potassium est de 48°C. La temperature provoque des changements dans les parametres spectraux tels la frkquence et la largeur des bandes qui caracterisent ce point Krafft en tant que 1 transition de phase i partir d'une conformation ordonnee, ma1 hydratee dans la phase solide, jusqu'a une phase micellaire isotropique. La transition de phase de ce "pseudo" savon s e produit au dessus d'un intervalle de temperature de 10°C environ refletant ainsi I'hydratation progressive du solide lors de la formation des micelles, c'est un comportement typique des savons.

The biological efficacy of vitamin C depends on its redox abilities as given by the relations between ascorbic acid, semidehydroascorbic acid, and dehydroascorbic acid. It is shown by means of proton magnetic resonance spectroscopy that the enzymatic (by ascorbate oxidase) as well as non-enzymatic (by iodine) oxidation of ascorbic acid is, in principle, reversible despite the hydration and structural changes during the formation of dehydroascorbic acid. The strong redox activity of semidehydroascorbic acid which results in a fast disproportionation to ascorbic acid and dehydroascorbic acid is inferred from an inversion of the electrochemical potentials of the vitamin C redox system. The capacity of this is maintained by a fast reduction of dehydro­ ascorbic acid e.g. by reduced glutathione, preventing its delactonization and further degradation.

Dehydro-ʟ-ascorbic acid, the oxidation product of L-ascorbic acid, is unstable in aqueous solution and decomposed by a hydrolytic disruption of its ring structure to 2,3-diketo-ʟ-gulonic acid. It is shown by means of UV, 1H-, and 13C-NMR spectroscopy that after decarboxylation 3,4,5-trihydroxy-2-keto-ʟ-valeraldehyde, an α-ketoaldehyde, is formed. This substance is oxidized further to ʟ-erythroascorbic acid, coupled with a reduction of 2,3-diketo-ʟ-gulonic acid to L-ascorbic acid.

The stacking interaction between nucleic acid bases has been investigated by the determination of the self-association of 6-methylpurine in various mixtures of water and nonaqueous solvents in order to elucidate the solvent effect. The parameters of stacking association as well as of local solvent-solute interactions have been measured by means of NMR technique. The influences of local hydration and of solvent-solvent interactions on the stacking ability are discussed.

The selfassociation of N6,N9-dimethyladenine and N6-dimethyl-N9-ethyladenine has been studied by means of NMR technique. The thermodynamic quantities have been calculated using an isodesmic NMR model with three NMR parameters (the monomer shift deltaM and two complex shifts delta2 and delta3). The dependence of the thermodynamic quantities on the NMR parameters is discussed. Special attention is given to the determination of deltaM and its temperature dependence. Calculations with delta3 = 2 - delta2 and deltaM taken independently of temperature result in an average entropy deltaS = - 17.9 +/- 1.8 e.u. for N6,N9-dimethyladenine and deltaS = - 16.7 +/- 1.7 e.u. for N6-dimethyl-N9-ethyladenine and in an average enthalpy deltaH = - 7.2 +/- 0.6 kcal - mol-1 for both substances investigated.