sample calculations above, the fractional disorder required at each position to produce a particular loss of intensity is not a very strong function of the power dependence assumed in eq 1, for values of p close to 1 that occur in slightly disordered phases.From a biological perspective, the current results suggest that membranes with high levels of cholesterol, such as mammalian cells, may contain more conformational order than might be anticipated from the original fluid mosaic model of membrane str~cture.~' We are currently examining native membranes enriched in a single acyl chain length to quantitatively explore this hypothesis. (27) Singer, S. J.; Nicolson, G. L. Science 1972, 175, 720.Finally, previous experiments from this laborat~ryI~-'~ have introduced spectral parameters for the study of conformational disorder in phospholipid liquid crystalline phases. The CD2 rocking modes of specifically deuterated acyl chains provide a depth-dependent probe of trans-gauche isomerization, especially in the L, phase. However, in ordered phases, the bands that reveal gauche rotamer formation are extremely weak and yield band intensities of low precision. The approach presented in the current investigation thus complements the earlier studies and permits a more detailed evaluation of the whole range of conformational order available to hydrocarbon chains.In order to shed light on the mechanism of proton-transfer reactions, a kinetic and ab initio study of the antioxidant action (intermolecular proton transfer) of vitamin E derivatives has been carried out. The second-order rate constants (k,'s) for the reaction of tocopherols (TocH's) with variously substituted phenoxy1 radicals (Ph0"s) in ethanol were measured with a stopped-flow spectrophotometer. The half-wave reduction potentials (Ellis) of Ph0"s were obtained by using a cyclic voltammetry technique. The result indicates that k, increases as the total electron-donating capacity of the alkyl substituents at the aromatic ring of TocH or the electron-withdrawing capacity of the substituent of Pho' increases. k, for the reaction of deuterated tocopherol derivatives (TocDs) with a PhO' in deuterated ethanol (C2H50D, ethanol-d,) was also measured. A substantial deuterium kinetic isotope effect on k, is observed. In the reactions of each Pho' with various TocH's, a plot of log k, vs peak oxidation potential (E,) of TocH is found to be linear. The slope of its plot for TocDs is close to that for TocHs. In the reactions of each TocH with various PhWs, a plot of log k, vs Ell2 of Pho' is found to be linear. The geometries of TocH's were optimized with the semiempirical modified neglect of diatomic overlap (MNDO) method. The Koopmans' theorem first ionization energies (IP) for those geometries were calculated with the ab initio method. In the reactions of a PhO' with various TocH's, plots of log k, vs IP, the activation energy (Eaa) vs IP, and E, vs IP are also found to be linear.From these results, it is considered that both the charge transfer and the proton tunneling play...

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The electronic-state dependence of the intramolecular proton transfer of o-hydroxybenzaldehyde in the vapor phase has been studied by means of emission spectroscopy. High-resolution fluorescence spectra were obtained by excitation at several bands of the absorption from the ground state to the second excited '( , *) (S)*1) state in the vapor phase at room temperature.The decay rates at several vibronic levels of were evaluated from the analysis of line width of the individual vibronic bands in the fluorescence-excitation spectrum in a supersonic free jet. It is considered that the intramolecular proton transfer does not play an important role in the decay process from the state contrary to the case of the first excited '( , *) (Sj*1) state. The observed electronic-state dependence of the intramolecular proton transfer of o-hydroxybenzaldehyde is consistent with the explanation that in contrast to the case of the S0 and S2T' states the enol tautomer is stabilized in the S1*1 state owing to the character of the wave function.

We have studied the proton spin-lattice relaxation times (Td of a series of benzoic acid (BAC) derivatives and decanoic acid (DAC) over a wide range of temperature and analyzed the results in terms of the double proton switching along the hydrogen bonds. The proton T, in the high temperature region are analyzed using the classical jump model and the barrier heights for the proton transfer are determined. The thermodynamic parameters for the equilibria between the two configurations in the solid state are also determined by the Ff -IR measurements. It is shown that the energetics and dynamics of the proton transfer in DAC and the para-and meta-substituted BAC are all similar, but they are very different in the ortho-substituted ones. It is suggested that the low temperature behavior of the proton T, of the dimers of carboxylic acid is due to the tunneling and the asymmetry of the potential brings in a small activation energy.

Studies of the kinetics of the proton- or hydrogen-transfer reactions concerning vitamin E in solutions and in micellar dispersions by means of stopped-flow and absorption spectroscopy indicated that proton tunneling plays an important role in the antioxidant and regeneration reactions that are advantageous in vivo but not as a part of the harmful prooxidant action.