Deuterium (2H) nuclear magnetic resonance (NMR) quadrupole splittings and relaxation times have been measured for a variety of specifically deuterated lipids intercalated in lamellar-multibilayer dispersions and single-bilayer vesicles of egg lecithin and lecithin-cholesterol mixtures. The deduced order parameters and relaxation times vary with position of deuteration, acyl chain length, unsaturation, and temperature. The order parameters and spinlattice relaxation times T1 indicate rapid intramolecular motions of restricted amplitude in both the choline head group and hydrocarbon chains. The ordering profile for the acyl chains is similar to that predicted by statistical-mechanical theory. The order parameters yield estimates of the bilayer thickness and linear coefficient of expansion in close agreement with the x-ray determinations. A comparison of the deuterium and electron spin resonance spinprobe order parameters demonstrates the perturbation of the bilayer by the bulky nitroxide probe. The transverse relaxation time T2 for single-bilayer vesicles is quantitatively accounted for by a simple modification of classical relaxation theory which takes into account the modulation of the static quadrupole interaction by rapid local molecular motions and the modulation of the residual quadrupole interaction by the slower overall tumbling of the vesicle. It is unambiguously demonstrated that molecular motion and order in single-bilayer vesicles are very similar to those in lamellar multibilayers. Significant differences occur only for a few segments near the terminal methyl groups of the acyl chains, where the order parameters for vesicles are 10-30% smaller than those found for lamellae. The incorporation of cholesterol in lecithin bilayers is shown to increase the degree of orientational order in vesicles and lamellae, and to increase the hydrodynamic radius of vesicles. Thus, single-bilayer vesicles and multilamellar dispersions of phospholipids are equally useful models for biological membranes. They yield equivalent information about the internal organization and mobility of lipid bilayers, when the spectral manifestations of overall vesicle motion are correctly taken into account.
The chromic acid oxidation of oxalic acid (H»C204) has been reinvestigated over a wide range of conditions. Oxalic acid is oxidized by chromium(VI) by two different mechanisms, each involving a different oxalic acidchromic acid complex. The complete mechanism (Scheme III) leads to the rate law v = [CrT][OxH2] {kptl + (kK¡ KnKa°*H2[OxH2])/(l + Ao/KaH=Cr0« + / ,[ ,]/ 0 + KiKu'K.,°3 H j[OxH2]2/ 0) 1. where [OxH2] is the concentration of undissociated oxalic acid and [CrT] the sum of the concentrations of all chromium(VI) species present. Kinetic evidence for the formation of a chromium(VI)-oxalic acid 1:1 and 1:2 complexes has been obtained and the equilibrium constants for their formation have been determined. The 1:1 complex exists as a neutral species and is most probably a cyclic anhydride. The 1:2 complex is stable as a dianion and exists most likely in an open chain form in which the usual coordination number of four for chromium(VI) is retained. The reactive intermediate in the second-order reaction is the monoanion of the 1:2 complex, HCbCCCTCrCbCOCO»". It is proposed that this intermediate decomposes directly into [Cr(H20)6]3+, three molecules of CO» and a free radical C02H, in a onestep three-electron oxidation reaction. The formation of free-radical intermediates has been demonstrated. In the presence of an excess of acrylamide, the yield of carbon dioxide is reduced to a limiting value which is in agreement with the proposed mechanism. An unexpected exchange between [Cr(H20)6]3+ and oxalic acid under the reaction conditions has been observed.In the first paper of this series1 we reported what we believe to be the first documented case of a threeelectron oxidation in organic chemistry.2 3
The chromic acid oxidation of a mixture of oxalic acid and isopropyl alcohol proceeds much faster than that of either of the two substrates alone. It is shown that both substrates undergo oxidation. In the presence of free-radical scavengers an exactly 1:1 ratio of acetone and carbon dioxide is formed, indicating that the alcohol undergoes a two-electron oxidation and oxalic acid a one-electron oxidation. The rate law governing the oxida-
The chromic acid oxidation of glycolic acid follows the rate law -d[Cr(VI)]/d? = K¡ [HCr04-] [S](k2 + LjjS]), where [S] is the concentration of glycolic acid and [Cr(VI)] the total analytical concentration of chromium(VI); K¡ = 2.2, k2 = 3.0 X 10-3 M~1 sec-1, k-¡ = 1.3 X 10-3 M~2 sec-1. The first term corresponds to a reaction leading to the formation of glyoxylic acid, formaldehyde and carbon dioxide in a 2:1:1 ratio. Glyoxylic acid is formed by two-electron oxidation with chromium(VI) and chromium(V) while chromium(IV) is responsible for the oxidative cleavage to formaldehyde and a CO2-radical. The second term of the rate law corresponds to a three-electron oxidation in which a 2:1 glycolic acid-chromic acid complex decomposes directly to chromium(III), glyoxylic acid, and a HOCHCO2H radical. No cleavage to formaldehyde takes place under conditions under which the second term of the rate law is dominant. The free radicals formed in either step react with chromium(VI) to yield an observable chromium(V) intermediate. The chromium(V) oxidation of glycolic acid yields glyoxylic acid only. The kinetic isotope effect in the oxidation of HOCD2CO2H is kn/kr> = 6.15 for k2 and kn/ko > 36.5 for k3. The high value of the isotope effect of the rate constant k-¡ for the second term provided convincing proof that two carbon-hydrogen bonds are broken simultaneously and that the reduction of chromium(VI) to chromium(III) does take place in a single three-electron step.
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