A spin-probe method is described that can detect changes in the relative aggregation numbers in SDS micelles with a precision of about one molecule. The method is based on the fact that the 14 N hyperfine coupling constant is sensitive to the average fraction of the volume occupied by water in the region of the nitroxide moiety that is located on average near the micelle surface. Defining A 0 (N A ) to be the 14 N hyperfine coupling constant at an aggregation number N A , we find A 0 (N A ) ) A 0 (0) + (∂A 0 /∂N A )N A , where N A is controlled by varying either the SDS or the NaCl concentrations. For the spin probe 5-doxylstearic acid ester(5DSE), by combination of the results of experiments in which the SDS and/or the NaCl concentrations were varied, linear least-squares fits gave A 0 (0) ) (15.498 ( 0.009) G and ∂A 0 /∂N A ) -3.99 ( 0.02 mG/molecule (constant). A 0 (N A ) depends only on the aggregation number despite the fact that a given value of N A may be prepared by choosing different combinations of NaCl and SDS concentrations. This means that, for a given micelle size, neither interactions between micelles nor the ionic strength of the solution influence the value of A 0 . A geometric model, based on a simple model in which a spherical hydrocarbon core is surrounded by a concentric spherical polar shell, is developed to predict the volume fraction of the polar shell occupied by water as a function of N A . This water fraction is written in terms of a hydration number per surfactant molecule, N(H 2 O). A 0 (N A ) is related to N(H 2 O) by employing the nonempirical polarity index introduced by Mukerjee et al. (Mukerjee, P.; Ramachandran, C.; Pyter, R. A. J. Phys. Chem. 1982, 86, 3189). The value of N(H 2 O) decreases as the micelle grows because the volume per surfactant molecule in the polar shell decreases. Once N(H 2 O) is specified at a particular value of N A , no further adjustable parameters enter into the model. The variation with micelle size of the theoretical polarity is in excellent agreement with experiment for values of N(H 2 O) that are comparable to those found from transport properties. The sphere-rod transition is observed as a rather sharp transition in which the detected water volume fraction becomes constant. Detailed future tests of the model are outlined.

Recently cytochrome c has been mentioned as an important mediator in the events of cellular oxidative stress and apoptosis. To investigate the influence of charged interfaces on the conformation of cytochrome c, the CD and magnetic circular dichroic behavior of ferric and ferrous cytochrome c in homogeneous medium and in phosphatidylcholine/phosphatidylethanolamine/cardiolipin and dicetylphosphate liposomes was studied in the 300 -600 and 200 -320 nm wavelength region. EPR spectra demonstrate that the association of cytochrome c with membranes promotes alterations of the crystal field symmetry and spin state of the heme Fe 3؉ . The studies also include the effect of P i , NaCl, and CaCl 2 . Magnetic circular dichroism and CD results show that the interaction of both ferrous and ferric cytochrome c with charged interfaces promotes conformational changes in the ␣-helix content, tertiary structure, and heme iron spin state. Moreover, the association of cytochrome c with different liposomes is sensitive to the heme iron valence state. The more effective association with membranes occurs with ferrous cytochrome c. Dicetylphosphate liposomes, as a negatively charged membrane model, promoted a more pronounced conformational modification in the cytochrome c structure. A decrease in the lipid/protein association is detected in the presence of increasing amounts of CaCl 2 , NaCl, and P i , in response to the increase of the ionic strength.

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We report electron-paramagnetic resonance ͑EPR͒ studies at ϳ9.5 GHz ͑X band͒ and ϳ34 GHz ͑Q band͒ of powder and single-crystal samples of the compound Cu 2 ͓TzTs͔ 4 ͓N-thiazol-2-yl-toluenesulfonamidatecopper͑II͔͒, C 40 H 36 Cu 2 N 8 O 8 S 8 , having copper͑II͒ ions in dinuclear units. Our data allow determining an antiferromagnetic interaction J 0 = ͑−113Ϯ 1͒ cm −1 ͑H ex =−J 0 S 1 • S 2 ͒ between Cu͑II͒ ions in the dinuclear unit and the anisotropic contributions to the spin-spin coupling matrix D ͑H ani = S 1 • D • S 2 ͒, a traceless symmetric matrix with principal values D / 4=͑0.198Ϯ 0.003͒ cm −1 and E / 4=͑0.001Ϯ 0.003͒ cm −1 arising from magnetic dipole-dipole and anisotropic exchange couplings within the units. In addition, the singlecrystal EPR measurements allow detecting and estimating very weak exchange couplings between neighbor dinuclear units, with an estimated magnitude ͉JЈ͉ = ͑0.060Ϯ 0.015͒ cm −1 . The interactions between a dinuclear unit and the "environment" of similar units in the structure of the compound produce a spin dynamics that averages out the intradinuclear dipolar interactions. This coupling with the environment leads to decoherence, a quantum phase transition that collapses the dipolar interaction when the isotropic exchange coupling with neighbor dinuclear units equals the magnitude of the intradinuclear dipolar coupling. Our EPR experiments provide a new procedure to follow the classical exchange-narrowing process as a shift and collapse of the line structure ͑not only as a change of the resonance width͒, which is described with general ͑but otherwise simple͒ theories of magnetic resonance. Using complementary procedures, our EPR measurements in powder and single-crystal samples allow measuring simultaneously three types of interactions differing by more than three orders of magnitude ͑between 113 cm −1 and 0.060 cm −1 ͒.