Arnold Wishnia

The intrinsic structural asymmetry of highly curved phospholipid bilayer membranes.

Phosphorus-31 NMR studies of solutions of small L-alpha-dipalmitoyl phosphatidylcholine bilayer vesicles containing sodium dimethyl phosphate uniformly distributed between the continuous external and the intravesicular aqueous spaces, with the paramagnetic shift reagent Pr3+ present only in the external space, are reported. These studies give the distribution both of dipalmitoyl phosphatidylcholine in the vesicle inner and outer monolayers and of dimethyl phosphate in the aqueous spaces. With the third necessary parameter obtained from the vesicle sedimentation coefficient, the very different packing parameters of dipalmitoyl phosphatidylcholine in inner and outer monolayers can be determined. The vesicle outer radius is 109 A. Although the total bilayer thickness is virtually identical to that of planar bilayers, the outer monolayer is thicker (20 A) and the inner monolayer thinner (15 A). The area per head group at the inner surface, 68 A2, is like the planar value, but the tails are much more folded, so as to decrease the radial lengths and increase the tangential spreat (to 94A2). The reverse is true in the outer layer: the surface per head group is 76 A2, tapering to 51 A2 in the tail region, so that outer layer tails are relatively extended. The difference is equivalent to a shift of about two 2g1 kinks from outer to inner layers; the uneven packing certainly affects fluidity, and may have important biological consequences.

Spin transfer between laser-polarized 129Xe nuclei and surface protons

We have demonstrated a large polarization transfer from highly polarized gaseous 129Xe to protons in a silicone surface coating. The proton polarization enhancement of ∼ 104–105 over the thermal equilibrium polarization at 0.2 T makes possible the detection of the previously unobservable resonance. We expect that this technique may allow high-resolution NMR to become a viable tool in the study of surfaces.

Aqueous shift reagents for high-resolution cation NMR. V. Thermodynamics of interaction of DyTTHA3− with Na+, K+, Mg 2+ , and Ca 2+

Abstract Triethylenetetraminehexaacetate complexes of Dy(II) or Tm(III) (DyTTHA3− and TmTTHA3−, introduced as NMR shift reagents for alkali metal cations, Chu et al., J. Magn. Reson. 56, 33 (1984) bind to the four major biological inorganic cations: Na+, K+, Mg2+, and Ca2+. New 23Na and 39K NMR shift displacement data, obtained over wide and different ranges of concentration, were combined with the previous results (including some 25Mg data) for computer analysis. With a proper treatment of the relevant activity coefficients using Pitzers formulas, it is established that only mono complexes of the shift reagent and a cation need be considered. The cations bind competitively, with nearly identical limiting shifts of 159 and 155 ppm for Na+ and K+; the shift for 25Mg is 3 5 as large. The thermodynamic formation constants for M-LnTTHA are 11 and 18 M−1 with Na+ and K+ (enthalpy of binding, −54 kJ) , and 130 and 3100 M−1 with Mg2+ and Ca2+ at 3°C. The results suggest that all the cations bind at the same site, with Mg2+ probably forming a solvent-separated complex. The formation constants expected at 37°C indicate that, at useful shift reagent concentrations, and at physiological cation concentrations, DyTTHA3− can be used not only to distinguish among cation pools but also for quantitative studies of cation relationships. In particular, 23Na signals might be used to report free extracellular Ca2+ concentrations in vivo.