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Effect of calcium on the dynamic behavior of sialylglycerolipids and phospholipids in mixed model membranes a proton and phosphorus nmr study



Effect of calcium on the dynamic behavior of sialylglycerolipids and phospholipids in mixed model membranes a proton and phosphorus nmr study



Biochemistry 30(43): 10542-10550



DTSL, a sialic acid bearing glyceroglycolipid, has been deuterated at the C3 position of the sialic acid headgroup and at the C3 position of the glycerol backbone. The glycolipid was studied as a neat dispersion and in multilamellar dispersions of DMPC (at a concentration of 5-10 mol % relative to phospholipid), using 2H and 31P NMR. The quadrupolar splittings, .DELTA.nu.Q, of the headgroup deuterons were found to differ in the neat and mixed dispersion, suggesting different headgroup orientations in the two systems. In DTSL-DMPC liposomes, two quadrupolar splittings were observed, indicating that the axial and equatorial deuterons make different angles with respect to the axis of motional averaging. The splittings originating from the equatorial and axial deuterons were found to increase and decrease with increasing temperature, respectively, indicating a temperature-dependent change in average headgroup orientation. Longitudinal relaxation times, T1Z, wre found to be short (3-6 ms). The field dependence of T1Z suggests that more than one motion governs relaxation. At 30.7 MHz a T1Z minimum was observed at approximately 40.degree. C, At 46.1 MHz the T1Z values were longer and increased with temperature, demonstrating that the dominant rigid-body motions of the headgroup at this field are in the rapid motional regime (> 108 s-1). DTSL labeled at the glycerol C3 position was studied in DMPC multilamellar dispersions. Whereas two quadrupolar splittings have been observed for other glycolipids labeled at this position, only a single .delta.nu.Q was observed. This shows that the orientation of the C2-C3 segment of DTSL relative to the bilayer normal differs from that of other glycolipids. T1Z values were short (3-7 ms) and increased with temperature, demonstrating that motion is in the rapid motional regime. Quadrupolar splittings and T1Z values were also obtained for the headgroup-labeled DTSL in the presence of 5 and 50 mM Ca2+. As the Ca2+ concentration was increased, the ratio of outer to inner quadrupolar splittings increased, suggesting a small challenge in headgroup orientation. From the longitudinal relaxation times the rate of the dominant headgroup motion(s) appeared to decrease. The DTSL-DMPC liposomes were also studied by 31P NMR and by two-dimensional solid-state 31P NMR, the latter technique giving information on the orientational exchange of phospholipid molecules. DTSL appeared to alter the headgroup orientation of DMPC and also to increase the rate of orientational exchange. The latter most likely reflects an increase in the rate of lateral diffusion of the phospholipid. Ca2+ was found to reverse both of these effects partially.

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Accession: 007242031

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