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NMRLipids / lipid_ionINTERACTION

Files related to the NMRlipids II project
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electrometer concept using cationic lipids (Figure 8 and appendix B) #30

Open ohsOllila opened 8 years ago

ohsOllila commented 8 years ago

This is comment by Claire Loison copied from https://github.com/NMRLipids/lipid_ionINTERACTION/pull/29

  1. Is the membrane charge only described as the lipid charge ? I would expect counter-ion binding, just as for zwitterionic membranes...comparing delta S directly as a function of lipid content, one assumes then that the Cl- binding on cationic membranes is well described by the model. maybe you can mention this assumption explicitely. Something like "one assumes that the description of Cl- binding is much less critical that Na+ binding" (why?)
  2. How to understand the contradiction between the two experimental data (dotab and others), which one is the reference ? as the simulations considered as ok with experiments or overestimating the effect ? for me, the comparison with [31] is misleading for the discussion. I would only compare the PC/TAP mixture for exp. and simulations.
  3. Maybe you can put vertical arrows on the figure 8 at the charge 0.34 e/lipid between exp and theoretical lines, to show what your 0.04 and 0.06 stand for. It took me a while to follow where these numbers come from...
ohsOllila commented 8 years ago
  1. This is a good point. It is now mentioned (see new text below).
  2. I have now clarified this. The manuscript now reads: "The order parameter changes from simulations overestimate the changes measured from DMPC/DOTAP mixtures [47] especially with larger amphiphile concentrations, but are in good agreement with experimental line from various amphiphiles with saturated acyl chains measured by Scherer et al. [31]. The origin of the difference in order parameter changes between DOTAP and amphihiles with saturated chains is not known. It may arise from the differecens in acyl chain saturation level or from differences in headgroup. In the used simulation data the acyl chains are similar to data from [31] but the headgroup is similar to the data from [47]. Also Cl- binding affinity may affect the comparison. Thus we cannot fully conclude how well the headgroup response to bound charge is reproduced in simulation.

To estimate the maximum error we take the maximum amount of bound charge from Fig. 4 (\approx 0.5 q/nm2) and assume the area per lipid of 0.68 nm2. This gives for maximum amount of bound charge per lipid X+max = 0.5 q/nm2 * 0.68nm2 = 0.34 q/lipid, which is shown as dashed line in Fig. 8. The maximum overestimations of order parameter decrease with this amount of bound charge per lipid are \approx 0.04 and \approx 0.06 for \beta and \alpha order parameter changes, respectively. The numbers are smaller with less amount of bound cations. In principle, these value could explain the overestimated order parameter change due to the presence of CaCl2 in Berger model but not in the presence of NaCl (see Fig. 2)."

  1. The lines are added. The text is clarified (see above).