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Comments Strength of van der Waals Attraction between Mica Surfaces across Lipid Bilayers Is the powerful van der Waals attraction between mica surfaces in water made comparatively negligible when mica is coated by phosphatidylcholine bilayers? My recent calculation1 concluded that the mica contribution should not be neglected when one is comparing interactions between bilayers in water with those between bilayercoated mica surfaces. In fact, the added attraction of mica might explain a puzzling disparity between bilayerbilayer attraction measured by pipet aspiration (PA) and the much larger bilayer attraction inferred from the coated-mica surface force apparatus (SFA). Could this conclusion have been different, as implied in ref 2, had different starting numbers been used in van der Waals computation? Let us assign bilayers a refractive index of 1.46 to 1.49rather than the 1.41 to 1.42 value used earlier, take the Hamaker coefficientof a hydrocarbon liquid across water to be (7-9) x J, recently suggested as more realistic,2 rather than the earlier 4.26 x J. The result is clear in the figure. There is no significant change. The mica still shines through the bilayer coatings. According to the analysis in ref 3, the values of the ratio m(w)lb(w)would have to be a flat line equal to 1.0. Microscopic details might matter a t close range but will not dominate over the large range of separations where measurements are reported. Mica van der Waals interactions can still be a significant part of the discrepancy between PA and SFA results. This should not be surprising given the fact that the refractive index of mica is so much greater than that of water or hydrocarbon (ref 1);the “Hamaker Constant” of bilayers is not a primary factor in assessing the action of mica itself. It is of course conceivable that many other factors might also bear on the PAISFA contradiction. Concerning the undulations and protrusions mentioned in ref 2: Undulations are not a t issue; the PAISFA contradiction was still evident after correction for undulatory repulsion.6 Protrusions? If mica can suppress the motion of individual molecules on the far side of a bilayer to the extent suggested in ref 2, one might ask why the surface (1)Parsegian, V. A. Langmuir 1993,9,3625-3628. (2)Israelachvili, J. N.Langmuir 1994,10,3369-3370. (3)Marra, J.;Israelachvili, J. N. Biochemistry 1985,24,4608-4618. (4)Leneveu, D.M.; Rand, R. P.; Parsegian,V. A.; Gingell, D.Biophys. J. 1977,18,209-230. (5)Parsegian, V. A,; Ninham, B. W. J. Colloid Interface Sci. 1971, 37,332-341. (6)Evans, E. A.; Parsegian, V. A. PNAS 1986,83,7132-7136.Evans, E. A,; Rawicz, W. Phys. Rev.Lett. 1990,64,2094-2097.Evans, E. A. Langmuir 1991,7, 1900-1908. (7)Attard, P.; Parker, J. L. Phys. Rev.A 1992,46,7959.Parker, J. L.; Attard, P. J . Phys. Chem. 1992,96,10398. (8)Horn, R.G.; Israelachvili, J. N.; Marra, J.;Parsegian,V. A.; Rand, R. P. Biophys. J . 1988,54,1185-1186. (9)Parsegian, V. A.; Gershfeld, N. L. Biophys. J . 1993,64,A222. (10)Kekicheff, P.; Marcelja, S.;Senden, T. J.;Shubin,V. E. J. Chem. Phvs. 1993.99.6098-6113. ill)Pashley, R.;McGuiggan, P.; Horn, R.; Ninham, B. W. J . Colloid Interface Sci. 1988,126, 569-578. (12)Tsao, Y.-H.; Yang, S. X.; Evans, D. F. Langmuir 1992,8,11881194. __ .. .
(13)Farrell, B.; Bailey, A. I.; Chapman, D. Appl. Opt., in press. (14)Helm, C. A,; Israelachvili, J. N.; McGuiggan, P. M. Biochemistry 1992,31. 1794-1805.
4.0
3.5
I h=30A
1.0
1 0
io
20
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40
50
I 60
w , water t h i c k n e s s ( A n g s t r o m s ) Figure 1. Ratio of van der Waals attraction energy computed between bilayer-coated mica to vdW attraction computed between bilayers in water. Any deviation of m(w)lb(w)from 1.0 indicatesa contribution ofmica to the van der Waals interaction energy. (a,b, and c are different mica spectra, details in ref 1.) The range of separations w is taken to be that over which measurements were made with bilayer-coated mica (Figure3, ref 3). To satisfy both the 1.48refractive index and 8 x J Hamaker coefficient conditions suggestedin ref 1,hydrocarbon was assigned a 12-eV relaxation frequency. The approximate “combining relation” used in this computation is only qualitatively accurate for the zero-frequency contribution.With the starting numbers used here, this contributionis relatively less important than it was in the earlier computation. To be consistent with the model used by Israelachvili and Mar~-a,~ when they replaced the bilayer-plus-micasurface by a semiinfinite layer with hydrocarbonpolarizabilities(evenreaching out over the polar groups,cf. Figure 13,ref 3). The phospholipid bilayer here is treated as a hydrocarbon layer but one whose thickness is in keeping with measured bilayer capacitance and with X-ray diffraction data (cf. ref 1). More elaborate models (e.g.,4where a polar group layer is included 01.5 which more thoroughly treats the zero-frequencycontribution)can be more exhaustivelyexamined but will not give qualitatively different results.
force apparatus ever be used to study bilayers. Indeed, in addition to the effects of mica on bilayers mentioned in ref 2,one can consider other features that have recently been noted with the SFA.’-13 In particular, immobilization onto mica may create a completely different object; bilayer adsorption might create attractive forces and lateral stresses quite different from those seen between free bilayers in aqueous s01ution.l~ None of this alters the need to inquire into gentler ways to explain anomalies in mica-adsorbed bilayers. Micamica attraction is one such possibility that can be instructively examined by computation rather than speculation.
V. A. Parsegian Laboratory of Structural Biology, DCRT and Section on Molecular Forces, DIR INIDDK, National Institutes of Health, Bethesda, Maryland 20892-5626 Received June 16, 1994 In Final Form: December’l2, 1994
This article not subject to U.S.Copyright. Published 1995 by the American Chemical Society
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