11: Bent Fatty Acid Chains in Lecithin Bilayers
The “flexibility gradient” in lipid bilayers was discovered in early work by Wayne Hubbell (138). (Later work by Joachim and Anna Seelig measured the flexibility gradient using deuterium labeled fatty acid lipids.) There was accordingly much interest understanding the molecular origin of this gradient. In a paper titled “Bent fatty acid chains in lecithin bilayers” (142), Betty Gaffney and I described an analysis of the paramagnetic resonance spectra of two different spin label lipids in bilayers, and we made an unfortunate error in describing our results. The spin label magnetic resonance part of the paper was interesting and straightforward. The spectra showed that one label had a persistent nitroxide group tilt of about 30 deg relative to the bilayer normal (0 deg), and the lifetime of this tilt was about 0.01- 0.1 microseconds. Most labels show rapid axial motion about the bilayer normal, and no evidence of tilt. Our argument was that there is no way that one can have a persistent tilt of a labeled lipid without the bilayer having a 0.01- 0.1 microsecond configuration that supported this tilt. (For a more detailed discussion see (176).) Our “mistake” was to include a figure of a small group of cooperatively tilted fatty acid chains, tilted at about 30 degrees, and to make the comment that such tilting restricted motion near the polar head groups provided for more motional freedom near the terminal methyl groups. Our paper was strongly attacked by Dill and Flory, who in part said our picture implied long range order in the bilayer. How anyone could believe that a configuration of molecules with a lifetime of 0.01 - 0.1 microseconds could show long range order is beyond me. Separately, I heard that Maurice Wilkins (London) was furious at me for publishing his idea which had arisen in a conversation between the two of us. I would certainly be sympathetic to his distress if there were long-range order, for which there is no evidence. Unfortunately if one looks at our figure, and does not read the text, one might infer we were describing long-range tilt order.