Llombart, PabloDe la Arada, IgorGonzález Ramírez, Emilio J.Alonso, AliciaGonzález Mac-Dowell, LuisGoñi, Félix M.2025-04-022025-04-022025-04-01Llombart, P.; Arada, I. D. L.; González-Ramírez, E. J.; Alonso, A.; MacDowell, L. G.; Goñi, F. M. Long-Term Memory in Lipid Assemblies: Rate-Independent Hysteresis in the Ripple-to-Liquid-Disordered Transition of Sphingomyelin Bilayers. The Journal of Chemical Physics 2025, 162 (13), 135101. https://doi.org/10.1063/5.0252051.0021-9606https://hdl.handle.net/20.500.14352/119157This work was funded in part by the Spanish Ministry of Science, Innovation, and Universities (MCIU), Agencia Estatal de Investigación (AEI), Fondo Europeo de Desarrollo Regional (FEDER) (Grant Nos. PID2021-124461NB-I00 and PID2023-151751NB-100), the Basque Government (Grant No. IT1625-22), Fundación Biofísica Bizkaia, and the Basque Excellence Research Center (BERC) program of the Basque Government.Sphingomyelin (SM) is the most abundant sphingolipid in mammalian cells. It contains a phosphorylcholine headgroup, which makes SM an analog of the (glycerol-containing) phosphatidylcholines. Palmitoyl (C16:0) SM bilayers in excess water exhibit a thermotropic transition from the ripple to the fluid phase centered at ≈41 °C. In phosphatidylcholines, as in most phospholipids, the ripple-to-fluid transition is fully reversible and virtually free of hysteresis. In this paper, however, the corresponding transition was assessed in aqueous SM by infrared (IR) spectroscopy, a technique detecting molecular vibrations. Vibrational spectra as a function of temperature revealed thermotropic phase transitions. When the samples were successively heated up and cooled down, a clear hysteresis was detected. The cooling transition started at the same temperature as the heating one, but the end-point, in terms of IR band position, was clearly different. Hysteresis was particularly visible in the shift of the IR Amide I band, associated with the lipid polar headgroup, and it was rate-independent, within a wide range of heating/cooling rates (from 5.5 °C/min to less than 0.05 °C/min). Atomistic computer simulations of the molecular dynamics provided information consistent with the IR data. In addition, it showed that the in-plane arrangement of SM bilayers displays a significant amount of hexatic order, and that the hexatic order parameter, reflecting primarily polar headgroup ordering, exhibited the same kind of hysteresis described by IR. Rate-independent hysteresis allows the development of durable memories; therefore, the observations in this paper could lead to novel applications of lipid assemblies.engAttribution 4.0 Internationalhttp://creativecommons.org/licenses/by/4.0/journal article1089-7690https://doi.org/10.1063/5.0252051https://pubs.aip.org/aip/jcp/article/162/13/135101/3341534/Long-term-memory-in-lipid-assemblies-Rateopen access544Lipid membranePhospholipidsInfrared spectroscopyMolecular simulationHysteresisCiencias2210 Química Física