Giancarlo Franzese
Department de Física de la Matèria Condensada, Universitat de Barcelona, Barcelona, Spain
Water properties under extreme confinement can be very different from those in bulk [1] and can be related to important functions in bio-membranes activity [2]. We study, by all-atom molecular dynamics simulations, water confined in stacked phospholipid membranes with hydration from poor to complete [3,4]. We find that the water dynamics is dramatically slowed down by reducing the hydration level, as in the experiments. We identify three different regions in the interface where water molecules exhibit distinctive dynamical behavior. We show that the slow-down is a consequence of (1) the robustness of water-lipid hydrogen bonds, which are more frequent the lower the hydration of the membrane, and (2) the longer lifetime of water-water hydrogen bonds the lower the hydration. By adopting a sensitive local order metric recently proposed by Martelli et al., measuring the degree of overlap of the local environment with the structures of perfect ice, we find that water acquires a high intermediate range order (IRO) within 1 nm from the membrane interface, i.e. with the same distance where its dynamics slows down [5]. Surprisingly, we show that at distances as far as ≃ 3 nm from the interface, although the bulk-like dynamics is recovered, the IRO of water is still slightly higher than that in the bulk under the same thermodynamic conditions. Therefore, the water-membrane interface has a structural effect at ambient conditions that propagates further than the often-invoked 1 nm length scale. We compare these results with the case of water in a rigid graphene sieve [6]. We find that the diffusion of water confined between parallel walls depends on the plate distance in a non-monotonic way and is related to the water structuring, crystallization, re-melting, and evaporation for decreasing inter-plate distance. Our results provide much-needed insight for theories and experiments about water-mediated surface interactions and the behavior of interfacial water under extreme confinement [7, 8].
References:
[1] L. Fumagalli et al. Anomalously low dielectric constant of confined water, Science 360, 1339–1342 (2018).
[2] E. W. Martin and T. Mittag, Dwelling at membranes promotes decisive signaling, Science 363, 1036-1037 (2019).
[3] C. Calero and G.Franzese, Membranes with different hydration levels: the interface between bound and unbound hydration water, J. Mol. Liq. 273, 488 (2019).
[4] C. Calero and H.E. Stanley and G. Franzese, Structural Interpretation of the Large Slowdown of Water Dynamics at Stacked Phospholipid Membranes for Decreasing Hydration Level: All-Atom Molecular Dynamics, Materials 9, 319 (2016). Level: All-Atoms Molecular Dynamics, http://arxiv.org/abs/1603.02565[5]
[5] F. Martelli, H-Y. Ko, C. Calero Borallo, G. Franzese, Structural properties of water confined by phospholipid membranes, Front. Phys. 13, 136801 (2018).
[6] J Martí, C Calero, and G. Franzese, Structure and Dynamics of Water at Carbon-Based Interfaces, Entropy 19, 135 (2017).
[7] S. Samatas, C. Calero, F. Martelli, and G. Franzese, Water Between Membranes: Structure and Dynamics in “Biomembrane Simulations: Computational Studies of Biological Membranes”, M. L. Berkowitz ed. (CRC Press, 2019), Chapter 2.
[8] The author acknowledges funding from the Spanish MINECO Grants No. FIS2015-66879-C2-2-P, and the ICREA Foundation.