Water determines the properties of biological systems. Therefore, understanding the nature of the mutual interaction between water and biosystems is of primary importance for a proper assessment of any biological activity, e.g., the efficacy of new drugs or vaccines. A convenient way to characterize the interactions between biosystems and water is to analyze their impact on water density and dynamics in the proximity of the interfaces. It is commonly accepted that water bulk density and dynamical properties are recovered at distances of the order of 1 nm away from the surface of biological systems. This notion leads to the definition of hydration or biological water as the nanoscopic layer of water covering the surface of biosystems and to the expectation that all the effects of the water-interface interaction are limited to this thin region. Here, we review some of our latest contributions, showing that phospholipid membranes affect the water dynamics, structural properties, and hydrogen bond network at a distance that is more than twice as large as the commonly evoked $\sim 1\mathrm{nm}$ thick layer and of the order of 2.4 nm. Furthermore, we unveil that at a shorter distance $\sim 0.5\mathrm{nm}$ from the membrane, instead, there is an additional interface between lipid-bound and unbound water. Bound water has a structural role in the stability of the membrane. Our results imply that the concept of hydration water should be revised or extended and pave the way to a deeper understanding of the mutual interactions between water and biological systems.

中文翻译：

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重新定义软界面附近水合水的概念

水决定了生物系统的特性。因此，了解水和生物系统之间相互作用的性质对于正确评估任何生物活性（例如新药或疫苗的功效）至关重要。表征生物系统和水之间相互作用的一种方便方法是分析它们对界面附近水密度和动力学的影响。人们普遍认为，在距生物系统表面 1 nm 量级的距离处，水的体积密度和动力学特性可以恢复。这个概念导致了水合作用或生物作用的定义水作为覆盖生物系统表面的纳米级水层，并期望水-界面相互作用的所有影响都限于这个薄区域。在这里，我们回顾了我们的一些最新贡献，表明磷脂膜影响水动力学、结构特性和氢键网络的距离是通常诱发的两倍多。$～1\mathrm{纳米}$厚的层和 2.4 nm 的数量级。此外，我们揭示了在更短的距离$～0.5\mathrm{纳米}$从膜，相反，有脂质之间的附加接口结合和未结合的水。结合水对膜的稳定性具有结构作用。我们的研究结果意味着水化水的概念应该被修改或扩展，并为更深入地了解水和生物系统之间的相互作用铺平道路。