Abstract
The organization of biomolecules and bioassemblies is highly governed by the nature and extent of their interactions with water. These interactions are of high intricacy and a broad range of methods based on various principles have been introduced to characterize them. As these methods view the hydration phenomena differently (e.g., in terms of time and length scales), a detailed insight in each particular technique is to promote the overall understanding of the stunning “hydration world.” In this prospective mini-review we therefore critically examine time-dependent fluorescence shift (TDFS)—an experimental method with a high potential for studying the hydration in the biological systems. We demonstrate that TDFS is very useful especially for phospholipid bilayers for mapping the interfacial region formed by the hydrated lipid headgroups. TDFS, when properly applied, reports on the degree of hydration and mobility of the hydrated phospholipid segments in the close vicinity of the fluorophore embedded in the bilayer. Here, the interpretation of the recorded TDFS parameters are thoroughly discussed, also in the context of the findings obtained by other experimental techniques addressing the hydration phenomena (e.g., molecular dynamics simulations, NMR spectroscopy, scattering techniques, etc.). The differences in the interpretations of TDFS outputs between phospholipid biomembranes and proteins are also addressed. Additionally, prerequisites for the successful TDFS application are presented (i.e., the proper choice of fluorescence dye for TDFS studies, and TDFS instrumentation). Finally, the effects of ions and oxidized phospholipids on the bilayer organization and headgroup packing viewed from TDFS perspective are presented as application examples.
Highlights
The role of the hydration in maintaining the biological function of biomolecules and biomolecular aggregates is unquestionable (Disalvo, 2015; Biedermannová and Schneider, 2016)
Within the last decade the experimental advances in label free techniques together with computational methods allowed the development of a quantitative understanding of the hydration shell of biomembranes and proteins
Characteristic relaxation time τ can report directly on subtle changes in 1) the degree of hydration and 2) mobility, respectively, of the hydrated phospholipid or protein segment at the close vicinity of the fluorophore embedded in the bilayer
Summary
The role of the hydration in maintaining the biological function of biomolecules and biomolecular aggregates is unquestionable (Disalvo, 2015; Biedermannová and Schneider, 2016). Claims of a significant long-range modification of the structure and dynamics of water around biomolecules lack theoretical and experimental evidence.” This finding clarifies what are those three basic types of the interfacial hydration modes mentioned in the introduction: 1) bulk water, 2) water molecules directly interacting with lipids which are imbedded in 3) adjacent water layers. Despite that relatively low number of interfacial water molecules, it is undoubtedly accepted that the structure and function of the biological membranes are strongly affected by the dynamic properties of the hydration water layer It plays a pivotal role in transport and signalling functions, mediating membrane-membrane interactions, as well as the ones with ions, DNA or proteins (Disalvo and Bakás, 1986; Hamley, 2000; Berkowitz et al, 2006). We will illustrate this approach on mapping the organization of the lipid bilayers by TDFS, taking into consideration broader atomistic context
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