Local Hydration Research Articles

Expanding the Utility of 4‐cyano‐L‐phenylalanine as a Vibrational Reporter

Unnatural amino acids (UAAs) have become useful tools for understanding protein structure and local environments. One of the most commonly used vibrational reporter UAAs to study local protein hydration environments is 4‐cyano‐L‐phenylalanine (pCNF) due to the position and sensitivity of the nitrile symmetric stretching frequency as well as the ability to efficiently incorporate this UAA site‐specifically into proteins using the Amber codon suppression methodology. Additionally, pCNF provides promise to function as a distance reporter when coupled with two‐dimensional infrared (2D IR) spectroscopy. Here, a number of superfolder green fluorescent protein (sfGFP) constructs have been generated with pCNF site‐specifically incorporated at two unique sites simultaneously. The pairs of sites were selected to permit the nitrile symmetric stretching frequency of pCNF at each site to be spectrally resolved. The temperature dependent linear IR spectra of these sfGFP constructs with two pCNF UAAs incorporated will be presented, illustrating that the nitrile groups are in different local environments. Preliminary X‐ray crystal structures of these sfGFP constructs will be presented to probe if the UAAs resulted in a structural perturbation and to determine the distance between the nitrile groups of the two pCNF UAAs in each structure. These results will serve as the basis for and calibration of employing pCNF in future 2D IR spectroscopic studies to extract the distances between these two groups based upon coupling between them.

Bacterial age distribution in soil - Generational gaps in adjacent hot and cold spots.

Resource patchiness and aqueous phase fragmentation in soil may induce large differences local growth conditions at submillimeter scales. These are translated to vast differences in bacterial age from cells dividing every thirty minutes in close proximity to plant roots to very old cells experiencing negligible growth in adjacent nutrient poor patches. In this study, we link bacterial population demographics with localized soil and hydration conditions to predict emerging generation time distributions and estimate mean bacterial cell ages using mechanistic and heuristic models of bacterial life in soil. Results show heavy-tailed distributions of generation times that resemble a power law for certain conditions, suggesting that we may find bacterial cells of vastly different ages living side by side within small soil volumes. Our results imply that individual bacteria may exist concurrently with all of their ancestors, resulting in an archive of bacterial cells with traits that have been gained (and lost) throughout time-a feature unique to microbial life. This reservoir of bacterial strains and the potential for the reemergence of rare strains with specific functions may be critical for ecosystem stability and function.

Effect of Temperature on Local Hydration of Zn in Hematite

Iron (oxyhydr)oxides are common soil minerals that can be host phases to a large number of environmentally important elements, such as the micronutrient Zn. The structure and hydration level of these host phases change significantly if the soil is heated, as is the case during wildfires, for example. Steady increases in the frequency and scale of wildfires in recent decades thus raises the question of whether these events significantly impact the geochemical availability of trace elements incorporated within the reacting iron (oxyhydr)oxides. In this contribution, we focus on Zn-substitution into two abundant iron minerals in soil, hematite (α-Fe2O3) and goethite (α-FeOOH). Using ab initio molecular dynamics (AIMD)-informed extended X-ray absorption fine structure (EXAFS) spectra, we determine the local environment of Zn in hydrothermally precipitated material and after annealing at 500 and 800 °C. Results show that Zn was incorporated into the hematite structure by preferential formation of coupled O–Fe and protonated Fe vacancies (PFe). These defects increase in density around Zn upon heating the samples to 500 °C, particularly PFe. Heating to higher temperatures produced franklinite (ZnFe2O4), a highly refractory zinc iron spinel. This difference in structure and hydration of Zn sites suggests soil mineralogy and wildfire temperature impact the local coordination of trace elements, which may play a role in controlling the bioavailability of these elements.

Interactions between water and organic molecules or inorganic salts on surfaces

AbstractWater, as a ubiquitous and relatively inert medium, plays a vital role in nature. The interactions between water and organic molecules or inorganic salts are of fundamental interest in understanding hydration processes in chemistry. The combination of scanning probe microscopy and theoretical calculations provides a versatile tool to directly visualize and further rationalize such interactions as well as influences of water on organic molecules and inorganic salts. In this review, we provide a brief overview of the recent exciting progress in revealing the interactions between water and organic molecules as well as inorganic salts (including ions) on surfaces. Herein, we describe first steps of hydration of organic molecules followed by a microsolvation process on surfaces. Subsequently, water‐induced tautomerization and chiral separation of small biomolecules on surfaces are discussed for understanding the roles of water in driving biological self‐assembly processes. Moreover, water is also able to assist structural transformation globally by selectively interrupting the relatively weak hydrogen bonds within nanostructures. Based on the water‐incorporated molecular nanostructures, dissolution of sodium halides on wet surfaces is achievable using confined water, while hydrates of halide ions desorb from the surface. More interestingly, ion hydrates are also demonstrated to be artificially accessible, which enables atomic‐scale investigation into local ion hydration and transport at interfaces. This review provides new insights into the role of water in the hydration‐related molecular and ionic systems, with implications for hydration and solvent effects down to the single‐molecule level.

The deep Basel-1 geothermal well: an attempt assessing the predrilling hydraulic and hydrochemical conditions in the basement of the Upper Rhine Graben

Hydrogeological properties of fluid reservoirs in the brittle continental crust at 5 km have been deduced from hydraulic and chemical data provided by the Deep Heat Mining well Basel-1 in the south of the Upper Rhine rift valley (central Europe, Switzerland). The investigation was challenging because no direct temperature logs or fluid samples from the undisturbed reservoir exist. However, the properties of the undisturbed reservoir have been reliably reconstructed from short time hydraulic tests and the evolution of outflow water composition. The rock of the open hole sections (4629–5000 m) is predominantly coarse-grained undeformed poorly fractured quartz-monzodiorite. The permeability k = 5.8 × 10–18 m2 is characteristic for plutonic basement at 5 km depth. Fluid flow is restricted to few steeply dipping fracture zones in this section. Outflow water triggered by massive injection of river water contains predominantly NaCl. The total of dissolved solids (TDS) in the pristine reservoir at depth is about 45 g kg−1. The origin of the high salinity is probably fossil seawater. The water has been modified in the reservoir by desiccation reactions related to the partial and local hydration of the igneous reservoir rock. The estimated reservoir temperature of 185 °C using three different calibrations of standard fluid geothermometers is in excellent agreement with measured and extrapolated temperatures in the borehole. The consistent application of different fluid geothermometers confirms the rock control of the fluid composition.

Dual Time-Scale Proton Transfer and High-Energy, Long-Lived Excitons Unveiled by Broadband Ultrafast Time-Resolved Fluorescence in Adenine-Uracil RNA Duplexes.

In contrast to the immense amount of research on electronically excited DNA, surprisingly little has been done about the excited states of RNA. Herein, we demonstrate an ultrafast broadband time-resolved fluorescence and fluorescence anisotropy study to probe directly the intrinsic fluorescence and overall dynamics of the fluorescence from a homopolymeric adenine·uracil RNA duplex adopting the A-form structure. The results unveiled complex deactivation through distinctive multichannels mediated by states of varied energy, a character of charge transfer, and a lifetime from sub-picosecond to nanoseconds. In particular, we observed an unprecedented kinetic isotopic effect and participation of unusual proton transfer from states in two discrete energies and time domains. We also identified a high-energy nanosecond emission that we attributed to its fluorescence anisotropy to long-lived weakly emissive excitons not reported in DNA. These distinguishing features originate from the stacking, pairing, and local hydration environment specific to the A-form conformation of the adenine·uracil double helix.

Influence of Methylene Fluorination and Chain Length on the Hydration Shell Structure and Thermodynamics of Linear Diols.

The interplay between the local hydration shell structure, the length of hydrophobic solutes, and their identity (perfluorinated or not) remains poorly understood. We address this issue by combining Raman-multivariate curve resolution (Raman-MCR) spectroscopy, simulation, and quantum-mechanical calculations to quantify the thermodynamics and the first principle interactions behind the formation of defects in the hydration shell of alkyl-diol and perfluoroalkyl-diol chains. The hydration shell of the fluorinated diols contains substantially more defects than that of the nonfluorinated diols; these defects are water hydroxy groups that do not donate hydrogen bonds and which either point to the solute (radial-dangling OH) or not (nonradial-dangling OH). The number of radial-dangling OH defects per carbon decreases for longer chains and toward the interior of the fluorinated diols, mainly due to less favorable electrostatics and exchange interactions; nonradial-dangling OH defects per carbon increase with chain length. In contrast, the hydration shell of the nonfluorinated diols only contains radial-dangling defects, which become more abundant toward the center of the chain and for larger chains, predominantly because of more favorable dispersion interactions. These results have implications for how the folding of macromolecules, ligand binding to biomacromolecules, and chemical reactions at water-oil interfaces could be modified through the introduction of fluorinated groups or solvents.

Activator Anion Influences the Nanostructure of Alkali-Activated Slag Cements

Alkali-activated materials are promising low-carbon alternatives to Portland cement; however, there remains an absence of a fundamental understanding of the effect of different activator types on their reaction products at the atomic scale. Solid-state 27Al and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and 1H–29Si cross-polarization MAS NMR spectroscopy are used to reveal the effect of the activator anion on the nanostructure, cross-linking, and local hydration of aged alkali-activated slag cements. The main reaction product identified is a mixed cross-linked/non-cross-linked sodium-substituted calcium aluminosilicate hydrate (C–(N)–A–S–H) gel with a structure comparable to tobermorite 11 Å. Analysis of cross-polarization kinetics revealed that a higher content of soluble silicate in the activator promoted the incorporation of Al into the aluminosilicate chains of C–(N)–A–S–H gels, charge-balanced preferentially by protons within the gel interlayer. In sodium carbonate-activated slag cements, aluminosilicate chains of C–(N)–A–S–H gels are instead charge-balanced preferentially by Ca2+ or AlV ions. Hydrotalcite was observed as a secondary reaction product independent of the activator used and in higher quantities as the content of sodium carbonate in the activator increases. The presence of soluble silicates in the activator promotes the formation of an Al-rich sodium aluminosilicate hydrate (N–A–S–H) gel which was not identified when using sodium carbonate as the activator. These results demonstrate that the anion type in the activator promotes significant differences in the nanostructure and local hydration of the main binding phases forming in alkali-activated slag cements. This explains the significant differences in properties identified when using these different activators.

Concerted conformational dynamics and water movements in the ghrelin G protein-coupled receptor.

There is increasing support for water molecules playing a role in signal propagation through G protein-coupled receptors (GPCRs). However, exploration of the hydration features of GPCRs is still in its infancy. Here, we combined site-specific labeling with unnatural amino acids to molecular dynamics to delineate how local hydration of the ghrelin receptor growth hormone secretagogue receptor (GHSR) is rearranged upon activation. We found that GHSR is characterized by a specific hydration pattern that is selectively remodeled by pharmacologically distinct ligands and by the lipid environment. This process is directly related to the concerted movements of the transmembrane domains of the receptor. These results demonstrate that the conformational dynamics of GHSR are tightly coupled to the movements of internal water molecules, further enhancing our understanding of the molecular bases of GPCR-mediated signaling.

Sensing Hydration of Biomimetic Cell Membranes.

Biological membranes play a vital role in cell functioning, providing structural integrity, controlling signal transduction, and controlling the transport of various chemical species. Owing to the complex nature of biomembranes, the self-assembly of lipids in aqueous media has been utilized to develop model systems mimicking the lipid bilayer structure, paving the way to elucidate the mechanisms underlying various biological processes, as well as to develop a number of biomedical and technical applications. The hydration properties of lipid bilayers are crucial for their activity in various cellular processes. Of particular interest is the local membrane dehydration, which occurs in membrane fusion events, including neurotransmission, fertilization, and viral entry. The lack of universal technique to evaluate the local hydration state of the membrane components hampers understanding of the molecular-level mechanisms of these processes. Here, we present a new approach to quantify the hydration state of lipid bilayers. It takes advantage of the change in the lateral diffusion of lipids that depends on the number of water molecules hydrating them. Using fluorescence recovery after photobleaching technique, we applied this approach to planar single and multicomponent supported lipid bilayers. The method enables the determination of the hydration level of a biomimetic membrane down to a few water molecules per lipid.

Coarse-grained hybrid Monte Carlo/molecular dynamics simulations of the nanophase separated morphology of polysulfone grafted with phosphonated sidechain for proton exchange membrane fuel cells

Nanophase separated morphology is a widely accepted design for the balance of proton conductivity and integrity of proton exchange membrane for fuel cells (PEMFC). Since the coarse-grained (CG) modeling goes beyond the time- and space-scales accessible to the atomistic modeling and the Monte Carlo simulation (MC) overcomes the steric hindrance that molecular dynamics simulation (MD) faces during the rearrangements of polymeric chains, the CG hybrid MC/MD searches effectively the configurational space and can be applied to the investigation of nanophase separated morphology of proton exchange membrane. In this work, the morphological characteristics of proton exchange membrane of poly(vinyl phosphonic acid) grafted polysulfone (PSU-g-PVPA) are studied using CG hybrid MC/MD and analyzed using distance-based clustering algorithm. It is revealed that the PVPA hydrophilic sidechains and the PSU hydrophobic mainchains segregate, respectively, into hydrophilic/hydrophobic clusters resulting nanophase separated morphology upon hydration. It is concluded that a length of five repeat units for the PVPA sidechains is necessary for the hydrophilic/hydrophobic nanophase separation, and the hydrophilic clusters connect into continuous channels for the long-distance transport of protons. Finally, the effective local hydration degrees (λi/λ0) in the four largest hydrophilic clusters are evaluated to increase from 42.2% to 48.0% and 48.7% as the PVPA sidechain length increases from 1 to 5 and 10.

Origins of Clustering of Metalate-Extractant Complexes in Liquid-Liquid Extraction.

Effective and energy-efficient separation of precious and rare metals is very important for a variety of advanced technologies. Liquid-liquid extraction (LLE) is a relatively less energy intensive separation technique, widely used in separation of lanthanides, actinides, and platinum group metals (PGMs). In LLE, the distribution of an ion between an aqueous phase and an organic phase is determined by enthalpic (coordination interactions) and entropic (fluid reorganization) contributions. The molecular scale details of these contributions are not well understood. Preferential extraction of an ion from the aqueous phase is usually correlated with the resulting fluid organization in the organic phase, as the longer-range organization increases with metal loading. However, it is difficult to determine the extent to which organic phase fluid organization causes, or is caused by, metal loading. In this study, we demonstrate that two systems with the same metal loading may impart very different organic phase organizations and investigate the underlying molecular scale mechanism. Small-angle X-ray scattering shows that the structure of a quaternary ammonium extractant solution in toluene is affected differently by the extraction of two metalates (octahedral PtCl62- and square-planar PdCl42-), although both are completely transferred into the organic phase. The aggregates formed by the metalate-extractant complexes (approximated as reverse micelles) exhibit a more long-range order (clustering) with PtCl62- compared to that with PdCl42-. Vibrational sum frequency generation spectroscopy and complementary atomistic molecular dynamics simulations on model Langmuir monolayers indicate that the two metalates affect the interfacial hydration structures differently. Furthermore, the interfacial hydration is correlated with water extraction into the organic phase. These results support a strong relationship between the organic phase organizational structure and the different local hydration present within the aggregates of metalate-extractant complexes, which is independent of metalate concentration.

Towards real-life EEG applications: novel superporous hydrogel-based semi-dry EEG electrodes enabling automatically ‘charge–discharge’ electrolyte

Objective. A novel polyacrylamide/polyvinyl alcohol superporous hydrogel (PAAm/PVA SPH)-based semi-dry electrode was constructed for capturing electroencephalogram (EEG) signals at the hairy scalp, showing automatically ‘charge–discharge’ electrolyte concept in EEG electrode development. Approach. In this regard, PAAm/PVA SPH was polymerized in-situ in the hollow electrode cavity by freezing polymerization, which acted as a dynamic reservoir of electrolyte fluid. The SPH can be completely ‘charged’ with electrolyte fluid, such as saline, in just a few seconds and can be ‘discharged’ through a few tiny pillars into the scalp at a desirable rate. In this way, an ideal local skin hydration effect was achieved at electrode–skin contact sites, facilitating the bioelectrical signal pathway and significantly reducing electrode–skin impedance. Moreover, the electrode interface effectively avoids short circuit and inconvenient issues. Main results. The results show that the semi-dry electrode displayed low and stable contact impedance, showing non-polarization properties with low off-set potential and negligible potential drift. The average temporal cross-correlation coefficient between the semi-dry and conventional wet electrodes was 0.941. Frequency spectra also showed almost identical responses with anticipated neural electrophysiology responses. Significance. Considering prominent advantages such as a rapid setup, robust signal, and user-friendliness, the new concept of semi-dry electrodes shows excellent potential in emerging real-life EEG applications.

Hydrophobicity of CO2 gas diffusion electrodes

In Nature Communications, Hu, Feng, and their colleagues present a scalable, facile method to tune the hydrophobicity of CO2 reduction gas diffusion electrodes by adding polytetrafluoroethylene (PTFE) nanopowder into the catalyst ink. For a copper catalyst CO2 reduction electrode, the additive significantly enhances CO2 reduction performance and imparts persistent hydrophobicity.

Spectroscopy, Dynamics, and Hydration of S-Nitrosylated Myoglobin.

S-Nitrosylation, the covalent addition of NO to the thiol side chain of cysteine, is an important post-transitional modification that can alter the function of various proteins. The structural dynamics and vibrational spectroscopy of S-nitrosylation in the condensed phase are investigated for the methyl-capped cysteine model system and for myoglobin. Using conventional point charge and physically more realistic multipolar force fields for the -SNO group, it is found that the SN- and NO-stretch and the SNO-bend vibrations can be located and distinguished from the other protein modes for simulations of MbSNO at 50 K. The finding of stable cis- and trans-MbSNO agrees with experimental findings on other proteins as is the observation of buried -SNO. For MbSNO the observed relocation of the EF loop in the simulations by ∼3 Å is consistent with the available X-ray structure, and the conformations adopted by the -SNO label are in good overall agreement with the X-ray structure. Despite the larger size of the -SNO group compared with -SH, MbSNO recruits more water molecules in the first two hydration shells due to stronger electrostatic interactions. Similarly, when comparing the hydration between the A- and H-helices, they differ by up to 30% between WT and MbSNO. This suggests that local hydration can also be significantly modulated through nitrosylation.

5 Quality Improvement Project (QIP): A Teamwork Approach to Optimise Fluid Intake in Older Inpatients #ButFirstADrink

Abstract The shortfalls of hydration management have been widely exposed since the Frances report in 2013. Age-related changes create challenges for people to drink adequate fluid amounts (Nazarko, 2018), making dehydration prevention in older people an ongoing public health priority (NICE, 2016). Hydration needs of older people are complex and may not be fully understood. A proactive MDT approach to facilitate regular drinking opportunities seems an achievable solution to increase hydration awareness, knowledge and daily fluid intake in older inpatients. A QI methodology was used, including stakeholder engagement and PDSA learning cycles to influence, engage and educate staff on hydration management. Outcome measures: Fluid intake (ml/day), number of drinks/days based on fluid balance chart recordings. Interventions: Sharing the #ButFirstADrink social media hydration movement, which includes educational posters, flyers and a YouTube video. Face to face drop-in education sessions were implemented using local hydration education resources from Age UK Salford and GM nutrition and hydration programme as teaching tools. SPC charts were used to show a time-series data. In 15 weeks, 461/525 fluid charts were reviewed from 169 patients. Mean age was 84 years and 68% females. Average daily fluid intake increased by 25% (exceeding aim) indicating 176mls more fluid was offered on average/day. Mean number of drinks/days increased from 4.6 to 6 post interventions. Knowledge and attitude improved in some areas; however fluid output is not routinely recorded. Post intervention, 89% of staff felt that offering a drink at first contact is a suitable hydration promotion campaign. The project scored 58% using the NHS sustainability model (ACT Academy, 2018) suggesting reason for optimism. As it is a behaviour change project, further engagement and momentum is dependent on ongoing enthusiasm to drive the #ButFirstADrink approach. This intervention is free, simple and innovative and stimulates behaviour change, raises drinking awareness and increases hydration.

The Interplay of Ligand Properties and Core Size Dictates the Hydrophobicity of Monolayer-Protected Gold Nanoparticles.

The hydrophobicity of monolayer-protected gold nanoparticles is a crucial design parameter that influences self-assembly, preferential binding to proteins and membranes, and other nano-bio interactions. Predicting the effects of monolayer components on nanoparticle hydrophobicity is challenging due to the nonadditive, cooperative perturbations to interfacial water structure that dictate hydrophobicity at the nanoscale. In this work, we quantify nanoparticle hydrophobicity by using atomistic molecular dynamics simulations to calculate local hydration free energies at the nanoparticle-water interface. The simulations reveal that the hydrophobicity of large gold nanoparticles is determined primarily by ligand end group chemistry, as expected. However, for small gold nanoparticles, long alkanethiol ligands interact to form anisotropic bundles that lead to substantial spatial variations in hydrophobicity even for homogeneous monolayer compositions. We further show that nanoparticle hydrophobicity is modulated by changing the ligand structure, ligand chemistry, and gold core size, emphasizing that single-ligand properties alone are insufficient to characterize hydrophobicity. Finally, we illustrate that hydration free energy measurements correlate with the preferential binding of propane as a representative hydrophobic probe molecule. Together, these results show that both physical and chemical properties influence the hydrophobicity of small nanoparticles and must be considered together when predicting gold nanoparticle interactions with biomolecules.

A computational insight into the relationship between side chain IR line shapes and local environment in fibril-like structures.

Infrared spectroscopy is a widely used technique to characterize protein structures and protein mediated processes. While the amide I band provides information on proteins' secondary structure, amino acid side chains are used as infrared probes for the investigation of protein reactions and local properties. In this paper, we use a hybrid quantum mechanical/classical molecular dynamical approach based on the perturbed matrix method to compute the infrared band due to the C=O stretching mode of amide-containing side chains. We calculate, at first, the infrared band of zwitterionic glutamine in water and obtain results in very good agreement with the experimental data. Then, we compute the signal arising from glutamine side chains in a microcrystal of the yeast prion Sup35-derived peptide, GNNQQNY, with a fibrillar structure. The infrared bands obtained by selective isotopic labeling of the two glutamine residues, Q4 and Q5, of each peptide were experimentally used to investigate the local hydration in the fibrillar microcrystal. The experimental spectra of the two glutamine residues, which experience different hydration environments, feature different spectral signals that are well reproduced by the corresponding calculated spectra. In addition, the analysis of the simulated spectra clarifies the molecular origin of the experimentally observed spectroscopic differences that arise from the different local electric field experienced by the two glutamine residues, which is, in turn, determined by a different hydrogen bonding pattern.

Resolving the Equal Number Density Puzzle: Molecular Picture from Simulations of LiCl(aq) and NaCl(aq).

The change in number densities of aqueous solutions of alkali chlorides should be qualitatively predictable. Typically, as cations get larger, the number density of the solution decreases. However, aqueous solutions of lithium and sodium chloride exhibit at ambient conditions practically identical number densities at equal molalities despite different ionic sizes. Here, we provide an atomistic interpretation of this experimentally observed anomalous behavior using molecular dynamics simulations. The obtained results show that the rigidity of the Li+ first and second solvation shells and the associated compromised hydrogen bonding result in practically equal average water densities in the local hydration regions for Li+ and Na+ despite different sizes of the cations. In addition, in more distant regions from the cations, the water densities of these two solutions also coincide. These findings thus provide an atomistic interpretation for matching number densities of LiCl and NaCl solutions. In contrast, the number density differences between NaCl and KCl solutions as well as between LiCl and KCl solutions behave in a regular fashion with lower number densities of solutions observed for larger cations.

Cation enrichment in the ion atmosphere is promoted by local hydration of DNA.

Electrostatic interactions are central to the structure and function of nucleic acids, including their folding, condensation, and interaction with proteins and other charged molecules. These interactions are profoundly affected by ions surrounding nucleic acids, the constituents of the so-called ion atmosphere. Here, we report precise Fourier Transform-Terahertz/Far-Infrared (FT-THz/FIR) measurements in the frequency range 30–500 cm−1 for a 24-bp DNA solvated in a series of alkali halide (NaCl, NaF, KCl, CsCl, and CsF) electrolyte solutions which are sensitive to changes in the ion atmosphere. Cation excess in the ion atmosphere is detected experimentally by observation of cation modes of Na+, K+, and Cs+ in the frequency range between 70–90 cm−1. Based on MD simulations, we propose that the magnitude of cation excess (which is salt specific) depends on the ability of the electrolyte to perturb the water network at the DNA interface: In the NaF atmosphere, the ions reduce the strength of interactions between water and the DNA more than in case of a NaCl electrolyte. Here, we explicitly take into account the solvent contribution to the chemical potential in the ion atmosphere: A decrease in the number of bound water molecules in the hydration layer of DNA is correlated with enhanced density fluctuations, which decrease the free energy cost of ion-hydration, thus promoting further ion accumulation within the DNA atmosphere. We propose that taking into account the local solvation is crucial for understanding the ion atmosphere.