Effects of End Group Termination on Salting-Out Constants for Triglycine

Assessing Weak Anion Binding to Small Peptides

Since Hofmeister's seminal studies in the late 19th century, it has been known that salts and buffers can drastically affect the properties of peptides and proteins. These Hofmeister effects can be conceived of in terms of three distinct phenomena/mechanisms: water-salt interactions that indirectly induce the salting-out of a protein by water sequestration by the salt, and direct salt-protein interactions that can either salt-in or salt-out the protein. Unfortunately, direct salt-protein interactions responsible for Hofmeister effects are weak and difficult to quantify. As such, they are frequently construed of as being nonspecific. Nevertheless, there has been considerable effort to better specify these interactions. Here, we use pentapeptides to demonstrate the utility of the H-dimension of nuclear magnetic resonance (NMR) spectroscopy to assess anion binding using N-H signal shifts. We qualify binding using these, demonstrating the upfield shifts induced by anion association and revealing how they are much larger than the corresponding downfield shifts induced by magnetic susceptibility and other ionic strength change effects. We also qualify binding in terms of how the pattern of signal shifts changes with point mutations. In general, we find that the observed upfield shifts are small compared with those induced by anion binding to amide-based hosts, and MD simulations suggest that this is so. Thus, charge-diffuse anions associate mostly with the nonpolar regions of the peptide rather than directly interacting with the amide N-H groups. These findings reveal the utility of 1H NMR spectroscopy for qualifying affinity to peptides─even when affinity constants are very low─and serve as a benchmark for using NMR spectroscopy to study anion binding to more complex systems.

Ion-specific binding of cations to the carboxylate and of anions to the amide of alanylalanine

Studies of ion-specific effects on oligopeptides have aided our understanding of Hofmeister effects on proteins, yet the use of different model peptides and different experimental sensitivities have led to conflicting conclusions. To resolve these controversies, we study a small model peptide, L-Alanyl-L-alanine (2Ala), carrying all fundamental chemical protein motifs: C-terminus, amide bond, and N-terminus. We elucidate the effect of GdmCl, LiCl, KCl, KI, and KSCN by combining dielectric relaxation, nuclear magnetic resonance (¹H-NMR), and (two-dimensional) infrared spectroscopy. Our dielectric results show that all ions reduce the rotational mobility of 2Ala, yet the magnitude of the reduction is larger for denaturing cations than for anions. The NMR chemical shifts of the amide group are particularly sensitive to denaturing anions, indicative of anion-amide interactions. Infrared experiments reveal that LiCl alters the spectral homogeneity and dynamics of the carboxylate, but not the amide group. Interaction of LiCl with the negatively charged pole of 2Ala, the COO^- group, can explain the marked cationic effect on dipolar rotation, while interaction of anions between the poles, at the amide, only weakly perturbs dipolar dynamics. As such, our results provide a unifying view on ions' preferential interaction sites at 2Ala and help rationalize Hofmeister effects on proteins.

Association Equilibria of Organo-Phosphoric Acids with Imines from a Combined Dielectric and Nuclear Magnetic Resonance Spectroscopy Approach

Aggregates formed between organo-phosphoric acids and imine bases in aprotic solvents are the reactive intermediates in Brønsted acid organo-catalysis. Due to the strong hydrogen-bonding interaction of the acids in solution, multiple homo- and heteroaggregates are formed with profound effects on catalytic activity. Yet, due to the similar binding motifs-hydrogen-bonds-it is challenging to experimentally quantify the abundance of these aggregates in solution. Here we demonstrate that a combination of nuclear magnetic resonance (NMR) and dielectric relaxation spectroscopy (DRS) allows for accurate speciation of these aggregates in solution. We show that only by using the observables of both experiments heteroaggregates can be discriminated with simultaneously taking homoaggregation into account. Comparison of the association of diphenyl phosphoric acid and quinaldine or phenylquinaline in chloroform, dichloromethane, or tetrahydrofuran suggests that the basicity of the base largely determines the association of one acid and one base molecule to form an ion-pair. We find the ion-pair formation constants to be highest in chloroform, slightly lower in dichloromethane and lowest in tetrahydrofuran, which indicates that the hydrogen-bonding ability of the solvent also alters ion-pairing equilibria. We find evidence for the formation of multimers, consisting of one imine base and multiple diphenyl phosphoric acid molecules for both bases in all three solvents. This subsequent association of an acid to an ion-pair is however little affected by the nature of the base or the solvent. As such our findings provide routes to enhance the overall fraction of these multimers in solution, which have been reported to open new catalytic pathways.

Specific and non-specific interactions between metal cations and zwitterionic alanine tripeptide in saline solutions reported by the symmetric carboxylate stretching and amide-II vibrations

The "specific" interaction between metal cations (Na⁺, Ca²⁺, Mg²⁺, and Zn²⁺) and the charged COO^- group, and the "non-specific" interaction between these cations and the peptide backbone of a zwitterionic trialanine (Ala3) in aqueous solutions were examined in detail, using linear infrared (IR) absorptions of the COO^- symmetric stretching and the amide-II (mainly the C-N stretching) modes as IR probes. Different IR spectral changes in peak positions and intensities of the two IR probes clearly demonstrate their sensitivities to nearby cation distributions in distance and population. Quantum chemistry calculations and molecular dynamics simulations were used to describe the cation-peptide interaction picture. These combined results suggest that Na⁺ and Ca²⁺ tend to bind to the COO^- group in the bidentate form, while Mg²⁺ and Zn²⁺ tend to bind to the COO^- group in the pseudo-bridging form. The results also show that while all three divalent cations indirectly interact with the peptide backbone with large population, Ca²⁺ and Mg²⁺ can be sometimes distributed very close to the backbone. Such a non-specific cation interaction can be moderately sensed by the C-N stretching of the amide-II mode when cations approach the polar amide C[double bond, length as m-dash]O group, and is also influenced by the NH₃⁺ charge group located at the N-terminus. The results suggest that the experimentally observed complication of the Hofmeister cation series shall be understood as a combined specific and non-specific cation-peptide interactions.

Molecular Mechanism for the Interactions of Hofmeister Cations with Macromolecules in Aqueous Solution

Ion identity and concentration influence the solubility of macromolecules. To date, substantial effort has been focused on obtaining a molecular level understanding of specific effects for anions. By contrast, the role of cations has received significantly less attention and the underlying mechanisms by which cations interact with macromolecules remain more elusive. To address this issue, the solubility of poly(N-isopropylacrylamide), a thermoresponsive polymer with an amide moiety on its side chain, was studied in aqueous solutions with a series of nine different cation chloride salts as a function of salt concentration. Phase transition temperature measurements were correlated to molecular dynamics simulations. The results showed that although all cations were on average depleted from the macromolecule/water interface, more strongly hydrated cations were able to locally accumulate around the amide oxygen. These weakly favorable interactions helped to partially offset the salting-out effect. Moreover, the cations approached the interface together with chloride counterions in solvent-shared ion pairs. Because ion pairing was concentration-dependent, the mitigation of the dominant salting-out effect became greater as the salt concentration was increased. Weakly hydrated cations showed less propensity for ion pairing and weaker affinity for the amide oxygen. As such, there was substantially less mitigation of the net salting-out effect for these ions, even at high salt concentrations.

Synergism between Specific Halide Anions and pH Effects during Nanosecond Laser Fragmentation of Ligand-Free Gold Nanoparticles

Gold nanoclusters (AuNCs) with diameters smaller than 3 nm are an emerging field of research because they possess interesting optical properties, such as photoluminescence. However, to date, it is still difficult to distinguish whether these properties originate from the cores of the nanoparticles or from the adsorbates on their surfaces. Hence, there is a high demand for ligand-free, ultra-small particles because they make it possible to study ligand and core effects separately. Pulsed laser fragmentation in liquids (LFL) is a convenient route for the synthesis of ligand-free AuNCs. The influence of physical parameters, such as melting and evaporation, on the LFL process is well understood both theoretically and experimentally. However, the impact of the chemical composition of the medium during LFL, which critically affects the particle formation process, has been less well examined. Therefore, in this work, we elucidate the extent to which the ionic strength, the pH value, and the nature of the halide anion that is present, that is, F^-, Cl^-, Br^-, or I^-, influence the particle size distribution of the LFL product and the mean yield of small particles (<3 nm) of the product. We showed that the yield of small particles can be enhanced by the synergism between pH and specific ion effects, which probably is attributable to the adsorption of specific anions. In addition, our findings indicated that anion-based stabilization depends critically on the type of anion. A direct Hofmeister effect was observed for anions in the neutral pH regime, whereas an indirect Hofmeister series was reported in alkaline solution, which probably was due to the more hydrophilic surfaces of the AuNCs that were formed.

Do soft anions promote protein denaturation through binding interactions? A case study using ribonuclease A

It has long been known that large soft anions like bromide, iodide and thiocyanate are protein denaturing agents, but their mechanism of action is still unclear. In this work we have investigated the protein denaturing properties of these anions using Ribonuclease A (RNase A) as a model protein system. Salt-induced perturbations to the protein folding free energy were determined using differential scanning calorimetry and the results demonstrate that the addition of sodium iodide and sodium thiocyanate significantly decreases the melting temperature of the protein. In order to account for this reduction in protein stability, we show that the introduction of salts that contain soft anions to the aqueous solvent perturbs the protein unfolding free energy through three mechanisms: (a) screening Coulomb interactions that exist between charged protein residues, (b) Hofmeister effects, and (c) specific anion binding to CH and CH2 moieties in the protein polypeptide backbone. Using the micellization of 1,2-hexanediol as a ruler for hydrophobicity, we have devised a practical methodology that separates the Coulomb and Hofmeister contributions of salts to the protein unfolding free energy. This allowing us to isolate the contribution of soft anion binding interactions to the unfolding process. The analysis shows that binding contributions have the largest magnitude, confirming that it is the binding of soft anions to the polypeptide backbone that is the main promoter of protein unfolding.

Specific Ion Effects on an Oligopeptide: Bidentate Binding Matters for the Guanidinium Cation

Ion-protein interactions are important for protein function, yet challenging to rationalize owing to the multitude of possible ion-protein interactions. To explore specific ion effects on protein binding sites, we investigate the interaction of different salts with the zwitterionic peptide triglycine in solution. Dielectric spectroscopy shows that salts affect the peptide's reorientational dynamics, with a more pronounced effect for denaturing cations (Li⁺ , guanidinium (Gdm⁺ )) and anions (I^- , SCN^- ) than for weakly denaturing ones (K⁺ , Cl^- ). The effects of Gdm⁺ and Li⁺ were found to be comparable. Molecular dynamics simulations confirm the enhanced binding of Gdm⁺ and Li⁺ to triglycine, yet with a different binding geometry: While Li⁺ predominantly binds to the C-terminal carboxylate group, bidentate binding to the terminus and the nearest amide is particularly important for Gdm⁺ . This bidentate binding markedly affects peptide conformation, and may help to explain the high denaturation activity of Gdm⁺ salts.

Anionic and cationic Hofmeister effects are non-additive for guanidinium salts

To understand specific ion effects on a molecular level we explore the effect of salts on the rotational mobility of a model amide using dielectric spectroscopy. Based on our previous studies on the effect of strong denaturing anions or cations, here we study the additivity of the anionic and cationic effect. Using salts consisting of denaturing spherical anions and spherical cations we find such salts to affect the amide according to what one expects based on the additive activity of the individual ions. The guanidinium (Gdm⁺) cation appears to be a notable exception, as our results suggest that GdmI (and accordingly GdmSCN) is less efficient in hindering the rotation of the amide than KI or GdmCl.

Beyond the Hofmeister Series: Ion-Specific Effects on Proteins and Their Biological Functions

Ions differ in their ability to salt out proteins from solution as expressed in the lyotropic or Hofmeister series of cations and anions. Since its first formulation in 1888, this series has been invoked in a plethora of effects, going beyond the original salting out/salting in idea to include enzyme activities and the crystallization of proteins, as well as to processes not involving proteins like ion exchange, the surface tension of electrolytes, or bubble coalescence. Although it has been clear that the Hofmeister series is intimately connected to ion hydration in homogeneous and heterogeneous environments and to ion pairing, its molecular origin has not been fully understood. This situation could have been summarized as follows: Many chemists used the Hofmeister series as a mantra to put a label on ion-specific behavior in various environments, rather than to reach a molecular level understanding and, consequently, an ability to predict a particular effect of a given salt ion on proteins in solutions. In this Feature Article we show that the cationic and anionic Hofmeister series can now be rationalized primarily in terms of specific interactions of salt ions with the backbone and charged side chain groups at the protein surface in solution. At the same time, we demonstrate the limitations of separating Hofmeister effects into independent cationic and anionic contributions due to the electroneutrality condition, as well as specific ion pairing, leading to interactions of ions of opposite polarity. Finally, we outline the route beyond Hofmeister chemistry in the direction of understanding specific roles of ions in various biological functionalities, where generic Hofmeister-type interactions can be complemented or even overruled by particular steric arrangements in various ion binding sites.

An NH moiety is not required for anion binding to amides in aqueous solution

Herein, we use a combination of thermodynamic and spectroscopic measurements to investigate the interactions of Hofmeister anions with a thermoresponsive polymer, poly(N,N-diethylacrylamide) (PDEA). This amide-based polymer does not contain an NH moiety in its chemical structure and, thus, can serve as a model to test if anions bind to amides in the absence of an NH site. The lower critical solution temperature (LCST) of PDEA was measured as a function of the concentration for 11 sodium salts in aqueous solutions, and followed a direct Hofmeister series for the ability of anions to precipitate the polymer. More strongly hydrated anions (CO3(2-), SO4(2-), S2O3(2-), H2PO4(-), F(-), and Cl(-)) linearly decreased the LCST of the polymer with increasing the salt concentration. Weakly hydrated anions (SCN(-), ClO4(-), I(-), NO3(-), and Br(-)) increased the LCST at lower salt concentrations but salted the polymer out at higher salt concentrations. Proton nuclear magnetic resonance (NMR) was used to probe the mechanism of the salting-in effect and showed apparent binding between weakly hydrated anions (SCN(-) and I(-)) and the α protons of the polymer backbone. Additional experiments performed by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy found little change in the amide I band upon the addition of salt, which is consistent with very limited, if any, interactions between the salt ions and the carbonyl moiety of the amide. These results support a molecular mechanism for ion-specific effects on proteins and model amides that does not specifically require an NH group to interact with the anions for the salting-in effect to occur.

Aqueous Cation-Amide Binding: Free Energies and IR Spectral Signatures by Ab Initio Molecular Dynamics

Understanding specific ion effects on proteins remains a considerable challenge. N-methylacetamide serves as a useful proxy for the protein backbone that can be well characterized both experimentally and theoretically. The spectroscopic signatures in the amide I band reflecting the strength of the interaction of alkali cations and alkaline earth dications with the carbonyl group remain difficult to assign and controversial to interpret. Herein, we directly compute the infrared (IR) shifts corresponding to the binding of either sodium or calcium to aqueous N-methylacetamide using ab initio molecular dynamics simulations. We show that the two cations interact with aqueous N-methylacetamide with different affinities and in different geometries. Because sodium exhibits a weak interaction with the carbonyl group, the resulting amide I band is similar to an unperturbed carbonyl group undergoing aqueous solvation. In contrast, the stronger calcium binding results in a clear IR shift with respect to N-methylacetamide in pure water.