Effect of buffer on heparin binding and sensing in competitive aqueous media

Synthesis, characterization, and heparin-binding study of a self-assembled p-cymene-Ru(II) metallocycle based on a 4-amino-1,8-naphthalimide Tröger's base supramolecular scaffold

We report the very first example of a self-assembled p-cymene-Ru(II) metallocycle based on a green emitting 4-amino-1,8-naphthalimide Tröger's base (TBNap) supramolecular scaffold. A new cleft-shaped TBNap-derived di-4-picolyl donor was synthesized and reacted in a 2 : 2 stoichiometry ratio with a dinuclear Ru(II) acceptor (Ru-A) to generate a [2 + 2] self-assembled metallocycle (TBNap-Ru-MC) in good yield. Both TBNap and TBNap-Ru-MC showed positive solvatochromism in different solvents with varying polarities. In addition, the binding propensity of cationic TBNap-Ru-MC toward the heparin polyanion was determined using fluorescence titration studies. The initial fluorescence emission of TBNap-Ru-MC was quenched upon the gradual addition of the heparin polyanion, and the Stern-Volmer quenching constant (K_SV) was calculated to be 3.97 × 10⁵ M^-1.

The Influence of Small Biomolecules, Salts and Buffers on the Molecular Recognition of Amide Naphthotube in Aqueous Solutions

We found the binding affinities of amide naphthotube to neutral organic molecules in water are not influenced by most of small biomolecules, inorganic salts, and PBS and Tris buffers but are reduced in HEPES buffer through competitive binding. Nevertheless, salts do change the binding affinities of amide naphthotube to charged molecules through a screening effect.

An indicator displacement assay-based optical chemosensor for heparin with a dual-readout and a reversible molecular logic gate operation based on the pyranine/methyl viologen

We have reported an optical indicator displacement assay (IDA) for heparin with a UV-vis absorbance and fluorescence dual-readout based on pyranine/methyl viologen (MV²⁺). Upon introducing heparin, pyranine/MV²⁺ shows a clearly observable increase in UV-vis absorbance and a turn-on of the fluorescence signal. We have demonstrated that the ionic nature of buffers significantly affects the pyranine displacement and the zwitterionic HEPES was most suitable for heparin sensing. After careful screening of experimental conditions, the pyranine/MV²⁺-based optical chemosensor exhibits a fast, sensitive, and selective response toward heparin. It shows dynamic linear concentration of heparin in the ranges of 0.1-40 U·mL^-1 and 0.01-20 U·mL^-1 for the absorptive and fluorescent measurements, respectively, which both cover the clinically relevant levels of heparin. As with the animal experiments, the optical chemosensor has been demonstrated to be selective and effective for heparin level qualification in rat plasma. The chemosensor is readily accessible, cost-effective, and reliable, which holds a great promise for potential application on clinical and biological studies. Furthermore, this IDA system can serve as an IMPLICATION logic gate with a reversible and switchable logical manner.

Buffer and Salt Effects in Aqueous Host-Guest Systems: Screening, Competitive Binding, or Both?

There are many open questions regarding the supramolecular properties of ions in water, a fact that has ramifications within any field of study involving buffered solutions. Indeed, as Pielak has noted (Buffers, Especially the Good Kind, Biochemistry, 2021, in press. DOI:10.1021/acs.biochem.1c00200) buffers were conceived of with little regard to their supramolecular properties. But there is a difficulty here; the mathematical models supramolecular chemists use for affinity determinations do not account for screening. As a result, there is uncertainty as to the magnitude of any screening effect and how this compares to competitive salt/buffer binding. Here we use a tetra-cation cavitand to compare halide affinities obtained using a traditional unscreened model and a screened (Debye-Hückel) model. The rule of thumb that emerges is that if ionic strength is changed by >1 order of magnitude─either during a titration or if a comparison is sought between two different buffered solutions─screening should be considered. We also build a competitive mathematical model showing that binding attenuation in buffer is largely due to competitive binding to the host by said buffer. For the system at hand, we find that the effect of competition is approximately twice that of the effect of screening (∼RT at 25 °C). Thus, for strong binders it is less important to account for screening than it is to account for competitive complexation, but for weaker binders both effects should be considered. We anticipate these results will help supramolecular chemists unravel the properties of buffers and so help guide studies of biomacromolecules.

Photobleaching of Erythrosine B in Aqueous Environment Investigation Beyond pH†

In the scientific literature, the term aqueous environment is loosely employed as it encompasses a broad range of different buffering agents. While there is an increasing number of experimental evidence that point toward specific buffer effects extending far beyond pH, the impact of the chemical nature of the buffering ions is often disregarded, especially in photochemical studies. Herein, we highlighted the importance of buffer specific effects on both the photobleaching and the singlet oxygen quantum yields of a dye in aqueous environments. For this study, we chose erythrosine B (EB) as our model photosensitizer as its photochemistry and photobleaching are well documented in the literature. We followed EB's photobleaching via absorption spectroscopy in four different aqueous solvents, including pure water, phosphate, Tris and HEPES buffer. These buffer systems were selected because they are commonly used in biochemical and biological applications. Our results show that specific buffer effects cannot be neglected. Indeed, the singlet oxygen quantum yield for EB is significantly different in HEPES compared to the other solvents. Furthermore, we showed that EB's photoproduct is highly dependent on the nature of the chemical buffer being used.

Buffer Influence on the Amino Acid Silica Interaction

Protein-surface interactions are exploited in various processes in life sciences and biotechnology. Many of such processes are performed in presence of a buffer system, which is generally believed to have an influence on the protein-surface interaction but is rarely investigated systematically. Combining experimental and theoretical methodologies, we herein demonstrate the strong influence of the buffer type on protein-surface interactions. Using state of the art chromatographic experiments, we measure the interaction between individual amino acids and silica, as a reference to understand protein-surface interactions. Among all the 20 proteinogenic amino acids studied, we found that arginine (R) and lysine (K) bind most strongly to silica, a finding validated by free energy calculations. We further measured the binding of R and K at different pH in presence of two different buffers, MOPS (3-(N-morpholino)propanesulfonic acid) and TRIS (tris(hydroxymethyl)aminomethane), and find dramatically different behavior. In presence of TRIS, the binding affinity of R/K increases with pH, whereas we observe an opposite trend for MOPS. These results can be understood using a multiscale modelling framework combining molecular dynamics simulation and Langmuir adsorption model. The modelling approach helps to optimize buffer conditions in various fields like biosensors, drug delivery or bio separation engineering prior to the experiment.

Specific Buffer Effects on the Intermolecular Interactions among Protein Molecules at Physiological pH

BSA and lysozyme molecular motion at pH 7.15 is buffer-specific. Adsorption of buffer ions on protein surfaces modulates the protein surface charge and thus protein-protein interactions. Interactions were estimated by means of the interaction parameter k_D obtained from plots of diffusion coefficients at different protein concentrations (D_app = D₀[1 + k_DC_protein]) via dynamic light scattering and nuclear magnetic resonance. The obtained results agree with recent findings confirming doubts regarding the validity of the Henderson-Hasselbalch equation, which has traditionally provided a basis for understanding pH buffers of primary importance in solution chemistry, electrochemistry, and biochemistry.

Dynamic light scattering studies of the effects of salts on the diffusivity of cationic and anionic cavitands

Although alkali halide salts play key roles in all living systems, the physical models used to describe the properties of aqueous solutions of salts do not take into account specific ion–ion interactions. To identify specific ion–ion interactions possibly contributing to the aggregation of proteins, we have used dynamic light scattering (DLS) to probe the aggregation of charged cavitands. DLS measurements of negatively charged 1 in the presence of a range of alkali metal halides reveal no significant aggregation of host 1 as a function of the nature of the cation of the added salt. Only at high concentrations could trace amounts of aggregation be detected by ¹H NMR spectroscopy. Contrarily, 1 was readily aggregated and precipitated by ZnCl₂. In contrast, although fluoride and chloride did not induce aggregation of positively charged host 2, this cavitand exhibited marked aggregation as a function of bromide and iodide concentration. Specifically, bromide induced small but significant amounts of dimerization, whilst iodide induced extreme aggregation. Moreover, in these cases aggregation of host 2 also exhibited a cationic dependence, with an observed trend Na⁺ > Li⁺ > K⁺ ≈ Cs⁺. In combination, these results reveal new details of specific ion pairings in aqueous solution and how this can influence the properties of dissolved organics.

Effect of buffer at nanoscale molecular recognition interfaces – electrostatic binding of biological polyanions

The electrostatic binding of polyanionic heparin by cationic receptors is highly dependent on the buffer in which the binding assay is carried out.