Probing driving forces for binding between nanoparticles and amino acids by saturation-transfer difference NMR

Abstract

As nanotechnology becomes increasingly used in biomedicine, it is important to have techniques by which to examine the structure and dynamics of biologically-relevant molecules on the surface of engineered nanoparticles. Previous work has shown that Saturation-Transfer Difference (STD)-NMR can be used to explore the interaction between small molecules, including amino acids, and the surface of polystyrene nanoparticles. Here we use STD-NMR to further explore the different driving forces that are responsible for these interactions. Electrostatic effects are probed by using zwitterionic polystyrene beads and performing STD-NMR experiments at high, low, and neutral pH, as well as by varying the salt concentration and observing the effect on the STD buildup curve. The influence of dispersion interactions on ligand-nanoparticle binding is also explored, by establishing a structureactivity relationship for binding using a series of unnatural amino acids with different lengths of hydrophobic side chains. these results will be useful for predicting which residues in a peptide are responsible for binding and for understanding the driving forces for binding between peptides and nanoparticles in future studies.

Solution NMR methods for structural and thermodynamic investigation of nanoparticle adsorption equilibria

“…For example, STD NMR has been used to screen libraries of small molecules against NP targets, 184,185 to determine the affinities of NP-ligand adducts, 184,186 and to investigate the forces driving small molecule adsorption. 187 It is important to note that when using the STD NMR for NPs, the concentration of the receptor (noted as [R] in eqn (25)) should be the total concentration of binding sites. As discussed above, when introducing the Langmuir model, this quantity is not easily measurable for NP receptors and introduces ambiguities in the interpretation of the experimental data.…”

Analytical Methods for Characterization of Nanomaterial Surfaces

“…NMR can be used to characterize surface ligands in several ways: (1) to confirm successful modification of nanomaterials by ligands, 19 (2) to study ligand structure, 20−22 (3) to differentiate between bound and unbound ligands, 23 (4) to quantify bound ligands, 23−25 (5) to understand the ligand binding mode and dynamics of the bound ligands, 26,27 and (6) to study the interaction of surface-functionalized nanomaterials with biomolecules. 28,29 Here, we focus on examples of solution phase NMR analysis, 30 although solid-state NMR has also been used to characterize nanomaterial surfaces. 31,32 Advanced 2D-NMR techniques such as diffusion ordered spectroscopy (DOSY), nuclear Overhauser effect spectroscopy (NOESY), total correlated spectroscopy (TOCSY), heteronuclear single-quantum correlation spectroscopy (HSQC), and rotating frame nuclear Overhauser effect spectroscopy (ROESY) are particularly useful to obtain additional information on nanomaterial surfaces.…”

“…Other NMR techniques such as 13 C, 19 F, 29 Si, and 31 P NMR are also used to characterize surface ligands. Carbons that are closest to the nanoparticle surface have broad peaks due the wide distribution of chemical environments within the sample.…”

“…NMR can be used to characterize surface ligands in several ways: (1) to confirm successful modification of nanomaterials by ligands, (2) to study ligand structure, − (3) to differentiate between bound and unbound ligands, (4) to quantify bound ligands, − (5) to understand the ligand binding mode and dynamics of the bound ligands, , and (6) to study the interaction of surface-functionalized nanomaterials with biomolecules. , Here, we focus on examples of solution phase NMR analysis, although solid-state NMR has also been used to characterize nanomaterial surfaces. , …”

Dual Fluorescence and NMR Study for the Interaction between Xanthene Dyes and Nanoparticles

“…In recent years, various nuclear magnetic resonance (NMR) spectroscopy experiments have been developed to gain insight into structural information regarding small molecules and proteins on the surface of NPs. − NMR experiments are nondestructive and can provide atomic-level resolution. These experiments include measuring diffusion coefficients , of free and bound ligands using either pulsed-field gradient or diffusion-ordered spectroscopy (DOSY) experiments, exploiting the different relaxation times in free and bound ligands, − ,− methods that exploit different rotational correlation times between the small ligand and large receptor such as NOESY and waterLOGSY, , and methods that rely on saturation transfer such as saturation-transfer difference (STD)-NMR and dark-state exchange saturation transfer (DEST). ,, STD-NMR , is particularly effective to probe ligand binding on an NP surface. − In each STD-NMR experiment, two spectra, one on-resonance and one off-resonance spectrum, are collected. The difference between these two spectra is the location of the saturation frequency.…”