Electrostatic Influence on Rotational Mobilities of Sol−Gel-Encapsulated Solutes by NMR and EPR Spectroscopies

Abstract

The rotational mobilities of small solute molecules encapsulated in tetramethyl orthosilicate (TMOS) sol-gels have been investigated by EPR spectroscopy of encapsulated nitroxide probes and by high-resolution NMR spectroscopic measurements of transferred NOE's (trNOE's), of T(1)'s, and of T(1)'s in the rotating frame (T(1)rho). The two spectroscopic methods are sensitive to motions on different time scales and hence, are nicely complementary. Suites of neutral, positively, and negatively charged nitroxide probes (EPR) and of simple diamagnetic small molecules (NMR) were selected to disclose influences of electrostatic interactions with the sol-gel walls and to probe the presence of multiple populations of molecules in distinct regions of the sol-gel pores. For neutral and negatively charged solute probes, both techniques disclose a single population with a significantly increased average rotational correlation time, which we interpret at least in part as resulting from exchange between free-volume and transiently immobilized surface populations. The electrostatic attraction between cationic probes and the negatively charged sol-gel walls causes the positively charged probes to be more effectively immobilized and/or causes a greater percentage of probes to undergo this transient immobilization. The EPR spectra directly disclose a population of cationic probes which are immobilized on the X-band EPR time scale: tau(c) greater than or approximately equal 10(-7) s. However, NMR measurements of trNOE's and of T(1)rho demonstrate that this population does exchange with the free-volume probes on the slower time scale of NMR. This approach is equally applicable to the study of solutes within other types of confined spaces, as well.

Comparative Study of Guest Charge−Charge Interactions within Silica Sol−Gel

“…A large difference in these two opposing processes leads to predominantly solvated or adsorbed molecules. Although hydrophobic interactions are often assumed to be the sole force controlling adsorption behavior, other forces that contribute to adsorption include the electrical double layer, , electrostatic interactions, , dispersion forces, , and hydrogen bonding. , Some of the representative methods used to investigate the extent to which these forces contribute to dye adsorption include using NMR, − varying the host composition and structure, ,,,− including additives, ,, varying the guest molecule identity, modifying the silicate surfaces, , and altering the solvents in contact with the sample. ,, …”

“…It is worth noting that, although time-resolved fluorescence anisotropy examines pico- to nanosecond sample responses, − electron paramagnetic resonance and NMR examine micro- to millisecond responses; , our classification scheme follows changes in fluorescence polarization occurring at time scales that range from hundreds of milliseconds to seconds and even minutes. Our ability to determine molecular mobility at the single-molecule level also helps eliminate the influence of guest−host interactions on fluorescence quantum yield that, if not corrected for, will inevitably skew an ensemble-measured mobility distribution in favor of a population with stronger fluorescence.…”

Entrapment and characterization of functional allosteric conformers of hemocyanin in sol–gel matrices

“…Electrostatic interactions between the embedded protein and the sol-gel matrix were described as pivotal for blocking a given protein conformation. 5,31 The role of net-charge with respect to the entrapment properties of a solgel matrix was also previously reported, 46 showing that positively charged molecules are more strongly hindered in their motion. Thus, one would expect that screening of charges leads to less effective encapsulation.…”

Biodoped Ceramics: Synthesis, Properties, and Applications

“…The electrostatic attraction between the cationic solute molecule and the silica wall restricts rotational mobility to a greater extent. X‐band EPR spectrum of entrapped cationic solute molecules showed that a fraction of it was completely immobilized due to attractive electrostatic forces 74 . Although these studies were conducted on solute molecules, observations may very well be applied to positively charged proteins and negatively charged silica walls at pH 7.…”