An Assessment of the Use of Mesoporous Silica Materials to Improve Pulsed Dipolar Spectroscopy

Abstract

Protein immobilization in mesoporous silica nanoparticles has attracted much attention due to its wide range of applications. However, it remains largely unexplored how the use of mesopores can alter the spatial distribution of encapsulated biomolecules so as to improve pulsed dipolar spectroscopy sensitivity. Here, we performed electron spin resonance measurements for three different spin-labeled biomolecules (including two different peptides and a protein) encapsulated in various types of mesoporous materials differing in textural properties such as nanochannel length (e.g., 0.2–4 μm) and average pore diameter (e.g., 6–11 nm, approximately). Our results show that biomolecules are clustered somewhat upon the encapsulation into mesopores, and that due to the clustering, instantaneous diffusion plays an important role in the spin relaxation in nanochannels. The extent of molecular clustering exhibits a clear positive correlation with the length of nanochannels, whereas it shows little correlation with pore diameters. Among the materials studied, mesoporous materials with the shortest length of nanochannels are most effective to reduce spin clustering, thus suppressing the unwanted instantaneous diffusion and enhancing spin–spin relaxation time. This study has opened a possibility of improving the quality of pulsed dipolar spectroscopy with mesoporous silica nanoparticles.