Tunneling dynamics of side chains and defects in proteins, polymer glasses, and OH-doped network glasses

TL;DR

This paper investigates how defects and side chains influence tunneling dynamics in glasses and proteins, revealing that extrinsic double-well potentials significantly alter low-temperature properties and can explain experimental observations.

Contribution

It extends the Standard Tunneling Model to include defect-induced extrinsic DWP's, providing a unified framework for understanding low-temperature glass and protein dynamics.

Findings

Defects create extrinsic DWP's with high barriers and localized dynamics.

OH-impurities in network glasses are accurately modeled by the extended STM.

Side groups in polymers and proteins induce extrinsic DWP's, explaining experimental anomalies.

Abstract

Simulations on a Lennard-Jones computer glass are performed to study effects arising from defects in glasses at low temperatures. The numerical analysis reveals that already a low concentration of defects may dramatically change the low temperature properties by giving rise to extrinsic double-well potentials (DWP's). The main characteristics of these extrinsic DWP's are (i) high barrier heights, (ii) high probability that a defect is indeed connected with an extrinsic DWP, (iii) highly localized dynamics around this defect, and (iv) smaller deformation potential coupling to phonons. Designing an extension of the Standard Tunneling Model (STM) which parametrizes this picture and comparing with ultrasound experiments on the wet network glass $a$-B$_2$O$_3$ shows that effects of OH-impurities are accurately accounted for. This model is then applied to organic polymer glasses and proteins. It is suggested that side groups may act similarly like doped impurities inasmuch as extrinsic DWP's are induced, which possess a distribution of barriers peaked around a high barrier height. This compares with the structurlessly distributed barrier heights of the intrinsic DWP's, which are associated with the backbone dynamics. It is shown that this picture is consistent with elastic measurements on polymers, and can explain anomalous nonlogarithmic line broadening recently observed in hole burning experiments in PMMA.