SCN as a Local Probe of Protein Structural Dynamics

TL;DR

This study uses -SCN labels attached to alanine residues in lysozyme to investigate local protein dynamics through infrared spectroscopy, revealing structural changes, environmental interactions, and hydration dynamics.

Contribution

It demonstrates the potential of -SCN as a vibrational probe for detailed analysis of protein structural dynamics in both high- and low-frequency infrared spectra.

Findings

-SCN labels show consistent frequency shifts indicating environmental interactions.

Labeling at specific residues causes local structural changes.

Hydration and reorientation dynamics of the label are observable.

Abstract

The dynamics of lysozyme is probed by attaching -SCN to all alanine-residues. The 1-dimensional infrared spectra exhibit frequency shifts in the position of the maximum absorption by 4 cm$^{-1}$ which is consistent with experiments in different solvents and indicates moderately strong interactions of the vibrational probe with its environment. Isotopic substitution $^{12}$C $\rightarrow ^{13}$C leads to a red-shift by $-47$ cm$^{-1}$ which is consistent with experiments with results on CN-substituted copper complexes in solution. The low-frequency, far-infrared part of the protein spectra contain label-specific information in the difference spectra when compared with the wild type protein. Depending on the positioning of the labels, local structural changes are observed. For example, introducing the -SCN label at Ala129 leads to breaking of the $\alpha-$helical structure with concomitant change in the far-infrared spectrum. Finally, changes in the local hydration of SCN-labelled Alanine residues as a function of time can be related to angular reorientation of the label. It is concluded that -SCN is potentially useful for probing protein dynamics, both in the high-frequency (CN-stretch) and far-infrared part of the spectrum.