SH—It happens: S–H bonds as intrinsic 2D-IR labels in proteins

Abstract

Cysteine S–H bonds have a spectroscopically convenient stretching frequency of ∼2550 cm−1. However, their cross section is low, and the band can be strongly broadened in heterogeneous environments, making detection very challenging. With two-dimensional infrared (2D-IR) setups achieving ever higher sensitivities in recent years, systematic use of the weak cysteine sulfhydryls (Cys–SHs) absorption band is now within reach, even at low millimolar protein concentrations. Here, we demonstrate the capabilities of Cys–SH as an intrinsic 2D-IR label in pyruvate oxidase from E. coli, an enzyme with ten cysteines in its native sequence. 1D-IR measurements on the wild-type and individual cysteine knock-out variants show that two such residues have especially narrow SH signatures, caused by their intrahelical hydrogen bonding. 2D-IR analysis of these bands reveals an extraordinarily high anharmonicity (∼110 cm−1) and a long vibrational lifetime (∼4 ps). This allows monitoring spectral diffusion via center line slope analysis for up to 10 ps—separately for both the ground and excited states. The unique spectroscopic features and its ease of introduction make Cys–SH a useful IR spectroscopic label.