(A) The La antigen contains cysteine residues (C232, C245) sensitive to electrophiles. When covalently modified, these residues perturb nearby 13C-labeled leucine residues (L249, L294, L296). PDB ID: 1OWX (Jacks et al., 2003). (B) Portions of the [1H,13C]-HMQC spectra of the La antigen reporter protein in the presence and absence of DTT. Test compounds are incubated with the La antigen reporter protein in physiological buffer and compared to vehicle control (DMSO). With excess DTT present, thiol-reactive compounds (positive control, PC) are scavenged by DTT (blue contours); with DTT absent, thiol-reactive compounds can covalently modify cysteine thiols (red contours) which manifests as chemical shifts and/or attenuation of peak intensity. Z-axis, relative units.