Hypothesis: Restoration of damaged hair structure by replacing lost keratin is still of paramount interest. On account of the fact that native keratin is a highly cross-linked protein with numerous disulfide bonds but just a few nucleophilic thiol groups, binding affinity to hair is comparatively low. Hence, the design of reactive keratin derivatives bearing free sulfhydryl groups that are optionally S-protected and preactivated should enhance permanent binding to hair fibers. Experiments: Keratin was extracted from human Caucasian hair and reduced with NaBH4 to obtain free sulfhydryl groups (keratin-SH). These thiol groups were S-protected via disulfide linkage to 2-mercaptonicotinic acid (keratin-MNA). Hair fibers were either utilized in their natural form or chemically damaged by bleaching. Amount of keratin derivatives being bound to hair fibers was quantified by fluorescence detection of fluorescein isothiocyanate labeled keratins. Findings: Both modifications induced higher affinity of keratin to hair fibers, resulting in up to 1.7-fold (keratin-MNA) improved binding to natural hair and up to 3.6-fold (keratin-MNA) improved binding to bleached hair. Confocal laser microscopy confirmed the accumulation of keratin derivatives in distinct regions of the cuticle layer. Thiol functionalization seems therefore to be a promising strategy for efficient durable binding of keratin to hair. (C) 2018 Elsevier Inc. All rights reserved.