Biotin is a water-soluble B-complex vitamin (vitamin B7 or vitamin H)
It binds very tightly (irreversibly) to Streptavidin and Strep-Tactin® and with very high affinity (but reversible) to Strep-Tactin®XT. Therefore biotin allows the efficient elution of Strep-tagged proteins from Strep-Tactin®XT, MagStrep "type3" XT beads. Biotin binds irreversible to Strep-Tactin®, therefore the resin cannot be regenerated. Strep-Tactin®XT can be regenerated after elution with biotin by addition of Buffer XT-R.
How to prepare a 50 mM biotin solution for elution? https://www.iba-lifesciences.com/tips-and-tricks.html