ALFA Selector ST (for "super tight") is an innovative, high affinity resin designed for the highly efficient pull-down of ALFA-tagged target proteins. For general information about the ALFA system click here. ALFA Selector ST is based on a single-domain antibody (sdAb), which is covalently immobilized on 4% cross-linked agarose beads and binds ALFA-tagged target proteins with extraordinarily high affinity (Kd = 26 pM). Target proteins are efficiently released from ALFA Selector ST under acidic (e.g. 0.1 M glycin pH2.2) or denaturing conditions. Due to the extremely tight interaction with ALFA-tagged proteins, competitive elution using ALFA elution peptide is inefficient. For competitive elution under physiological conditions we recommend using ALFA Selector PE (Cat. No. N1510) instead. The innovative, oriented and highly selective attachment of the active sdAbs via a flexible linker guarantees an optimal accessibility for target proteins and at the same time largely eliminates batch-to-batch variations. Due to the single-chain nature of sdAbs and their covalent attachment, no "leakage" of light and heavy chains from IgGs is observed even during elution with strongly denaturing and reducing conditions (e.g. SDS sample buffer). ALFA Selector ST thus features high affinity and superior capacity for ALFA fusion proteins while showing negligible non-specific background. ALFA Selector ST is compatible not only with physiological buffers but also with high stringency buffers (see Product Data Sheet). ALFA Selector ST thus provides great freedom to adjust the binding and washing conditions to the experimental needs.