This depends on the nature of the specific ligand to be immobilized and the desired downstream application.
-The most frequently used surface-activated Dynabeads magnetic beads for protein isolation are the Tosyl-activated Dynabeads magnetic beads M-280. These beads are hydrophobic, easy to handle and ideal for covalent binding of antibodies for immunoprecipitation. Other ligands could also be covalently bound to these beads, but they have to tolerate conditions like neutral to high pH and high temperatures (required for covalent bond formation).
-Dynabeads magnetic beads M-270 Epoxy are used when the ligand to be immobilized needs to be treated gently and will not tolerate harsh binding conditions like high temperature or pH. Proteins/peptides (other than antibodies) and enzymes are often coupled onto these beads.
-Dynabeads magnetic beads M-270 Amine is often used in combination with crosslinkers to create specific surface groups on the beads. Hetro-bifunctional crosslinkers with an amine-reactive NHS group at one end and another chemical group of your choice at the other end are most frequently used. For example, Dynabeads magnetic beads M-270 Amine can be reacted with a hetero-bifunctional crosslinker containing a NHS group at one end and maleimide at the other end to create Dynabeads magnetic beads with a maleimide surface. Since maleimide reacts specifically with sulfhydryl groups, these modified Dynabeads magnetic beads can be used for applications where binding of sulfhydryl groups are desired. Dynabeads magnetic beads M-270 Amine may also be used for direct ligand coupling via aldehyde or ketone groups by Schiff base (imine) formation and reductive amination. In addition carboxylic acid groups on a ligand can be activated with a carbodiimide like EDC, which results in a direct amide bond formation between the beads and the ligand. Alternatively, a crosslinker may be introduced to the ligand. After activation of the ligand with crosslinker, the free end on the crosslinker has to be amine reactive.
-Dynabeads magnetic beads M-270 Carboxylic Acid and Dynabeads magnetic beads MyOne Carboxylic Acid can also be used for immobilizing proteins. The carboxylic acid groups on these beads need to be activated with a carbodiimide before coupling. The negatively-charged surface of these beads may attract positively charged proteins and cause nonspecific binding. This needs to be considered if these beads are going to be used for immunoprecipitation.
Find additional tips, troubleshooting help, and resources within our Protein Assays and Analysis Support Center.