Biotinylated compounds

A compound of formula I or formula II:

• wherein:
• each B is independently
• X is (C2-C8)alkyl, (C2-C8)alkenyl or (C2-C8)alkynyl, wherein one or more carbon atoms of the (C2-C8)alkyl, (C2-C8)alkenyl or (C2-C8)alkynyl may be optionally replaced with one or more groups independently selected from O, S, C(═O) and NRa provided that the O, S or NRa are not adjacent to any other O, S or NRa, and wherein any (C2-C8)alkyl, (C2-C8)alkenyl or (C2-C8)alkynyl of X is optionally substituted with one or more groups independently selected from halogen, ORb and NRb2;
• Y is (C2-C28)alkyl, (C2-C28)alkenyl or (C2-C28)alkynyl, wherein one or more carbon atoms of the (C2-C28)alkyl, (C2-C28)alkenyl or (C2-C28)alkynyl may be optionally replaced with one or more groups independently selected from O, S, C(═O) and NRc provided that the O, S or NRc are not adjacent to any other O, S or NRc, and wherein any (C2-C28)alkyl, (C2-C28)alkenyl or (C2-C28)alkynyl of Y is optionally substituted with one or more groups independently selected from halogen, ORd and NRd2;
• W is O or S;
• each Ra is independently selected from H and (C1-C6)alkyl;
• each Rb is independently selected from H and (C1-C6)alkyl;
• each Rc is independently selected from H and (C1-C6)alkyl;
• and
• each Rd is independently selected from H and (C1-C6)alkyl;
• or a salt thereof.