Modified nucleotides methods and kits

A compound having the structure (II):

• or a salt, conjugate base, tautomer, or ionized form thereof, where
• Base* is a purine or pyrimidine base;
• R is H, OH, CH3, or a hydroxyl protecting group;
• Alk is a connecting group having the structure -//—(CH2)m—Y—//- wherein m is an integer ranging from 3 to 6 inclusive, Y is a bond or bond forming group selected from
• and
• the leftmost bond is to Base* and the rightmost bond is to Lnk;
• Lnk is a linking group having the structure
• where n is an integer ranging from 2 to 48 inclusive,
• A1 is a bond forming group selected from
• A2 is a bond forming group selected from
• A3 when present is a bond forming group selected from
• X is a cleavable group that can undergo silicon-carbon cleavage, nucleophilic cleavage, redox cleavage, photochemical cleavage, enzymatic cleavage, or exchange-based cleavage;
• Z is a branching group that contains a modifying molecule (Mod);
• and
• the leftmost bond is to Alk and the rightmost bond is to Obs;
• and
• Obs is an observable label moiety.