Methods for labeling nucleotides, labeled nucleotides and useful intermediates

A method for labeling a nucleotide comprising:

• providing a label;
• providing a spacer means comprising a spacer reactive moiety at one end and an electron donating moiety at the other end of the spacer means , wherein the spacer reactive moiety is capable of coupling the spacer means to a label when the spacer reactive moiety is reacted with the label, wherein the spacer comprises a chain having at least four carbon atoms and at least one oxygen atom in the chain, and wherein the spacer comprises no more than twenty carbon atoms;
• providing a nucleotide;
• providing a linker having formula I,
• wherein X represents an aliphatic diamine, and A and B represent the same or different linker reactive moieties capable of reacting with the electron donating group of the spacer means or with the nucleotide, thereby attaching the spacer means or the nucleotide to the linker;
• reacting the spacer reactive moiety with the label, thereby coupling the spacer means to the label;
• reacting the electron donating moiety of the spacer means with one of the linker reactive moieties, thereby attaching the spacer means to the linker;
• and
• reacting the nucleotide with the other linker reactive moiety, thereby attaching the nucleotide to the linker.