Hydrazide modified oligonucleotides are able to react with aldehydes under neutral or slightly acidic conditions to form stable hydrazone conjugates in a fast and efficient way. Thus, a conjugation of oligonucleotides to other (bio)molecules can be achieved easily under physiological conditions. Contrary to standard amino functions (e.g. 5‘-Aminolink), hydrazides react with N-hydroxy succinimidyl esters as well at neutral pH values. Especially for reactions of molecules, which are labile in alkaline solutions, this strategy may be interesting.
The production of hydrazide modified oligonucleotides can be achieved during automated oligo synthesis by coupling a protected hydrazide phosphoramidite to the 5‘-end of the growing oligonucleotide chain.
Altenatively, oligonucleotides can be synthesized with 5‘-aldehydes which react with appropriate hydrazides (dyes, biomolecules, surfaces).

Lit:
1. Raddat, S., Mueller-Ibeler, J., Kluge, J., Wäß, L., Burdinski, G., Havens, J.R., Onofrey, Tom J., Wang, D. & Schweitzer, M., Hydrazide oligonucleotides: new chemical modification for chip array attachment and conjugation, Nucleic Acids Research, 2002, Vol. 30, 4793-4802.
2. Antsypovich, S. I.; Oretskaya, T. S.; von Kiedrowski, G., New phosphoramidite derivatives for the preparation of oligonucleotides containing a hydrazide group in any specified position of the oligonucleotide chain, Rus. Chem. Bull. Int. Ed. 2005, 54, 2671-2681.