l)ethyl modifications exhibited intriguing properties such as the change in the structure of the tetrazole residue between a protonated and a deprotonated form. The Tm experiments of various oligodeoxynucleotides having a 2′-O-(1H-tetrazol-5-yl)ethyl-modified adenosine showed reduced hybridization affinity in comparison to the unmodified oligonucleotides toward their complementary oligodeoxynucleotides