Combinatorial libraries of nucleic acids are developing into novel sources for lead compounds in drug development. In order to diversify the pool of ss DNA sequences, we have used a modified nucleotide, 5-(1-pentynyl)-2′-deoxyuridine, in place of thymidine in a random nucleic acid library and screened this library against human thrombin. Previously, we described this screening method to identify a novel structural inhibitor an aptamer) of the coagulation protease thrombin Bock, L. et. al.(1992) Nature 355 564 – 566). Using the modified nucleic acid library, we have now isolated a second pool of thrombin inhibitors with strikingly different sequence composition compared to the selection using natural bases. This second class of aptamers is dependent on the presence of the modified nucleotide for protein binding and clotting inhibition. Our method represents a potential strategy to enhance the diversity of libraries for in vitro selection, and thereby increasing the utility of this technique in the identification of molecules with novel biochemical properties.

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