Heterochiral DNA and RNA heptamers, which contained an unnatural L-nucleotide, were synthesized, and thermodynamic analyses of their hybridization properties with complementary DNA and RNA strands were systematically conducted by UV melting experiments. The results clearly demonstrated that the incorporation of an L-ribonucleotide into the RNA strand leads to more significant destabilization of the duplexes than that of an L-deoxyribonucleotide into the DNA strand, regardless of whether the complementary strand is DNA or RNA. The destabilization of the duplexes by the substitution Of D-thymidine with L-thymidine in the DNA strand is entropically driven, whereas that by the substitution Of D-uridine with L-uridine in the RNA strand is enthalpically driven. The thermodynamic characteristic that the stability of homochiral duplex is far superior to that of heterochiral duplex is much more remarkable in RNA than in DNA. Thus, RNA might have been a self-replicating system superior to DNA to exclude the chiral antipode. (C) 2003 Elsevier Inc. All rights reserved.