The proton signals of a 50 mM solution of thymidylyl-(3'-5')-thymidylyl-(3'-5')-2'-deoxyadenosine, d(T-T-A), in 2H2O at 65 degrees C and p2H=7.4, were detected by nuclear magnetic resonance at 360 MHz. Complete assignment of the 21 ribose proton signals was achieved by extensive decoupling experiments and computer simulations. The coupling constant data show that the ribose rings prefer to adopt the S (2'-endo) conformation. The presence of a substantial amount of a right handed helical structure of the d(-T-A) fragment is indicated from chemical shift considerations. The molecule displays a high degree of conformational purity along the sugar-phosphate backbone.