Nearest‐neighbor and next‐nearest‐neighbor effects in the proton NMR spectra of the oligoribonucleotides ApGpX and CpApX

The variable‐temperature proton nmr spectra of the oligoribonucleotides in the series CpAp X and the series ApGp X , X = A, G, C, U, together with the parent dimers CpA and ApG have been measured. A complete analysis of all the nonexchangeable base proton resonances and ribose H‐1′ proton resonances was made. The presence of trends in the shielding abilities of the various bases at both the nearest‐neighbor and next‐nearest‐neighbor positions were identified. The observed shieldings could be used to predict the chemical shifts of protons in related systems. Based on the empirical results from ribodinucleoside monophosphates, the temperature‐dependent behavior of the J 1′2′ coupling constants of the triribonucleotides suggested that the compounds in the CpAp X series stacked from the 5′‐end to the 3′‐end, while those in the ApGp X series stacked from the 3′‐end to the 5′‐end.