A · U and G · C base pairs in synthetic RNAS have characteristic vacuum UV CD bands

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Abstract
The vacuum UV CD spectra of GpC, GpC, GpG, poly[r(A)], poly[r(C)], poly[r(U)], poly[r(A‐U)], poly[r(G) · r(C)], poly[r(A) · r(U)], and poly[r(A‐U) · r(A‐U)] were measured down to at least 174 nm. These spectra, together with the published spectra of poly[r(G‐C) · r(G‐C)], CMP, and GMP, were sufficient to estimate the CD changes upon base pairing for four double‐stranded RNAs. The vacuum UV CD bands of poly[r(A)], poly[r(C)], and the dinucleotides GpC and CpG were temperature dependent, suggesting that they were due to intrastrand base stacking. The dinucleotide sequence isomers GpC and CpG had very different vacuum UV CD bands, indicating that the sequence can play a role in the vacuum UV CD of single‐straded RNA. The vacuum UV CD bands of the double‐stranded (G · C)‐containing RNAs, poly[r(G) · r(C)] and poly[r(G‐C) · r(G‐C)], were larger than the measured or estimated vacuum UV CD bands of their constituent single‐stranded RNAs and were similar in having an exceptionally large positive band at about 185 nm and negative bands near 176 and 209 nm. These similarities were enhanced in difference‐CD spectra, obtained by subtracting the CD spectra of the single strands from the CD spectra of the corresponding double strands. The (A · U)‐containing double‐stranded RNAs poly[r(A) · r(U)] and poly[r(A‐U) · r(A‐U)] were similar only in that their vacuum UV CD spectra had a large positive band at 177 nm. The spectrum of poly[r(A) · r(U)] had a shoulder at 188 nm and a negative band at 206 nm, whereas the specturm of poly[r(A‐U) · r(A‐U)] had a positive band at 201 nm. On the other hand, difference spectra of both of the (A · U)‐containing polymers had positive bands at about 177 and 201 nm. Thus, the difference‐CD spectra revealed CD bands characteristic of A · U and G · C base pairing. A · U base pairs induced two positive bands at about 177 and 201 nm, whereas G · C base pair induced a large positive band at 186 nm and negative bands at about 175 and 202 nm.