Table 3.

Comparison of the Cleavage Rates of the Modified DNA/RNA Ribonucleotide and Tethered Substrates in the Presence of the Ribozyme (pH 5) or Absence of the Ribozyme (pH 14)

	pKa	base- catalyzeda	ribozyme- catalyzed
ribonucleotide, WT	13.5 ± 0.1b	6.9 × 10−2	1.7 × 10−1
ribonucleotide, abasic	14.5 ± 0.1	2.0 × 10−1	4.4 × 10−2
R= CF3	13.5 ± 0.1	3.7 × 10−4	1.3 × 10−3
R = CH3	14.5 ± 0.2	1.1 × 10−4	4.9 × 10−4
abasic/CH3c		540	90
WT/CF3c		627	133

^aRates extrapolated from the best-fit curve to pH 5.

^bBase-catalyzed reactions measured with N₋₁ = U and ribozyme-catalyzed reaction observed with N₋₁ = C. A previous investigation suggested that the nucleophile pK_a is insensitive to the identity of the N₋₁ nucleobase.³¹

^cRatios of rates given for sterically distinct substrates with similar nucleophile pK_a’s.