Table 2.

Detection of the unnatural base pair by MS

[dXTP] (µM)	Expected incorporation	Δmmu dκ	Δmmu dX
1	<1% (<10 pmol)	3.2	nd
10	10% (100 pmol)	0.2	0.5
50	25% (250 pmol)	0.3	3.9
100	35% (350 pmol)	0.6	0.2
500	>50% (>500 pmol)	0.7	1.0
MIXTURE A	<35% (<350 pmol)	1.5	0.7
MIXTURE B	<35% (<350 pmol)	0.1	2.5
MIXTURE C	<35% (<350 pmol)	0.7	0.6

dκ was detected as the [MH]⁺ ion and dX as the [BH]⁺ ion (the free base form of dX). [dXTP] refers to the amount used in the first step of the selection scheme. The experiments were performed with 1 nmol of DNAκ, and the expected amount of extended DNA was estimated from prior kinetic studies; for the mixtures, the incorporation of dXTP is expected to be significantly lower due to competition by the inactive dNTPs. Although the data were not quantified, visual inspection of the autoradiograms used to locate the products indicated that the amount of extension was similar to the predicted amount. Δmmu refers to the difference between the detected mass and the known mass in millimass units. nd, not detected.