Table 1.

Relative k_obs values for the bimolecular HD2 deoxyribozyme with 25 mM l-histidine or various analogs (k_obs for l-histidine = 0.11 min⁻¹)

Cofactor	Fold discrimination
l-Histidine methyl ester	1.1
l-Histidine benzyl ester	1.3
α-Methyl-dl-histidine	24
l-Histidinamide	41
Glycylhistidine	185
l-Histidinol	357
3-Methyl-l-histidine	455
d-Histidine	714
l-β-Imidazolelactic acid	1,235
1-Methyl-l-histidine	1,250
Histamine	5,263
Imidazole	6,667
Urocanic acid	>10,000
dl-1′,2′,4′-Triazolyl-3-alanine	>10,000
β-2-Thienyl-l-alanine	>10,000
Imidazole-4-acetic acid	>10,000

In general, deoxyribozyme activity diminishes in proportion to the level of disruption of the carboxyl and amino groups of l-histidine. Samples of 1- and 3-methyl-l-histidine may contain as much as 10 μM unmodified l-histidine as a contaminant. This concentration of preferred cofactor is more than sufficient to produce the level of HD2 activity seen with these analogs.