Table 3. Distribution of the Number of Tautomers Predicted Per Molecule by Our Ring–Chain Rules, CACTVS Prototropic Rules, and Combined Application of Both Sets of Rules to the AMS (Aldrich Market Select) and CSDB (Chemical Structure DataBase) Databases (See Text)^a.

	ring–chain tautomerism, 11 rules, AMS		prototropic tautomerism, 21 rules, AMS		prototropic tautomerism, 21 rules, CSDB		both types of tautomerism, 32 rules, AMS
	count	%	count	%	count	%	count	%
no tautomers (single molecule)	5 297 864	92.05	1 393 612	24.21	9 756 186	14.06	1 364 937	23.72
one tautomer	101 890	22.26	1 235 979	28.34	10 721 845	17.99	1 093 477	24.90
2–10 tautomers	304 245	66.47	2 428 781	55.68	33 532 284	56.25	2 271 956	51.75
11–50 tautomers	37 267	8.14	584 842	13.41	13 492 899	22.63	722 423	16.45
51–100 tautomers	3905	0.85	72 832	1.67	1 136 066	1.91	136 558	3.11
101–200 tautomers	7078	1.55	35 901	0.82	565 199	0.95	151 384	3.45
201–500 tautomers	3017	0.66	3486	0.08	157 260	0.26	14 734	0.34
501–1000 tautomers	308	0.07	141	0.00	6088	0.01	105	0.00

^aThe reduction of the number of AMS molecules with 501–1000 tautomers when going from 21 to 32 rules stems from the fact that we cut off tautomer generation at 1000 generated tautomers (see text), and more rules pushed more molecules beyond this limit.