Structure and Stability of Chemically Modified DNA Bases: Quantum Chemical Calculations on 16 Isomers of Diphosphocytosine

Table 2

Thermodynamic parameters (kcal/mol)^a for diphosphocytosine tautomers/rotamers at different levels in gas phase.

Species	B3LYP				M06-2X				MP2				MP4(SDQ)^b
	Δ𝐸0	Δ𝐸0^b	Δ𝐻298	Δ𝐺298	Δ𝐸0	Δ𝐸0^b	Δ𝐻298	Δ𝐺298	Δ𝐸0	Δ𝐸0^b	Δ𝐻298	Δ𝐺298	Δ𝐸0
PC1	3.70	7.01	5.78	4.66	6.84	10.26	8.99	7.83	9.90	13.27	11.61	11.38	7.19
PC2	0.00	0.62	0.50	0.46	0.00	0.72	0.58	0.50	0.40	0.42	0.42	0.38	0.46
PC3	0.47	0.00	0.00	0.00	0.40	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
PC4	24.73	28.43	26.96	25.99	25.98	29.52	28.11	27.05	27.46	31.80	30.46	30.01	24.82
PC5	24.55	28.53	26.87	25.63	26.02	29.68	28.21	27.01	28.17	32.75	26.78	26.68	25.51
PC6	22.28	25.73	24.30	23.46	23.58	26.75	25.49	24.67	24.72	28.90	26.94	26.91	22.23
PC7	22.11	25.64	24.15	23.54	23.14	26.49	25.13	24.44	24.87	29.09	26.94	0.04	22.17
PC8	9.97	16.68	13.65	12.64	12.76	19.53	16.50	15.40	15.91	23.12	19.50	0.03	13.96
PC9	12.00	19.21	15.95	14.82	14.16	21.46	18.21	16.73	17.82	25.48	21.66	21.11	15.68
PC10	5.37	8.90	7.56	6.44	8.83	12.65	11.21	9.95	11.78	15.27	13.56	13.24	9.63
PC11	0.92	0.48	0.50	0.46	0.967	0.56	0.58	0.50	0.40	0.42	0.42	0.38	0.46
PC12	0.47	0.00	0.00	0.00	0.400	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
PC13	23.47	27.01	25.57	24.70	24.31	27.87	26.48	25.13	26.13	30.61	28.41	27.86	23.42
PC14	25.18	29.14	27.50	26.56	25.96	29.81	28.25	27.26	27.68	32.49	30.12	0.00	25.17
PC15	20.56	23.71	22.39	21.86	21.26	24.50	23.17	22.55	22.98	26.92	24.90	0.00	20.05
PC16	21.76	25.23	23.75	23.23	22.43	25.76	24.39	23.81	23.96	28.21	26.04	25.99	21.24

aCorrected for zero-point energy and thermal energy; ^buncorrected energy.