. Author manuscript; available in PMC: 2017 Feb 1.

Published in final edited form as: J Pharm Sci. 2016 Jan 29;105(2):664–672. doi: 10.1002/jps.24539

Table 1.

K_H and pK_a values ± standard error (SE) for 1–4 at 25°C, ionic strength 0.15.

Compound	pK_a^oxo± SE ^a	pK_a^hyd± SE ^a	K_H^oxo± SE ^a	K_H^ion± SE ^a	pK_a^oxo ± SE ^b	pK_a^hyd± SE ^b	pK_a^oxo± SE ^c	pK_a^hyd± SE ^c
1	1.79 ± 0.06	3.23 ± 0.04	2.353 ± 0.000	0.087 ± 0.002	1.70 ± 0.09	3.27 ± 0.04	1.71 ± 0.04 ^d 1.72 ± 0.04 ^e 1.64 ± 0.02 ^d, ^f 1.65 ± 0.04 ^e, ^f	3.25 ± 0.03 ^d 3.29 ± 0.02 ^e 3.24 ± 0.03 ^d, ^f 3.26 ± 0.03 ^e, ^f
2	1.60 ± 0.09	3.29 ± 0.06	1.786 ± 0.183	0.037 ± 0.002	1.39 ± 0.06	3.12 ± 0.10	1.37 ± 0.02 ^d 1.38 ± 0.02 ^e	3.27 ± 0.01 ^d 3.19 ± 0.05 ^e
3	1.68 ± 0.05	3.35 ± 0.04	0.285 ± 0.011	0.006 ± 0.002	1.70 ± 0.07 ^g 1.68 ± 0.04 ^g	3.28 ± 0.04 ^g 3.36 ± 0.11 ^g	1.59 ± 0.08^d 1.59 ± 0.02 ^e	3.30 ± 0.07 ^d 3.26 ± 0.08 ^e
4	ND	ND	ND	ND	ND	ND	1.82 ± 0.13 ^d 1.82 ± 0.15 ^e	ND

^(a)

The equilibrium constant of hydration K_H^oxo, K_H^ion and dissociation K_a^oxo and K_a^hyd were determined from measurements of the degrees of oxo and hydrated form at different pH values by ¹H-NMR. Equilibrium constants were calculated using Eqs 1, 7 and 8, as described above

^(b)

Determined by the observed chemical shift (δ_obs) of the signals arising from methyl (1), methine (2) and methylene (3) associated with the hydrated and oxo forms, using ¹H-NMR. The pK_a values were calculated using eq. 14, as described above

^(c)

Determined by ¹³C-NMR, by δ_obs of the signals arising from carbon in position 1

^(d)

and position 2

^(e)

associated with the hydrated and oxo forms. The pK_a values were calculated using eq. 14, as described above

^(f)

Determined in pure water using methanol as external reference

^(g)

Signal for methylene hydrogens is split into a doublet

(ND) not determined