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Hydration energies of protonated and sodiated thiouracils.

Wincel H - J. Am. Soc. Mass Spectrom. (2014)

Bottom Line: The water binding energies of protonated thiouracils, [2SU]H(+) and [6Me2SU]H(+), were found to be of the order of 51 kJ/mol for the first, and 46 kJ/mol for the second water molecule.For [4SU]H(+), these values are 3-4 kJ/mol lower.The structural aspects of the precursors for hydrated complexes are discussed in conjunction with available literature data.

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Affiliation: Institute of Physical Chemistry, Polish Academy of Sciences, 01-224, Warsaw, Poland, wincel@ichf.edu.pl.

ABSTRACT
Hydration reactions of protonated and sodiated thiouracils (2-thiouracil, 6-methyl-2-thiouracil, and 4-thiouracil) generated by electrospray ionization have been studied in a gas phase at 10 mbar using a pulsed ion-beam high-pressure mass spectrometer. The thermochemical data, ΔH(o)n, ΔS(o)n, and ΔG(o)n, for the hydrated systems were obtained by equilibrium measurements. The water binding energies of protonated thiouracils, [2SU]H(+) and [6Me2SU]H(+), were found to be of the order of 51 kJ/mol for the first, and 46 kJ/mol for the second water molecule. For [4SU]H(+), these values are 3-4 kJ/mol lower. For sodiated complexes, these energies are similar for all studied systems, and varied between 62 and 68 kJ/mol for the first and between 48 and 51 kJ/mol for the second water molecule. The structural aspects of the precursors for hydrated complexes are discussed in conjunction with available literature data.

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Correlation between hydration free energy, –ΔGo, at 298 K and corresponding proton affinity, PA, of uracil and thiouracils for the first (●) and second (○) water molecule. For 2SU, 4SU, and 6Me2SU, the PA values are taken from Reference [18]; for U from Reference [44]; see Table 1
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Fig3: Correlation between hydration free energy, –ΔGo, at 298 K and corresponding proton affinity, PA, of uracil and thiouracils for the first (●) and second (○) water molecule. For 2SU, 4SU, and 6Me2SU, the PA values are taken from Reference [18]; for U from Reference [44]; see Table 1

Mentions: The reaction enthalpies (–ΔHon = 1,2) for the hydration of the [2SU]H+ and [6Me2SU]H+ complexes are very similar to that of [U]H+, whereas for [4SU]H+ is slightly lower (Table 1). The free energy values (–ΔGon = 1,2 ) for the hydration of these complexes follow the order [U]H+>[2SU]H+>[4SU]H+>[6Me2SU]H+. Although the differences between these values are small and generally fall within the combined experimental uncertainty, the general trend is evident. The hydration free energy decreases as the proton affinity (PA) of the nucleobase increases (Fig. 3). This trend is consistent with the correlation between the binding energies of hydrogen-bonded complexes [B]H+…OH2, where B is an organic base, and the proton affinity difference ΔPA=PA(B) – PA(H2O) [45]. This results from the fact that when the PA of neutral B increases, the residual charge on the proton in the BH+…OH2 complex becomes smaller, resulting in a weaker electrostatic interaction of H2O with [B]H+. Such a correlation has been experimentally observed for many protonated systems [39, 45–49]. The lower values of –ΔGo2 compared with –ΔGo1 can be attributed to the charge delocalization on the binding site with the addition of the second water molecule.Figure 3


Hydration energies of protonated and sodiated thiouracils.

Wincel H - J. Am. Soc. Mass Spectrom. (2014)

Correlation between hydration free energy, –ΔGo, at 298 K and corresponding proton affinity, PA, of uracil and thiouracils for the first (●) and second (○) water molecule. For 2SU, 4SU, and 6Me2SU, the PA values are taken from Reference [18]; for U from Reference [44]; see Table 1
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4221615&req=5

Fig3: Correlation between hydration free energy, –ΔGo, at 298 K and corresponding proton affinity, PA, of uracil and thiouracils for the first (●) and second (○) water molecule. For 2SU, 4SU, and 6Me2SU, the PA values are taken from Reference [18]; for U from Reference [44]; see Table 1
Mentions: The reaction enthalpies (–ΔHon = 1,2) for the hydration of the [2SU]H+ and [6Me2SU]H+ complexes are very similar to that of [U]H+, whereas for [4SU]H+ is slightly lower (Table 1). The free energy values (–ΔGon = 1,2 ) for the hydration of these complexes follow the order [U]H+>[2SU]H+>[4SU]H+>[6Me2SU]H+. Although the differences between these values are small and generally fall within the combined experimental uncertainty, the general trend is evident. The hydration free energy decreases as the proton affinity (PA) of the nucleobase increases (Fig. 3). This trend is consistent with the correlation between the binding energies of hydrogen-bonded complexes [B]H+…OH2, where B is an organic base, and the proton affinity difference ΔPA=PA(B) – PA(H2O) [45]. This results from the fact that when the PA of neutral B increases, the residual charge on the proton in the BH+…OH2 complex becomes smaller, resulting in a weaker electrostatic interaction of H2O with [B]H+. Such a correlation has been experimentally observed for many protonated systems [39, 45–49]. The lower values of –ΔGo2 compared with –ΔGo1 can be attributed to the charge delocalization on the binding site with the addition of the second water molecule.Figure 3

Bottom Line: The water binding energies of protonated thiouracils, [2SU]H(+) and [6Me2SU]H(+), were found to be of the order of 51 kJ/mol for the first, and 46 kJ/mol for the second water molecule.For [4SU]H(+), these values are 3-4 kJ/mol lower.The structural aspects of the precursors for hydrated complexes are discussed in conjunction with available literature data.

View Article: PubMed Central - PubMed

Affiliation: Institute of Physical Chemistry, Polish Academy of Sciences, 01-224, Warsaw, Poland, wincel@ichf.edu.pl.

ABSTRACT
Hydration reactions of protonated and sodiated thiouracils (2-thiouracil, 6-methyl-2-thiouracil, and 4-thiouracil) generated by electrospray ionization have been studied in a gas phase at 10 mbar using a pulsed ion-beam high-pressure mass spectrometer. The thermochemical data, ΔH(o)n, ΔS(o)n, and ΔG(o)n, for the hydrated systems were obtained by equilibrium measurements. The water binding energies of protonated thiouracils, [2SU]H(+) and [6Me2SU]H(+), were found to be of the order of 51 kJ/mol for the first, and 46 kJ/mol for the second water molecule. For [4SU]H(+), these values are 3-4 kJ/mol lower. For sodiated complexes, these energies are similar for all studied systems, and varied between 62 and 68 kJ/mol for the first and between 48 and 51 kJ/mol for the second water molecule. The structural aspects of the precursors for hydrated complexes are discussed in conjunction with available literature data.

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