Thermochemical Properties of Xanthine and Hypoxanthine Revisited

Article pubs.acs.org/jced

Thermochemical Properties of Xanthine and Hypoxanthine Revisited Vladimir N. Emel’yanenko,*,†,‡ Dzmitry H. Zaitsau,† and Sergey P. Verevkin*,† †

Department of Physical Chemistry and Department of “Science and Technology of Life, Light and Matter”, University of Rostock, Dr.-Lorenz-Weg 2, 18059, Rostock, Germany ‡ Department of Physical Chemistry, Kazan Federal University, Kremlevskaya Straße 18, 420008 Kazan, Russia S Supporting Information *

ABSTRACT: The standard molar enthalpies of formation of xanthine and hypoxanthine were measured by using highprecision combustion calorimetry. The standard molar enthalpies of sublimation of these compounds at 298.15 K were derived by the quartz-crystal microbalance technique. Limited thermodynamic data available in the literature are compared with our new experimental data. In addition, we use the G4 method to calculate the molar enthalpies of formation of xanthine and hypoxanthine in the gas phase. There is good agreement between the evaluated experimental data and the quantum-chemical calculations.

1. INTRODUCTION Xanthine (Figure 1a,b) and hypoxanthine (Figure 1c,d) are two well-known purine-based nucleotides, which are important

cytosine, thimine, and uracil. A good agreement between experimental and theoretical (G3 and G4 methods) values for gas-phase enthalpies of formation was achieved. In the course of these studies we have noticed, that combustion experiments were performed on samples of xanthine and hypoxanthine with ill-defined purity.1 In the present work, enthalpies of combustion and formation of highly pure crystalline samples of xanthine and hypoxanthine were determined by using a highprecision combustion calorimeter. Moreover, enthalpies of sublimation of both xanthine and hypoxanthine were determined by using QCM method. These new experimental data have been combined to derive the gas-phase enthalpies of formation. The composite G4 method was used for mutual validation of the experimental and theoretical results in order to ascertain ability of the G4 method toward biologically relevant compounds.

Figure 1. Molecules studied in this work: xanthine keto- and enol forms (1a and 1b) and hypoxanthine keto- and enolforms (1c and 1d).

2. EXPERIMENTAL SECTION 2.1. Materials. Purity and provenance of commercial samples of xanthine and hypoxanthine are given in Table S1. The samples were additionally purified by a fractional sublimation under reduced pressure. A gas chromatograph equipped with a flame ionization detector and a capillary column HP-1 was used for determining the final degree of samples purity. No impurities were detected in the samples used for combustion experiments and transpiration measurements. The Karl Fischer titrations of both materials have shown no discernible amounts of water.

sources of energy that drive most metabolic reactions. The last experimental thermodynamic study of both purine bases (including analysis of available before 1935 data) was conducted by Stiehler and Huffman.1,2 They derived standard molar enthalpies of formation by using combustion calorimetry1 and performed heat capacity measurements (down to 85 K) by using the adiabatic calorimetry.2 The standard molar sublimation enthalpy of hypoxanthine was determined with the quartz-crystal microbalance (QCM) technique.3 No data on the sublimation enthalpy of xanthine have been found in the literature. Recently, we reported4,5 a series of thermochemical and quantum-chemical studies of primary nucleobases: adenine, © 2017 American Chemical Society

Special Issue: Memorial Issue in Honor of Ken Marsh Received: January 25, 2017 Accepted: June 30, 2017 Published: July 13, 2017 2606

DOI: 10.1021/acs.jced.7b00085 J. Chem. Eng. Data 2017, 62, 2606−2609

Journal of Chemical & Engineering Data

Article

Table 1. Thermochemical Data of Xanthine and Hypoxanthine at T = 298.15 K (p° = 0.1 MPa, in kJ·mol−1)a compounds 1 xanthine hypoxanthine

ΔcHom(cr)

ΔfHom(cr)

ΔgcrHomb

ΔfHom(g)exp

ΔfHom(g)G4c

2

3

4

5

6

−2162.4 ± 1.61 −2152.5 ± 2.9 −2430.7 ± 1.41 −2427.9 ± 1.3

(−376.9 ± 1.8)1 −386.7 ± 2.9 −108.5 ± 1.71 −111.3 ± 1.5 −110.1 ± 1.1d

175.5 ± 1.3

−211.2 ± 3.2

−214.8 ± 4.6

162.5 ± 0.9

52.4 ± 1.4

46.0 ± 4.6

a

All uncertainties in this table are expressed as twice the standard deviation. Values given in bold are recommended for thermochemical calculations. b From Table 2. cCalculated by G4 by using atomization reactions. The uncertainties of the G4 method are assessed21 from the 270 experimental enthalpies of formation and the average absolute deviation from experiment showed 4.6 kJ·mol−1. dWeighted average value. Values given in brackets were disregarded.

thermodynamic procedures10,13 implemented in the data output.

2.2. Combustion Calorimetry. The molar energies of combustion of purine bases were measured with a highprecision isoperibolic calorimeter equipped with a static bomb. The detailed procedure can be found elswhere.6 The samples were weighed with a microbalance with 10−6 g resolution. Small polythene cuts were used as an auxiliary material (see Table S2) in order to ensure completeness of combustion. The sample masses were reduced to vacuum by using density values given in Table S2. Benzoic acid (sample SRM 39j, NIST) was used for measurement of the energy equivalent of the calorimeter εcalor = 14885.6 ± 1.0 J·K−1. Correction for nitric acid formation was based on the titration with 0.1 mol·dm−3 NaOH (aq). The relative atomic masses of elements were calculated as the mean of the bounds of the interval of the standard atomic weights recommended by the IUPAC commission in 2011.7 The conventional procedure8 was applied for reducing to standard states and converting the energy of the actual bomb process to that of the isothermal process. 2.3. Quartz-Crystal Microbalance Technique. The experimental setup based on the quartz-crystal microbalance (QCM) was recently designed for studies of compounds with extremely low volatility.9 The QCM is placed directly over the measuring cavity containing the sample under study. The sample is exposed to vacuum (10−5 Pa) with the whole surface open (Langmuir evaporation). During the sublimation into vacuum at a constant temperature, a certain amount of compound is deposited on the quartz crystal, changing the vibrational frequency Δf. The latter is directly related to the mass deposition Δm on the crystal. The temperature dependence of the measured frequency rate (df/dt) allows for estimating the sublimation enthalpy by using the Clausius− Clapeyron equation. To reduce the statistical error by the joined treatment of the experimental frequency shifts with the Clausius−Clapeyron equation, the experimental values in the consequent series were scaled. The scaling factor was evaluated by minimization of the standard deviation of the final enthalpy of sublimation. Scaling factor takes into account the minor changes in the frequency shift rates due to the variations in position of the sample and the QCM crystal. These minor variations are due to cleaning of the QCM between experimental runs by disassembling and assembling the system. Table S5 (Supporting Information) contains primary results of the QCM studies on xanthine and hypoxanthine. 2.4. Computational Details. The Gaussian 09 series software10 was used for the quantum chemical calculations. The high-level G4 method11 was applied for the calculation of energies of tautomeric forms of molecules under study. Details on the computational approach were reported elsewhere.12 For each compound, the enthalpies, H298, were computed with the

3. RESULTS AND DISCUSSION 3.1. Experimental Enthalpies of Formation in the Crystalline State. The standard specific energies of combustion Δcu°(cr) of xanthine and hypoxanthine are given in Tables S3 and S4. They have been used to derive the standard molar enthalpies of combustion ΔcHom(cr) and the standard molar enthalpies of formation in the crystalline state ΔfHmo(cr). Values of Δcu° and ΔcHmo are referenced to reactions: C5H4O2 N4(cr) + 5O2 (g) = 5CO2 (g) + 2H 2O + 2N2(g)

(1)

C5H4ON4(cr) + 5.5O2 (g) = 5CO2 (g) + 2H 2O + 2N2(g)

(2)

Values of ΔfHmo(cr) of compounds (see Table 1) were calculated using standard molar enthalpies of formation of H2O (l) and CO2 (g) assigned by CODATA.14 Recommendations by Olofsson15 were used for calculation of uncertainties related to combustion experiments. The uncertainties of the standard molar energy of combustion correspond to expanded uncertainties of the mean (0.95 confidence level). The uncertainty of the molar enthalpy of combustion is expressed as the twice the overall standard deviation and includes the uncertainties from calibration, from ratio between the masses of compound and auxiliary materials and from the combustion energies of the auxiliary materials. The uncertainties of the enthalpies of formation are expressed as twice the overall standard deviation and includes the uncertainties of the reaction products H2O and CO2. Primary results from combustion calorimetry are given in Tables S3 and S4. We also used the specific energies of combustion Δcu°(cr) of xanthine and hypoxanthine given by Stiehler and Huffman1 and recalculated their values of ΔcHom(cr) and ΔfHom(cr) according to the same protocol as used with our own results (see Table 1). It has turned out that the crystal phase standard molar enthalpy of formation of hypoxanthine measured in this work ΔfHom(cr) = (−111.3 ± 1.5) kJ·mol−1 (see Table 1) is in a good agreement with the result (−108.5 ± 1.7) kJ·mol−1 derived from the combustion results by Stiehler and Huffman.1 However, for the xanthine our result ΔfHom(cr) = (−386.7 ± 2.9) kJ·mol−1 differs significantly from the value of (−376.9 ± 1.8) kJ·mol−1 obtained from data by Stiehler and Huffman.1 The reason for the disagreement is not quite apparent. Stiehler and Huffman1 described a few different pathways of xanthine sample purification and drying, but at those time a careful analytical purity attestation was not developed enough. In 2607



Article

chemical calculations have been used as an additional argument to support the reliability of the evaluated thermochemical results for this compound. We have used the composite G4 method11 for calculation of the gaseous enthalpy of formation of compounds for comparison with the experimental data. Optimized structures of xanthine and hypoxanthine are given in Figure 2. Both molecules are generally planar. Milliken

contrast, samples for combustion experiments used in the current study have been of impeccable purity as proven by GC and Karl Fischer titration. 3.2. Enthalpies of Sublimation from the QCM Method. The temperature dependence of the measured by the QCM frequency rate (df/dt) was fitted with the following equation:9 g o ⎛ df ⎞ 1 ⎞ Δcr C pm ⎛ T ⎞ a b⎛1 ln⎜ ln⎜ ⎟ T⎟ = − − ⎜ − ⎟ − ⎝ dt ⎠ R R ⎝T T0 ⎠ R ⎝ T0 ⎠

(3)

where ΔgcrCop m is the difference between the molar heat capacities of the gaseous and of the crystalline phases, respectively, and a and b are adjustable parameters. T0 appearing in eq 3 is considered as an arbitrarily chosen reference temperature (which has been chosen to be 298.15 K) and R is the molar gas constant. Values of ΔgcrCop m = −24.9 J· K−1·mol−1 for xanthine and ΔgcrCop m = −20.9 J·K−1·mol−1 for hypoxanthine is an empirical estimate from a procedure suggested by Chickos and Acree16 based on the experimental value of constant pressure heat capacitiy Copm(cr) at 298.15 K. We used Copm(cr) = 151.3 J·K−1·mol−1 for xanthine2 and Copm(cr) = 134.5 J·K−1·mol−1 for hypoxanthine.2 Enthalpies of sublimation of compounds measured by QCM at different temperatures are given in Table S5. Values of ΔgcrHom(298.15K) = b + 298.15·ΔcrgCpo m are listed in Table 2. Combined

Figure 2. Optimized planar structures of xanthine and hypoxanthine.

charges on the nitrogen atoms in xanthine and hypoxanthine (see Figure S1) are similar to those in pyrimidine and pyrazine (see Figure S2). Figure 1 shows only keto- and enol forms of xanthine and hypoxanthine but theoretically many different tautomers may exist for these compounds. However, from our experiences, very often only few most stable conformers significantly contribute to the theoretical enthalpy of formation, provided that differences in their energies do not exceed 1−3 kJ·mol−1. Conformers with the energy difference ≥10 kJ·mol−1 are practically not populated in the gas phase. As can be seen in Figure S3 the ΔfHom (g, 298.15 K) values for keto- and enolforms of xanthine and hypoxanthine are larger than 10 kJ·mol−1 and the contribution to the enthalpy of formation from the enol form can be considered as negligible for both compounds. Theoretical values of the gas-phase enthalpies of formation calculated by using the atomization procedure are given for comparison in column 6 of Table 1. For xanthine, the experimental and theoretical ΔfHom (g, 298.15 K)-values are in very good agreement. For hypoxanthine, the agreement is within of the combined uncertainties of the experimental and theoretical values. Such agreement demonstrates the good ability of the G4 to provide reliable gas phase enthalpies of formation of a large number of purine and pyrimidine nucleotides. This knowledge is indispensable for metabolic pathway predictions, which is currently a focus of our experimental and theoretical efforts.18

Table 2. Enthalpies of Sublimation of Xanthine and Hypoxanthine from QCM Measurements (in kJ·mol−1)a compound

T-range/K

ΔgcrHom/Tav

xanthine hypoxanthine

398−446 423−473 388−436

172.4 ± 1.0 158.1 ± 1.6 160.4 ± 0.6

ΔgcrHom/ 298.15 K 175.5 ± 1.3 161.2 ± 1.9 162.8 ± 1.0 162.5 ± 0.9b

ref this work 3 this work

a

Uncertainties in this table are expanded uncertainties (the confidence level 95%) and they derived according to the procedure reported in ref17 bWeighted mean value. Value in bold was recommended for further thermochemical calculations.

uncertainties of the sublimation enthalpies include uncertainties from the experimental conditions and uncertainties in the temperature adjustment to T = 298.15 K as described elsewhere.17 For xanthine the enthalpy of sublimation was measured for the first time. The sublimation enthalpy ΔgcrHom (298.15 K) = (162.8 ± 1.0) kJ·mol−1 of hypoxanthine determined in this work is in very good agreement with the results ΔgcrHom (298.15 K) = (161.2 ± 1.9) kJ·mol−1 also were derived from the QCM measurements.3 To establish more confidence, we calculated the mean average value ΔgcrHom (298.15 K) = (162.5 ± 0.9) kJ· mol−1 for hypoxanthine using the uncertainties of the individual sublimation enthalpy results as the weighing factor. This weighted mean value (see Table 2) was recommended for thermochemical calculations. 3.3. Enthalpies of Formation in the Gas-Phase. Results from combustion calorimetry (Table 1) together with the sublimation enthalpies of xanthine and hypoxanthine, evaluated in Table 2 have been used for calculation of the gas-phase standard enthalpy of formation, ΔfHom(g) at 298.15 K given in column 5 of Table 1. Taking into account that the significant discrepancy among available experimental enthalpies of formation for xanthine has been found, high-level quantum-

4. CONCLUSIONS Thermochemical properties of the highly pure xanthine and hypoxanthine have been measured by using the high-precision combustion calorimetry and the modern QCM method. Reliable experimental values of Δcrg Hmo (298.15 K) and ΔfHom(cr, 298.15 K) have been used to obtain the experimental ΔfHom(g, 298.15 K) values for the sake of testing the high-level composite method G4. Good agreement between experimental and theoretical values provides mutual validation of the ΔfHmo (g, 298.15 K) results. The G4 method with the 2608



Article

Biologically Relevant Adenine and Cytosine. A Combined Experimental and Theoretical Study. J. Phys. Chem. A 2015, 119 (37), 9680−9691. (6) Beckhaus, H.-D.; Kratt, G.; Lay, K.; Geiselmann, J.; Rüchardt, C.; Kitschke, B.; Lindner, H. J. Thermolabile Kohlenwasserstoffe, XIII. 3,4-Dicyclohexyl-3,4-Dimethylhexan − Standardbildungsenthalpie, Thermische Stabilität Und Struktur. Chem. Ber. 1980, 113 (11), 3441−3455. (7) Wieser, M. E.; Holden, N.; Coplen, T. B.; Böhlke, J. K.; Berglund, M.; Brand, W. A.; De Bièvre, P.; Gröning, M.; De Loss, R.; Meija, J.; et al. Atomic Weights of the Elements 2011 (IUPAC Technical Report). Pure Appl. Chem. 2013, 85 (5), 1047−1078. (8) Hubbard, W. N.; Scott, D. W.; Waddington, G.; Rossini, F. D. Standard States and Corrections for Combustions in a Bomb at Constant Vol. In Experimental thermochemistry: measurement of heats of reaction; Interscience Publishers: New York, 1956; Vol. 1, pp 75−128. (9) Verevkin, S. P.; Zaitsau, D. H.; Emel’yanenko, V. N.; Heintz, A. A New Method for the Determination of Vaporization Enthalpies of Ionic Liquids at Low Temperatures. J. Phys. Chem. B 2011, 115 (44), 12889−12895. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision D.01. Gaussian, Inc.: Wallingford, CT, USA, 2016. (11) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. Gaussian-4 Theory. J. Chem. Phys. 2007, 126 (8), 84108−84112. (12) Verevkin, S. P.; Emel’yanenko, V. N.; Notario, R.; Roux, M. V.; Chickos, J. S.; Liebman, J. F. Rediscovering the Wheel. Thermochemical Analysis of Energetics of the Aromatic Diazines. J. Phys. Chem. Lett. 2012, 3 (23), 3454−3459. (13) McQuarrie, D. A. Statistical Mechanics; Harper & Row: New York, 1975. (14) Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA Key Values for Thermodynamics; Hemisphere Pub. Corp.: New York, 1989. (15) Olofsson, G. Assignment of Uncertainties. In Combustion Calorimetry; Pergamon, 1979; Chapter 6, pp 137−161. (16) Chickos, J. S.; Acree, W. E., Jr. Enthalpies of Sublimation of Organic and Organometallic Compounds. 1910−2001. J. Phys. Chem. Ref. Data 2002, 31 (2), 537−698. (17) Verevkin, S. P.; Emel’yanenko, V. N.; Varfolomeev, M. A.; Solomonov, B. N.; Zherikova, K. V.; Melkhanova, S. V. Thermochemistry of Dihalogen-Substituted Benzenes: Data Evaluation Using Experimental and Quantum Chemical Methods. J. Phys. Chem. B 2014, 118 (49), 14479−14492. (18) Wangler, A.; Canales, R.; Held, C.; Luong, T. Q.; Winter, R.; Zaitsau, D. H.; Verevkin, S. P.; Sadowski, G. Towards a Quantitative Understanding of Co-Solvent Effects on Rate and Equilibrium Data of Enzymatic Reactions. Angew. Chem. 2017, submitted. (19) Marsh, K. N.; Månsson, M. Standard Molar Enthalpies of Formation of Triethoxymethane and Tetraethoxymethane by Rotating Bomb Calorimetry. J. Chem. Thermodyn. 1985, 17 (10), 995−1002. (20) Duarte-Garza, H. A.; Stouffer, C. E.; Hall, K. R.; Holste, J. C.; Marsh, K. N.; Gammon, B. E. Experimental Critical Constants, Vapor Pressures, and Vapor and Liquid Densities for Pentafluoroethane (R125). J. Chem. Eng. Data 1997, 42 (4), 745−753. (21) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. Gaussian-4 theory. J. Chem. Phys. 2007, 126, 084108.

atomization procedure can be recommended for calculation of the gas-phase enthalpies of nucleotides similar to the shape of xanthine and hypoxanthine, which are important for understanding the energetics of biological processes.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00085. Physical properties of the materials used in the present study; results for typical combustion experiments for xanthine and hypoxanthine; results of the temperature dependence of frequency shift rate df/dt using the QCM method for xanthine and hypoxanthine and sublimation enthalpies ΔgcrHom (T); supplemental figures as described in the text (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Vladimir N. Emel’yanenko: 0000-0002-8230-4037 Sergey P. Verevkin: 0000-0002-0957-5594 Funding

This work has been supported by the German Science Foundation (DFG) in the frame of the Project VE 265/12-1 “Glycolysis: thermodynamics and pathway predictions” This work has been also partly supported by the Russian Government Program of Competitive Growth of Kazan Federal University and Russian Foundation for Basic Research No. 1503-07475. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This contribution is a part of the special issue dedicated to the memory of Dr. Ken Marsh. Ken had an astonishingly large scope of scientific interests in chemical thermodynamics and phase equilibria; moreover, on the basis of his experiences in combustion calorimetry,19 and vapor pressure measurements,20 his critical and detailed comments as a Journal of Chemical and Engineering Data editor have significantly contributed to improvement of experimental methods and to the establishment of thermochemistry in Rostock during the late 1990s.

■

REFERENCES

(1) Stiehler, R. D.; Huffman, H. M. Thermal Data. IV. The Heats of Combustion of Adenine, Hypoxanthine, Guanine, Xanthine, Uric Acid, Allantoin and Alloxan. J. Am. Chem. Soc. 1935, 57 (9), 1734−1740. (2) Stiehler, R. D.; Huffman, H. M. Thermal Data. V. The Heat Capacities, Entropies and Free Energies of Adenine, Hypoxanthine, Guanine, Xanthine, Uric Acid, Allantoin and Alloxan. J. Am. Chem. Soc. 1935, 57 (9), 1741−1743. (3) Teplitskii, A. B.; Yanson, I. K. Effect of Substituents on the Heat of Sublimation of Nucleic Acid Nitrogenous Bases. Biofizika 1975, 20 (2), 189−193. (4) Emel’yanenko, V. N.; Verevkin, S. P.; Notario, R. Thermochemistry of Uracil and Thymine Revisited. J. Chem. Thermodyn. 2015, 87, 129−135. (5) Emel’yanenko, V. N.; Zaitsau, D. H.; Shoifet, E.; Meurer, F.; Verevkin, S. P.; Schick, C.; Held, C. Benchmark Thermochemistry for 2609


Thermochemical Properties of Xanthine and Hypoxanthine Revisited

Recommend Documents