Enthalpy of combustion of purine

o-57 .I. Chem. Thermodynamics 1984, 16, 633-641

Enthalpy

of combustion

DUANE R. KIRKLIN

of purine

and EUGENE S. DOMALSKI

Chemical Thermodynamics Division, Center for Chemical Physics, National Measurement Laboratory, National Bureau of Standards, Washington, D.C. 20234, U.S.A. (Received 22 June 1983; in revised form 3 January 1984) The enthalpy of combustion of a commercial purine sample of 99.8 moles per cent purity was measured in an aneroid adiabatic bomb calorimeter. The molar enthalpy of combustion at 298.15 K for the reaction: ~,HJUcr) + 60,(g) = 5COAg) + 2WW + 2NAg), is A,Hz = -(2708.62+2.31) kJ ‘mol-’ (p” = 101.325kPa). The corresponding molar enthalpy of formation of purine, CsH,N,, is ArHk = (169.41k2.65) kJ.mol-‘. The estimation of ArH”, for purine by three calculated paths yielded 180.15kJ.mol-r. Some reasons for the difference are proposed.

1. Introduction A limited amount of thermodynamic information is available on the fundamental properties of the purine bases which play an important role in biological processes as part of the building blocks of nucleic acids. Earlier studies have reported measurements on the enthalpies of combustion,“-3’ proton dissociation in aqueous solution,(4) sublimation,‘5* 6, and solution”-“’ of the important nucleic-acid bases. In this paper, we report on the measurement of the enthalpy of combustion and calculation of the enthalpy of formation for crystalline purine. As a result of examining data on the enthalpies of formation of purine and pyrimidine derivatives, we have attempted to estimate the enthalpy of formation of purine so that a comparison can be made with the experimentally determined value. It is hoped that as more key-experimental quantities are available, an estimation scheme which can predict the enthalpies of formation of important biological molecules will be developed.

2. Experimental MATERIALS

Purine was purchased as Lot No. 0201HE with a stated purity of 99 moles per cent from the Aldrich Chemical Company, Inc. The sample was dried for 48 h at 378 K and the purity checked by high-performance liquid chromatography (h.p.1.c.)and nuclear magnetic resonance (n.m.r.). The h.p.1.c. chromatogram showed only one

634

D. R. KIRKLIN

AND E. S. DOMALSKI

peak indicating no detectable impurities. The h.p.1.c.used an ultraviolet detector at 260 nm. The 400 MHz proton-resonance spectra also showed no evidence of impurities. The spectra were recorded at various attenuations in order to observe all of the spectral peaks. All of the observed peaks were identified as carbon-12 bands, carbon-13 satellite bands, or spinning side bands of purine. Since the h.p.1.c.and n.m.r. measurements showed no impurities, a quantitative estimate of the impurity level was obtained by duplicate carbon analyses. Quantitatively, the purine sample was estimated to be 99.8 moles per cent pure based upon an experimental carbon content of 49.89 mass per cent compared with a theoretical carbon content of 50.00 massper cent. The enthalpy of combustion of the dried purine sample was measured with no further purification. APPARATUS

The adiabatic rotating-bomb calorimeter was designed, constructed, and tested at N.B.S. by W. H. Johnson and E. J. Prosen. A more comprehensive description of this calorimetric system was given earlier by Kirklin and Domalski.(‘) The temperature-measuring station was modified for these experiments with the introduction of a high-precision automatic inductive ratio-arm bridge (a.c. automatic bridge) in order to carry out temperature measurementsusing a platinum resistance thermometer. PROCEDURE

The calorimeter was calibrated by a series of combustion experiments using benzoic acid obtained from the National Bureau of Standards’ Office of Standard Reference Materials as SRM 39i. The calorimetric standard, SRM 39i, has a certified specific energy of combustion of - (26434+ 3) J *g- ’ when burned under the certificate conditions. This reduces to - 26412.68 J. g- ’ at 299.15 K and standard-state conditions.? A sample mass of approximately 0.13 g was compressed into a pellet, weighed in a platinum crucible, and placed in the bomb which contained 0.3 cm3 of water. The bomb was filled with oxygen to a pressure of 3.2 MPa followed by the slow releaseof oxygen for purging purposes. The bomb was refilled with oxygen to a pressure of 3.2 MPa and transferred to the calorimeter system. The temperature of filling was observed, the calorimeter jacket was evacuated, the bomb was heated electrically to 298 K, and the system was left overnight with both the vacuum pumps and the adiabatic temperature controls in operation. Calculations showed that sample volatilization was negligible in these experiments. On the following day, calorimeter temperatures were observed at 5 min intervals during a 30 min initial-rating period and thereafter the sample was ignited by passing an electrical current through a platinum wire fuse in contact with the sample. After approximately 30 min, thermal equilibrium was re-established and calorimeter temperatures were observed at 5 min intervals in the 30 min final-rating 7 Throughout

this paper the standard pressure p0 is taken as 101.325 kPa.

ENTHALPY

OF COMBUSTION

635

OF PURINE

period. A steady rise in temperature of the calorimeter (approximately 8 x lo-’ K 1s- ‘) was observed during the rating periods because of the heating effect of the 1 mA current through the platinum resistance thermometer. The temperature rise due to the overall bomb process was obtained by extrapolation of the curves of time against temperature to the actual time of ignition. The bomb was removed from the calorimeter, the gaseous contents were released, and the bomb was opened. The aqueous solution in the bomb was transferred to a beaker and the solution was titrated with standard alkali using a pH meter to determine the amount of HNO, which had formed as a side-reaction in the combustion. For the purine experiments, approximately 0.12 g of purine powder was placed in a weighed platinum crucible. Approximately 0.01 g of benzoic acid was compressed into a pellet and placed on the purine powder to initiate and ensure complete combustion. The platinum wire fuse was placed in contact with the benzoic acid pellet. In addition, 2.00 cm3 of water was added to the bomb (rather than the usual 0.3 cm3) because of the increased amount of HNO, that would be formed.

3. Discussion The results of the calibration and purine experiments are presented in tables 1 and 2, respectively. The order of the table entries is identical in tables 1 and 2. However, the order of calculation of the terms in the calibration experiments is opposite that used in the purine experiments. The energy terms in table 1 have a negative sign when energy is added to the system to obtain the total energy that produced the observed temperature rise and the quantity of interest is the effective energy equivalent of the empty calorimeter. In table 2, the total observed energy is negative

TABLE Expt no. -

738

~,/t J. K - ’ ) C&ont.)/(J

L&J.

1. Results of calibration

K - ‘)

K - ‘1

A T/K

Q,,,lJ W@/J Q(HNW/J QwlJ

K)/(J.g-‘)

2519.45 4.83 2524.28 1.166664 2944.99 1.10 0.64 2.09 -0.01 0.00 2941.7 0.111354 26412.68

experiments with benzoic acid; p” = 101.325 kPa

741

742

2518.95 4.95 2523.91 1.404129 3543.89 1.03 0.42 2.55 -0.11 0.00 3540.00 0.134026 26412.60

(s&299.15

K))/(J

a Mean and standard deviation of the mean. 31

2518.78 4.97 2523.93 1.342911 3389.18 1.45 0.42 2.43 0.07 0.00 3384.81 0.128151 26412.69 K - ‘)

743 2519.85 4.98 2524.83 1.466938 3703.77 0.97 1.63 2.67 0.33 0.00 3698.17 0.140015 26412.68

744 2517.29 4.92 2522.2 1 1.333271 3362.79 0.97 0.48 2.41 0.16 0.00 3358.77 0.127165 26412.70

2519.14~0.45”

745 2518.48 4.96 2523.44 1.416855 3575.35 1.26 0.61 2.58 0.16 0.00 3570.74 0.135190 26412.70

746 2521.15 4.93 2526.08 1.349251 3408.32 1.40 0.48 2.45 0.51 0.00 3403.47 0.128858 26412.68

-

636

D. R. KIRKLIN

AND

E. S. DOMALSKI

TABLE 2. Results of combustion experiments with purine; p“ = 101.325 kPa Expt no.

128

&,,,,/(J. K - ‘)

C,(cont.)/(J.r-‘)

pa(J.K-

)

Q,,,iJ J%@l/J E:03)‘J QINT, ,,,,.)/J Q,.jJ Q&J m.ig

A,u”(299.15 K)/(J . g- ‘)

129

730

731

132

735

736

737

2519.14 2519.14 2519.14 2519.14 2519.14 2519.14 2519.14 2519.14 11.81 12.47 4.79 4.72 4.78 11.95 11.89 12.00 2530.95 2531.61 2523.93 2523.86 2523.92 2531.09 2531.03 2531.13 1.135933 2.580044 1.136631 1.046440 1.121368 1.442272 1.300369 1.541462 -2874.99 -6531.67 -2868.77 -2641.07 -2830.24 - 3650.52 -3291.27 -3901.65 1.24 1.39 1.17 1.32 1.24 1.60 1.11 1.17 13.87 37.37 14.31 11.81 11.92 18.90 15.00 19.48 3.16 7.32 2.16 1.98 2.15 4.02 3.64 4.32 - 0.03 1.19 -0.01 -0.12 0.06 0.29 0.19 0.25 326.23 406.35 395.88 343.06 372.09 393.63 403.20 383.03 - 2530.52 - 6078.05 -2455.26 - 2283.02 - 2442.78 - 3232.08 - 2868.13 - 3493.39 0.112081 0.269478 0.108676 0.101182 0.108045 0.143213 0.127201 0.154929 - 22577.68 - 22554.93 - 22592.46 - 22563.48 - 22609.02 - 22568.33 - 22547.99 - 22548.33 (A,u”(299.15 K))/(J.g-‘) -22570.28f7.69

while other energy terms have positive signs to indicate that they are corrections applied to the total observed energy to obtain only that energy produced from the combustion of purine. The quantity of interest in table 2 is the specific internal energy of combustion for purine. A brief explanation of the table entries is as follows: Est.+the effective energy equivalent of the standard empty calorimeter including the bomb and all internal platinum parts except the crucible; C,(cont.), the heat capacity of all materials added to the standard calorimeter including the crucible, sample, oxygen, and water; E.,~,,~,the effective energy equivalent of the actual fully loaded calorimeter at 299.15 K; AT, the observed increase in temperature of the system following ignition of the sample; Qtot, the total energy added to the system as a result of the bomb processes; kk&“, the electrical energy added to the system to ignite the sample; Q(HNO,), the chemical energy added to the system by the formation of nitric acid; Qw, the Washburn correction” ‘* ’ 4, applied to convert all reactants and products to their respective standard states at the selected final temperature; Q(Tfcorr.), a correction applied for deviation of the actual final temperature from the selected final temperature of 299.15 K; Q,,X, the energy evolved in the bomb process due to the combustion of benzoic acid to insure ignition and complete combustion of purine; Qreac(,the energy evolved by the reaction of interest with products and reactants in their respective standard states at the selected final temperature of 299.15 K; m,, the mass of the sample; A,u”(299.15 K), the specific energy of combustion of the sample at the selected final temperature of 299.15 K. All calculations were performed using a computer program originally prepared by Shomate(15*16) and later revised by Armstrong”‘) according to the method of Hubbard, Scott, and Waddington. (14) The computer program was developed to eliminate the tedious chore of keeping track of the molar heat capacities and molar quantities of all the reactants and products which are needed to calculate Q,, the Washburn correction. The results presented in the tables have been rounded for

ENTHALPY

OF COMBUSTION

637

OF PURINE

convenience and for this reason small differences may result by calculation from the tabulated values. The calculations required several auxiliary data. Thermochemical quantities for CO,(g) and H,O(l) were taken from Wagman et a1.(‘*’ The molar mass of purine was 120.1134g.mol-‘, based on the 1979 Table of International Atomic Weights.“‘) The molar energy of decomposition of HNO, into N,, O,, and H,O was taken as 59.7 kJ* mol- ‘. The density and specific heat capacity of benzoic acid are 1.32 g+cmw3 and 1.21 J. K- ’ . g- ‘, respectively.08) The same properties for purine are 1.22 g. cm - 3 and 0.89 J. K- ’ . g- ’ and were measured in this laboratory. Thermodynamic quantities have been calculated for the reactions:

GJLN&r) + 60,(g) = 5C02(g)+ 2WN)

+

2N2(g),

SC(cr, graphite) + 2H,(g) + 2N,(g) = C,H,N,(cr).

(1) (2)

The thermodynamic results for crystalline purine C,H,N,(cr) are A,Uk(298.15 K) = -(2711.10+2.31) kJ.mol-‘, A.,Hk(298.15 K) = -(2708.62+2.31) kJ.moll’, A,Hk(298.15 K) = (169.41k2.65) kJ.mol-‘. The total uncertainty was calculated according to the guidelines presented by Olofsson.‘20) The uncertainty assigned to ArHm is twice the overall standard deviation, 2s{A,Hm(C,H,N,)}, and is defined as: s{A,Hk(C,H,N,,

cr)}” = 52s{A,Hm(C02, g)}’ +22s{A,Hm(H20, l)}” +s{A,fGWWJ4, cr)>‘,

where

s{~K,GHA,

cd>’= ~(4 G,W-LN4, cr)}’ = (M,A~u~)~C{S((A~U~))/A~U~}~ + M&aJ)/~,td*+ M@c~d/Ac~d~ + {s((AC~BA))/A\cu~}~(mgA/rn~)~ +(0.~1)2 +(Q@W21.

The first two terms in the square brackets represent the squares of the relative standard deviations of the mean determined in the combustion of purine and in the calibration experiments, respectively. The third term is the square of the relative standard deviation of the mean for the calibration standard: benzoic acid NBS SRM 39i. The fourth term also contains the standard deviation of the mean for SRM 39i because it was used as an auxiliary substance in these measurements; the fourth term is calculated relative to the mean values from the purine experiments; mBA/mpis the ratio of the mass of benzoic acid to the mass of purine in the combustion sample. The fifth and sixth terms account for the uncertainties for possible organic impurities and the systematic error which were estimated as 0.01 per cent, respectively, of the mean value for the purine experiments. The numerical values used to calculate the overall standard deviation are tabulated in table 3.

638

D. R. KIRKLIN

AND E. S. DOMALSKI

TABLE 3. Components of the overall standard deviation, “k” mean

preceding the standard deviation of the (22570.279k 7.689)J. g- ’ (2519.137kO.4546)J.K-’ (26434.0’ k 2.935n*b)J K - 1 0.110213 kO.130 kJ.mol-” +0.042 kJ.mol-”

(A,W,H,W) (%td)

A,u(C,H,CO,H, certified) (m(C,H,CO,H)/m(C,H,N,)) s{l s{i a Taken from reference 21. b Student t removed from the random uncertainty. ’ Taken from reference 32.

4. Estimation of the molar enthalpy of formation of crystalline purine We have tried to estimate the molar enthalpy of formation of crystalline purine at 298.15 K using three paths. In developing these paths, we have assumed that the ArH; contributions from substituents in the 2- and 4-positions in pyrimidine derivatives are equivalent to the 2- and 6-positions in purine derivatives, as is illustrated below.

pyrimidine

purine

First, establishing the ArHm contribution of the amino group at the 6-position in crystalline adenine would allow us to estimate the A,Hm of crystalline purine according to Ar Hk(adenine) = A,Hm(purine) + Ar Hk(amino group, 6-position):

[y>

= [+>

(3)

+ [(C-NH,)-(C-H)]

Examination of the auxiliary data on crystalline purines and pyrimidines in table 4 allows one to develop the following set of equations: Af HjD,,(xanthine)= A,Hm(hypoxanthine) + Ar Hh(oxo group, 2-position):

(4)

639

ENTHALPY OF COMBUSTION OF PURINE TABLE 4. Auxiliary quantities for crystalline purines and pyrimidines; p0 = 101.325kPa Name

Formula

Cd&N,

ArHL(298.15 K)/(kJ ‘mol-r)

adenine guanine hypoxanthine xanthine cytosine uracil pyrimidine

CHNO 5 5 5 5 W-LAO WUW’ W-W,0 WJW, CAN,

+96.90+ 1.29 - 182.92f 0.84 - 109.79kO.75 -378.6lkO.92 -221.352.3 -424.4 k 2.5 + 134.71kO.88

Reference 1 2 2 2 3 3 23,24

The value obtained for A,H;(oxo group, 2-position) is -268.82 kJ. mol- l. ArHi(cytosine) = A,Hk(pyrimidine) + A,Hi(oxo group, 2-position) + A,Hi(amino group, 6-position): (5) NH* + [(C=O)-(C-H)]

+ [(C-NH,)-(C-H)].

The value for ArHk pyrimidine in the crystalline phase was obtained from the combustion results of TjebbesCz3)on the liquid and from a recent d.s.c. measurement of the molar enthalpy of fusion (11.9 kJ . mol- ’ at 295.2 K) at N.B.S.(24) From equation (5) we can derive a value for A,Hk(amino group) equal to - 87.19 kJ. mol-‘. Application of this value to equation (3) yields ArH; for crystalline purine equal to + 184.09 kJ *mole ‘. The second path provides an estimate of A,H~(purine) from a calculation of the A,Hi contribution of the 0x0 group at the 6-position in crystalline hypoxanthine, according to A,Hm(hypoxanthine) = Ar Hk(purine) + A,Hm(oxo group, 6-position):

(6)

0

=

cy)

+ [(C=O)-(C-H)].

The value obtained for A,Hi(oxo group, 6-position) is - 290.29 kJ. mol-‘. An estimate of the ArHi for the 0x0 group can be derived from At Hk(uraci1) = Af Hk(pyrimidine) + Af Hk(oxo group, 6-position) + Af H,Y,,(oxo group, 2-position): (7) 0

+ [(C=O)-(C-H)]+[(C=O)-(C-H)]. 0

D. R. KIRKLIN

640

AND E. S. DOMALSKI

Introducing the value - 290.29 kJ. mall’ for the 0x0 group in the 6-position into equation (6) gives ArHh for crystalline purine equal to + 180.50 kJ . mall ‘. A third path which can be taken to calculate A,Hm(purine) utilizes equation (3) where the A,Hm(amino group, 6-position) is estimated from the A,Hk(amino group, 2-position) from A,Hl(guanine) = A,HL(hypoxanthine)+ A,Hz(amino group, 2-position):

(8)

From a proportional comparison of ArHi(oxo group) ratio for the 2- and 6-positions and A,Hk(amino group) for the 2-position derived from equation (8) one can calculate A,Hm(amino group, 6-position) as follows: A,Hm(amino group, 6-position) = {A,Hk(oxo group, 6-position)/A,Hk(oxo group, 2-position)} x AL\,Hi(amino group, 2-position), or A,Hk(amino group, 6-position) = (( -290.29)/( - 268.82)) x ( - 73.13) = - 78.97 kJ. mol- ‘. This value, -78.97 kJ . mol-‘, can be introduced into equation (3) to give A,Hk for crystalline purine equal to + 175.87 kJ. mol-‘. An average value for the estimated ArHz for purine as obtained by three calculated paths is 180.15 kJ.moll’. This estimated average differs from our experimental value of (169.41+ 2.65) kJ *mol- ’ by 10.74 kJ +mol- ‘. The difference can be attributed to the assumption that the ArHk contributions from substituents in the 2- and 4-positions in pyrimidine derivatives are equivalent to the 2- and 6-positions in purine derivatives. Systematic errors which influence the agreement of between-laboratory measurements are also likely to be part of the differences. REFERENCES I. 2. 3. 4. 5.

Kirklin, D. R.; Domalski, E. S. J. Chem. Thermodynamics 1983, 15, 941. Stiehler, R. D.; Huffman, H. M. J. Am. Chem. Sac. 1935, 57, 1734. Wilson, S. R.; Watson, I. D.; Malcolm, G. N. J. Chem. Thermodynamics 1979, 11, 91 I. Zimmer, S.; Biltonen, R. L. J. Soln Chem. 1972, 1, 291. Yanson, I. K.; Teplitskii, A. B. Vzaidmodeisvice Konform. Mol., Twzisy Delk. Vses. Simp., 3rd 19’76, 1, 25. 6. Tephtskii, A. B.; Sukhodub, L. F.; Yanson, I. K. Fiz. Kondens. Sostayaniya 1974, 32, 68. 7. Kilday, M. V. J. Res. Nat. Bur. Stand. 1978, 83, 347. 8. Kilday, M. V. J. Rex Nat. Bur. Stand. 1978, 83, 529. 9. Kilday, M. V. J. Rex Nat. Bur. Stand. 1978, 83, 539. 10. Kilday, M. V. J. Res. Nat. Bur. Stand. 1978, 83, 547. Il. Kilday, M. V. J. Res. Nat. Bur. Stand. 1979, 84, 23 1. 12. Kilday, M. V. J. Res. Nat. Bur. Stand. 1981, 86, 367. 13. Washburn, W. W. J. Res. Nat. Bur. Stand. 1938, 10, 525.

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14. Hubbard, W. N.; Scott, D. W.; Waddington, C. J. J. Phys. C/rem. 1954, 58, 152. 15. Shomate, C. H. U.S. Naval Ordnance Test Station Technical Report 327. August 1963, China Lake, California. 16. Shomate, C. H. U.S. Naval Ordnance Test Station Report Code 5052. January 1967, China Lake, California. 17. Armstrong, G. T. Nat. Bur. Stand. Report 9803. 1968, chap. 3. 18. Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, 1.; Bailey, S. M.; Schumm, R. H. Nat, Bur. Stand. Tech. Note 270-3. January 1968. 19. Holden, N. E. Pure Appl. Chem. 1980, 52, 2349. 20. Olofsson, G. Combustion Calorimetry. Sunner, S.; Mansson. M.: editors, Pergamon Press: New York. 1979, chap. 6. 21. Churney, K. L.; Armstrong, G. T. J. Res. Nat. Bur. Stand. 1968, 72A, 453. 22. CODATA RecommendedKey Valuesfor Thermodynamics 1977. CODATA Bulletin 28. April 1978. 23. Tjebbes, J. Acta Chem. Stand. 1962, 16, 916. 24. D.s.c. measurements by M. M. Miller of the N.B.S. Chemical Thermodynamics Division, 1983.