Interaction of Some Aminophosphonic Acids with Urea in Aqueous

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Interaction of Some Aminophosphonic Acids with Urea in Aqueous Solutions at 298.15 K Bartłomiej Palecz,*,† Aleksandra Grala,† and Zbigniew Kudzin‡ †

Department of Physical Chemistry and ‡Department of Organic Chemistry, University of Lodz, 90-416 Lodz, Poland ABSTRACT: The enthalpies of solution of aminomethylphosphonic acid (GlyP), 1-aminoethylphosphonic acid (AlaP), and 1-aminopropylphosphonic acid (HalaP) have been measured in water and aqueous urea solutions at 298.15 K. From the obtained experimental results, the standard enthalpies of solution of aminophosphonic acids (AP) in water−urea solutions have been determined. These data were used to calculate the heterogeneous enthalpic pair interaction coefficients based on McMillan−Mayer's formalism. These values were interpreted in the terms of the hydrophobic effect of the side chains of aminophosphonic acids on their interactions with a molecule of urea in water.

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INTRODUCTION Aminophosphonic acids (AP) are the analogues of natural occurring natural amino acids (A) possessing the phosphonic group bonding to Cα instead of the carboxyl group. Taking into consideration the similar structure of aminophosphonic acids to natural α-amino acids, aminophosphonic acids reveal also significant biological activity.1,2 Because of their structural similarities amino phosphonic acids are present in many living organisms.3,4 These phosphorus amino acids are the competitic inhibitors of numerous enzymes, for example, carboxypeptidase A5 or leucine aminopeptidase.6 Aminophosphonic acids have particularly found applications as herbicides. The synthesis of phosphonomethylglycine (PMG, glyphosine, Round-up)7 which turned out to be a very effective herbicide, has initiated extensive interest of applying aminophosphonic acids such as herbicides.8,9 Many scientific centers conduct studies of phytotoxicity10 and accumulation11 of aminomethylphosphonic acid in plants as well as their attendance in the natural environment.12 The research of our laboratory focused previously on the recognition interaction between natural L-α-amino acids with urea13 and sodium14 or potassium15 chlorides which are the components of body fluids. The universal presence of aminophosphonic acids in nature as a result of human activity and because of their role as many enzymes inhibitors gave us a reason to undertake the investigation of these amino acids interaction with urea in aqueous solutions. The urea is present in living organisms as a nitrogen compound metabolic product. This organic compound is used for the production of many fertilizers, and its aqueous solutions are applied as a protein denaturation factor. The present paper demonstrates the enthalpies of solutions of aminophosphonic acids (aminomethylphosphonic acid, 1aminoethylphosphonic acid, and 1-aminopropylphosphonic acid) in water and in the aqueous solutions of urea at the temperature of 298.15 K. The obtained results let us to calculate the heterogeneous enthalpic pair interaction coefficients between aminophosphonic acids and urea molecules based on the McMillan−Mayer theory.16 The enthalpic pair interaction coefficients describe the energetic effects of © 2012 American Chemical Society

interactions between aminophosphonic acids molecules and urea molecule which happen in the presence of the competitive participation of water molecules.

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EXPERIMENTAL SECTION Materials. The aminophosphonic acids (AP) were prepared at the Department of Organic Chemistry, University of Lodz. The aminomethylphosphonic acid was prepared according to procedure of Soroka,17 and 1-aminoethylphosphonic acid and 1-aminopropylphosphonic acid were prepared based on the procedures of Kudzin and Stec.18,19 All 1-aminoalkylphosphonic acids, homogeneous in 31P NMR, were dried under reduced pressure at 323 K (over solid KOH and P4O10). Urea (U) (mass fraction purity Aldrich, > 99.5 %) was dried under reduced pressure at 323 K for 48 h. The structures of the aminophosphonic acids prepared and investigated in this work are given in Table 1. The water used in the experiments was deionized, twice-distilled, and degassed. Calorimetric Measurements. The enthalpies of solution (ΔHS) of aminophosphonic acids were measured in water and in aqueous solutions of urea using an “isoperibol” calorimeters.23,24 The temperature stability of the thermostat was better than 0.002 K. The uncertainties in the measured enthalpies did not exceed ± 1.0 % of the measured value. The examined aqueous solutions containing from (0 to 3.00) mol(U)·kg−1(H2O) and the aqueous solutions of aminophosphonic acids (0.001 to 0.002) mol(AP)·kg−1(solvent) were prepared by mass with an accuracy of ± 0.00002 g using a Mettler AE240 balance.

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RESULTS AND DISCUSSION The dissolution enthalpies (ΔHS) of aminomethylphosphonic acid (GlyP) (Table 2), 1-aminoethylphosphonic acid (AlaP) (Table 3), and 1-aminopropylphosphonic acid (HalaP) (Table 4) measured in water and aqueous solutions of urea at T = Received: January 13, 2012 Accepted: April 3, 2012 Published: April 18, 2012 1515

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Table 1. Names, Abbreviations, Stuctures, and Negative Logarithm Dissociation Constants of Aminophosphonic Acids Examined and Discussed in This Work

Table 2. Enthalpies of Dissolution of Aminomethylphosphonic Acid (GlyP) in Water (W) and in Urea (U)−Water Mixtures at 298.15 K in 0.5000 mol(U)·kg−1(W)

in water ΔsolH

0

m mol(AP)·kg−1(W)

m

kJ·mol−1

0.000800 13.35 0.001080 13.14 0.001576 13.05 0.001851 12.97 0.002403 12.99 0.003024 12.96 0.003400 12.90 in 2.0000 mol(U)·kg−1(W) 0.000760 10.51 0.001127 10.50 0.001347 10.49 0.001358 10.49 0.001479 10.48 0.001903 10.47

m

ΔsolH0m

mol(AP)·kg−1(W+U)

kJ·mol−1

in 1.000 mol(U)·kg−1(W)

in 1.5000 mol(U)·kg−1(W)

m

ΔsolH0m

m

ΔsolH0m

mol(AP)·kg−1(W+U)

kJ·mol−1

mol(AP)·kg−1(W+U)

kJ·mol−1

0.001180 0.001345 0.001560 0.001604 0.001717 0.002278

10.92 10.87 10.85 10.86 10.84 10.83

0.001066 12.13 0.001257 12.08 0.001327 12.10 0.001505 12.08 0.001804 11.99 0.002004 11.931 0.003010 11.87 in 2.5000 mol(U)·kg−1(W) 0.001138 10.56 0.001434 10.46 0.001696 10.49 0.001788 10.51 0.001875 10.46 0.001974 10.44

0.000945 11.10 0.001027 11.06 0.001130 11.01 0.001261 11.06 0.001653 11.01 0.001696 10.99 0.002202 10.96 in 3.0000 mol(U)·kg−1(W) 0.0007562 10.08 0.001230 10.01 0.0012316 10.00 0.001320 9.85 0.002104 9.74 0.002549 9.69

Table 3. Enthalpies of Dissolution of 1-Aminoethylphosphonic Acid (AlaP) in Water (W) and in Urea (U)−Water Mixtures at 298.15 K in 0.5000 mol(U)·kg−1(W)

in water m

ΔsolH0m

mol(AP)·kg−1(W)

kJ·mol−1

0.001248 3.97 0.001261 3.95 0.001301 3.94 0.001464 3.93 0.001517 3.94 0.001785 3.92 in 2.0000 mol(U)·kg−1(W) 0.001203 2.19 0.001217 2.19 0.001298 2.18 0.001324 2.17 0.001374 2.17 0.001553 2.16

m

ΔsolH0m

mol(AP)·kg−1(W+U)

kJ·mol−1

in 1.000 mol(U)·kg−1(W)

in 1.5000 mol(U)·kg−1(W)

m

ΔsolH0m

m

ΔsolH0m

mol(AP)·kg−1(W+U)

kJ·mol−1

mol(AP)·kg−1(W+U)

kJ·mol−1

0.001146 0.001293 0.001352 0.001465 0.001530 0.001785

2.39 2.33 2.32 2.29 2.25 2.24

0.001206 2.79 0.001258 2.72 0.001301 2.65 0.001451 2.63 0.001687 2.55 0.001903 2.51 in 2.5000 mol(U)·kg−1(W) 0.001046 1.84 0.001398 1.79 0.001428 1.76 0.001496 1.77 0.001641 1.75 0.002036 1.74

0.000853 2.56 0.001073 2.29 0.001272 2.34 0.001344 2.26 0.001528 2.23 0.001703 2.21 in 3.0000 mol(U)·kg−1(W) 0.001063 1.64 0.001258 1.61 0.001318 1.58 0.001366 1.55 0.001449 1.59 0.001623 1.56

298.15 K enabled us to determine the standard molar enthalpies of solution ΔH0S by extrapolating the values of the dissolution enthalpies to infinite dilution as a function of the molality. The standard dissolution enthalpies of aminophosphonic acids in water, ΔH0S(W), and aqueous solutions of urea (U), ΔH0S(W+U), with concentrations from (0.5 to 3) mol(urea)·kg−1

are listed in Table 5. They were used to calculate the enthapic heterogeneous coefficients of interactions between the molecules dissolved in water, using Desnoyers's equation.25 0 0 2 ΔHS(W + U) = ΔHS(W) + 2hAPU mU + 3hAPUU mU + ...

(1)

where h AP,U denotes the heterogeneous enthalpic pair interaction coefficient between aminoalkanephosphonic acid 1516

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Table 4. Enthalpies of Dissolution of 1-Aminopropylphosphonic Acid (HalaP) in Water (W) and in Urea (U)−Water Mixtures at 298.15 K in 1.0000 mol(U)·kg−1(W)

in water m

ΔsolH0m

mol(AP)·kg−1(W) 0.001440 0.002157 0.001805 0.001960 0.002058 0.002070

in 2.0000 mol(U)·kg−1(W)

m

ΔsolH0m

kJ·mol−1

mol(AP)·kg−1(W+U)

−0.34 −0.35 −0.35 −0.34 −0.37 −0.34

0.001164 0.001365 0.001521 0.001687 0.001767 0.001950

m

m

ΔsolH0m

kJ·mol−1

mol(AP)·kg−1(W+U)

kJ·mol−1

mol(AP)·kg−1(W+U)

kJ·mol−1

−1.29 −1.36 −1.37 −1.70 −1.41 −1.39

0.000754 0.001290 0.001335 0.001667 0.001794 0.001927

−1.60 −1.65 −1.68 −1.65 −1.66 −1.72

0.001147 0.001442 0.001538 0.001659 0.001705 0.001862

−1.93 −1.97 −1.95 −1.95 −1.98 −1.97

Table 5. Standard Enthalpies of Solution of αAminophosphonic Acids (AP) in Water (W) and Aqueous Urea (U) Solutions at 298.15 K

molecules under consideration. Therefore these interactions (hAP,U,U) will not be discussed in this paper. The enthalpic pair interaction coefficients (hAP,U) between a dissociated molecule of aminophosphonic acids and urea molecule in water as a measure of the energetic interactions between two heterogeneous molecules are the sum of the following effects: - direct interactions between aminophosphonic acid polar head and the polar urea molecule, - a partial dehydration of the hydration sheaths of ionic and polar groups interacting in aqueous solution, - dehydration effects which are reinforced by the hydrophobic hydration phenomenon that is caused by the side nonpolar substituents of aminophosphonic acids. The molecules of water in the direct surroundings of hydrophobic groups strengthen the interactions between themselves. As a result of the cooperativeness of hydrogen bonds, this effect is transferred onto the water molecules that hydrate ionic “aminophosphonic acid polar head” [(H3N+)CH PO(OH)O−] and polar urea molecules that interact with them. Consequently, the removal of polar or ionic groups of some number of water molecules, constituting a hindrance in their direct interactions, from the hydration sheaths requires an increased energy input. The pair interaction coefficients (hAP,U) determined (Table 6) have negative values. These indicate that, in the global interaction effect, a dominant position is maintained by the exothermic effects of the direct interactions of zwitterion of aminophosphonic acids [(H3N+)CH PO(OH)O−] with urea that prevail over the endothermic effects of the desolvation of their hydration sheaths. The values of the enthalpic coefficients (hAP,U) increase with increasing the side alkyl substituent: GlyP < AlaP < HalaP (Table 6). Figure 1 shows the dependence of the enthalpic coefficients (hAP,U) on the number of CH2 groups in the aminophosphonic acid side chain (according to Wood's26 suggestion that a CH3 group corresponds to 1.5 of the CH2 group). The dependence presented (Figure 1) is not rectilinear as in the case of the interactions between natural amino acids and urea13 or electrolytes15,27 in water. It indicates that the endothermic contribution made by CH2 groups more remote from aminophosphonic acid polar head to the global effect described by the enthalpic pair interaction coefficients (hAP,U) decreases with increasing the distance between them. The values of the enthalpic heterogeneous pair interaction coefficients between the dissociated aminophosphonic acid and urea molecule (hAP,U) were confronted with those of the enthalpic heterogeneous pair interaction coefficients between Lα-amino acid zwitterions with urea molecules (hA,U)28 (Table

ΔsolHm0(W + U)/(kJ·mol−1)

m −1

mol(U)·kg (W) 0 0.5 1 1.5 2 2.5 3

Gly 13.32 12.26 11.16 10.97 10.54 10.66 10.24

P

± ± ± ± ± ± ±

0.09 0.10 0.09 0.09 0.08 0.09 0.08

AlaP

HalaP

± ± ± ± ± ± ±

−0.31 ± 0.03

4.04 3.16 2.79 2.64 2.32 1.92 1.79

0.06 0.10 0.12 0.08 0.07 0.06 0.07

−1.17 ± 0.05 −1.58 ± 0.06 −1.88 ± 0.10

and urea molecules, hAP,U,U is the enthalpic triplet interaction coefficient, describing interactions between three molecules, and mU is the molal concentration of urea. According to eq 1 the equations describing the values of the standard dissolution enthalpies of studied aminophosphonic acids in water and in aqueous urea solutions (Table 5) in the function of urea concentration have been determined: (Gly P)Δsol H 0 m = 13320 − 2303mU + 449mU 2 (R2 = 0.9679) (Ala P)Δsol H 0 m = 4041 − 1332mU + 200mU 2 (R2 = 0.9794) (Hala P)Δsol H 0 m = −310 − 951mU + 144mU 2 (R2 = 0.9956)

The enthalpic heterogeneous interaction coefficients determined are given in Table 6. The enthalpic interaction coefficients of three compounds, beside the interactions between three molecules, also comprise the interactions that can proceed in the system of three Table 6. Values of the Heterogenous Pair and Triplet Interaction Coefficients for Aminophosphonic Acids with Urea in Water (hAP,U) and Heterogenous Pair Interaction Coefficients for Natural Amino Acids with Urea in Water (hA,U)28

GlyP AlaP HalaP

hAP,U

hAP,U,U

hA,U

J·kg·mol−2

J·kg2·mol−2

J·kg·mol−2

CH2

−1151 ± 92 −666 ± 69 −476 ± 32

150 ± 24 67 ± 18 48 ± 8

−390.2 −238.2 −185

0.5 1.5 2.5

Gly Ala Aba (Hala)

in 3.0000 mol(U)·kg−1(W)

ΔsolH0m

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Figure 1. Dependences of enthalpic pair coefficient interaction molecules of aminophosphonic acid and urea molecules on the number of CH2 groups in aminophosphonic side chains.

6). The dependence presented is rectilinear (see Figure 2) with coefficient R2 = 0.9994. It indicates the same change tendency

Figure 2. Relationship between the enthalpic pair interaction coefficients (hAP,U) of aminophosphonic acid zwitterions−urea molecules in water and the enthalpic pair interaction coefficients (hA,U) of natural amino acid zwitterions−urea molecules28 in water.

testifying to similar contributions made by the side amino acid substituents in the molecules discussed to the values of the enthalpic interaction coefficients described by the heterogeneous pair interaction coefficients: (hAP,U) and (hA,U).

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CONCLUSIONS The obtained enthalpic pair interaction coefficients between the dissociated aminophosphonic acids and urea molecules let us distinguish the aminophosphonic acid side chains with respect of their affinity toward water, determining their hydrophobic properties. Further investigations of aminophosphonic acids facilitate better understanding of the mechanism of interaction between aminophosphonic acids and the components of human and animal physiological fluids.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.P.). Notes

The authors declare no competing financial interest. 1518

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(22) Jezowska-Bojczuk, M.; Kiss, T.; Kozlowski, H.; Decock, P.; Barycki, J. Complexes of Aminophosphonates. Part 8. Copper(II) Complexes of N-(Phosphonomethyl)amino Acids and Related Compounds. J. Chem. Soc., Dalton Trans. 1994, 6, 811−817. (23) Palecz, B. The enthalpies of interaction of glycine with some amides and ureas in water at 25 °C. J. Solution Chem. 1995, 24, 537− 50. (24) Waliszewski, D.; Stepniak, I.; Piekarski, H.; Lewandowski, A. Heat capacities of liquids and their heats of solution in molecular liquids. Thermochim. Acta 2005, 433, 149−152. (25) Desnoyers, J. E.; Perron, G.; Avedikian, L.; Morel, J.-P. Enthalpies of the urea-tert-butanol-water system at 25 °C. J. Solution Chem. 1976, 5, 631−644. (26) Wood, R. H.; Hiltzik, L. H. Enthalpies of dilution of aqueous solution of formamide, acetamide, propionamide, and N,N-dimethylformamide. J. Solution Chem. 1980, 9, 45−57. (27) Palecz, B. Thermochemical properties of L-α-amino acids in electrolyte-water mixtures. Fluid Phase Equilib. 2000, 167, 253−261. (28) Palecz, B. Enthalpic pair interaction coefficient between zwitterions of L-α-amino acids and urea molecule as a hydrophobicity parameter of amino acid side chains. J. Am. Chem. Soc. 2005, 127, 17768−17771.

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