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Complex Formation Between Uranium(VI) Ion and some α-Aminoacids
531
Notes discussed above are probably present in considerable quantity. This is not surprising since (&H&P is much less basic that (CzHs)sN. HARRY H. SISLER* JAMES CLINTON BARRICK Departmentof Chemistry Universityof Florida Gainesville, FL 32611 USA. REFERENCES
‘H. H. Sisler, F. Neth and F. R. Hurley, .r. Am. Chem. Sot. 1954, 163909. *G. Omietanski, A. D. Kelmers, R. W. Shellman and H. H. Sisler, /. Am. Chem. Sot. 1956,?8,3874. *Author to whom correspondence should be addressed.
‘H. H. Sister, G. Omietanski and B. Rudner, Chem. Rev. 1957, 51, 1021. 4R. A. Rowe and L. F. Audrieth, J. Am. Chem. Sot. 1956,78, 563. ‘L. F. Audrieth and L. H. Diamond, .r Am. Chem. Sot. 1954.76, 4860. 61.T. Gilson and H. H. Sisler, Inorg. Chem. 1%5,4, 273. ‘G. Omietanski and H. H. Sisler, J. Am. Chem. Sot. 1956,78, 1211. *H. H. Sisler, A. Sarkis, H. Ahuja, R. S. Drag0 and N. L. Smith, .I. Am. Chem. Sot. 1959,81,2982. 9H. Prakash and H. H. Sisler, Allgem. IL prakt. Chem. 1970, 21(4), 123. ‘OR.Appel and A. Hauss, Chem. Ber. 1960,93,405. “C P Haber, P. L. Herring and E. A. Lawton, J. Am. Chem. Sic. ‘1958,80,2116. “H. H. Sisler, H. S. Ahuja and N. L. Smith, Znorg.Chem. 1%2, 1, 84.
Porrkrdmn Vd. 1, No. 6, pp. 537-539, 1962 Printed in Great Britain.
0?77-5387/82/060537-03$03.00/0 Pergunon Press Ltd.
Complex Formation Between Uranium(V1) Ion and some ahninoacids (Received 18 December 1981) Abstract-The formationconstants for the complexation of the VOP by a number of C-substituted glycines have been determined by potentiometric titrations.
INTRODUCTION Although there have been some reports on aminopolycmboxylic acid complexes with uranyl ion using solvent extraction or spectrophotometric methods’ little work has been published on aaminoacid complexes of uranium using a potentiometric titration technique. Typical of the formation constants found are those for uranyl complexes of nitrilotriacetic acid and glycine reported as Log K = 9.56f 0.03 and 7.53 respectively.2” We report here the results of a study for UO,” complexes of a series of a-aminoacids containing sulphur and oxygen atoms, having certain affinity to chelate with metal ions in cooperation with the aminoacid’s nitrogen. The aminoacids studied were a series of C-substituted glycines of general formula R-CH(NH2) CGGH, where R = -CH*OH, -CHzSH, -CHr, -CH2CH2SCHI, (CH&CH-, (CH&CHCHr and C2H$H(CH3 all able to form five membered chelate rings. For comparison, complexes of serine and cysteine were investigated. Concurrently complexes of serine showed greater stability than those of alanine due to the presence of an OH group in serine which also showed an affinity to form an extra bond with the metal ion. In contrast to valine or leucine, methionine must therefore form more stable complexes due to its molecule containing a sulphur atom. The only example of a related investigation so far reported has been by Cefola et al.* who studied complexes of glycine and UOP ions. Studies of mixed ligand complexes of uranyl ion with aminoacids and carboxylic acids or monocarboxylates have been published recently3-’ each suggesting relatively strong bond formation between uranyl ion and oxygen donors. EXPERlMgNTAL Measurement of protonation and complex formation constants; For the hydrogen and metal ions, formation constants were calculated from potentiometric titration curves’ obtained using a combined electrode calibrated in terms of hydrogen ion
concentration, (H] at 25°C. The electrode was also calibrated by 0.05M KHP before and after each titration and agreement was always better than 0.005PH units. Potentials were measured with a Mettler DK31 diaital oH meter. All solutions were made up in a background 07 KNO, (total I = 0.1 M). The aminoacids under experiment were converted to the fully protonated form by adding the calculated amount of standard nitric acid. The titration cell was thermostatically controlled (25?O.l”C). Carbonate free alkali were added by means of a Mettler DV 210 automatic burette dispensing 0.022 0.001ml of alkali at each reading. The titrations were reproducible to 0.007pH unit throughout. Solutions of 1:2 or I:5 metal to ligand concentration were titrated with alkali and at the beginning of each titration total volume of solution in the cell was always kept at 30ml. At higher pH values, when ligands titrated in the presence of a metal ion precipitation occured. In such instances- therefore, data before -precipitation only were considered and used to calculate KML, and KML, values. Throughout this study uranyl nitrate (Analar) was used for the preparation of metal ion solutions. The aminoacids alanine, methionine, serine, leucine, valine, isoleucine and cycteine were obtained from Merck (Germany). The proton and metal complex formatiom constant were calculated using a Fortran program. RESULTSANDDUXUSSION Calculated proton and uranyl complex formation constants are given in Table 1 with those for comparable ligands. The obtained results for proton complex formation constants show the trends expected from inductive effects of the substituents. This effect is more noticable with the protonation of the amino group (LogfiHL) than the carboxyl group(Log ,¶HzL), since the amine nitrogen is closer to the substituent. The replacement of an a-hydrogen atom with an alkyl is seen to produce a small regular increase in proton complex formation
Notes
538
Table 1. Proton and many1 complex formation constants for the amino acids at 25” and I = 0.1 M. Standard deviation (u values) in parentheses
DSerine DCysteine D-Methionine DL-Alanine DL-Vahne L-Leucine L-Isoleucine
log 8 HL 9.16(4) 9.12*
log B H2L 11.509(9)
8.244(4) 8.331 8.921(3) 9.052$ 9.592(3) 9.603(2) 9.621(2) 9.652(4)
10.552(8) 10.50t 11.271(7) 11.203$ 12.067(8) 12.242(9) 12.266(6) 12.350(9)
log
B 1
b-a2
8.66(3)
14.66(5)
5.84(2)
11.85(5)
6.41(l)
13.38(2)
7.33(2) 7.lo(lj 7.13(3) 7.02(3)
14.97(4) 14.72(2) 14.36(7) 14.66(8)
*Ref. [6]. tRef. [7]. *Ref. [8].
constant. This increase is to be expected as a result of the positive inductive effect of the aIky1 groups causing electron repulsion and therefore tending to make the a-carbon atom negatively charged. This effect will be transmitted to both the nitrogen atom and the carboxyl group resulting a decrease in acidity. The amino acids containing a sulphur or oxygen donor atom have lower proton complex formation constants than those of the other amino acids investigated in this work; even a-alanine with a simillar molecular structure forms a stronger proton complex than serine A Log /3HL = 0.43) with cysteine having a (A Log f3HL = 1.35). This is due to the large negative inductive effect of the attached corboxyl or sulphydryl groups. The possibility of a steric effect has been demonstrated by Martell et al? for L-Cysteine with Log PHL = 8.13 and DL-penicillamine Log 8 = 7.88 in 0.1 M potassium nitrate. For methionine the values are higher than those of cysteine due to a lower negative inductive effect of -SMe group. The proton complex formation constants for the series was found to be Dcysteine < Dmethionine < Dserine < DL-a-alanine < DL valine < L-leucine < L-isoleucine, respectively. With UOr*’ all ligands produced a light yellow coloured solution and the solubility decreased as the alkyl chain increased in length. Mixtures of UOz*+and serine were found to be the most soluble of all. The dearee of formation (A) of the amino acid complexes with uranyiion generally approached values 0.2-1.5 and complexes of serine were found to be markedly more stable, with A values approaching 1.85 and formation constants comparable to those previously reported for iminodiacetic acid” (Log f3= 8.93). Above A= 1.5 there was some evidence of the formation of tris-complexes but since the maximum M: L ratio used was 1:3 the formation constants calculated were not sufficiently reliable. A maximum coordination number of 8 has been suggested for uranium ion,” therefore a maximum of three bidentated ligand molecules are sutlicient to coordinate with UOr’+ to reach the specified coordination number. A quantitative comparison of the values for log BHrL-log /32 shows that the affinity of the ligands for protons and uranyl ion is comparable whetherthe ligand contains-extra donor atoms (S or 0) or not. The existing small differences can be explained in terms of steric factors and the fact that formation of a metal
*Author to whom correspondence should be addressed.
complex involves chelate formation of the N-U-O type while the proton complexes involve formation of separate N-H and @H bonds. In comparing the results for serine with those of alanine the proton complexes of serine (IogflHrL) tend to be weaker than those of alanine by 1.26log units while the metal complexes are only marginally weaker (e.g. A Log t3 = 0.31). This can be regarded as an indication of extra bonding in serine. A similar comparison of results for cysteine and those for alanine suggests only limited bonding between the metal ions and the sulphur atom. Methionine forms complexes more stable than cysteine, this could be explained in terms of the difference of inductive effects in the two molecules. Uranyl complex formation constants for other amino-acids, investigated in this work are also given in Table 1 followed by the pattern expected from the varying inductive effects of the substituent groups. Acknowledgement-The authors should like to thank Mrs. A. Elhami for her assistance during the course of the investigation. M. NOURMAND* N. MEISSAMI Nuclear Research Centre, P.O. Box 3327 Tehran Iran
REFRRRNCRS
‘M. J. C. Nastasi and F. A. Lima, I. Radioanalytical Chem. 1972, 35, 289. 2M. Cefola, R. C. Taylor, P. S. Gentile and A. V. Celiano, J. Phys. Chem. 1%2,66,790. 3P. V. Selvaraj and M. Santappa, J. fnorg, Nucl. Chem. 1977,39, 119. 4P. V. Selvaraj and M. Santappa, J. Inorg. Nucl. Chem. 197738, 837. ‘L. Magon, R. Protanova, B. Zarli and A. Bismondo, J. Jnorg. Nucl. Chem. 1972,34, 1971. 6E. V. Raju and M. B. Mathur, J. Inorg. Nucl. Chem. 1968,30, 2181. ‘D. D. Perrin and I. J. Sayce, J. Chem. Sot. (A), 1%8,53. *M. Israeli and L. D. Pet&, J. Inorg. Nucl. them. 1975,37,999. ‘G. R. Lenz and A. E. Martell. Biochem. J. 1964.3.745. ‘OK.S. Rajan and A. E. Mart& J. Jnorg. Nucl. &em. 1964,X 789. “S. H. Ebele, Komplexverbindungen der Acticiden mit organishen liganden. K.F.K 1136,Karlsruher, Oct. (1970).
Notes discussed above are probably present in considerable quantity. This is not surprising since (&H&P is much less basic that (CzHs)sN. HARRY H. SISLER* JAMES CLINTON BARRICK Departmentof Chemistry Universityof Florida Gainesville, FL 32611 USA. REFERENCES
‘H. H. Sisler, F. Neth and F. R. Hurley, .r. Am. Chem. Sot. 1954, 163909. *G. Omietanski, A. D. Kelmers, R. W. Shellman and H. H. Sisler, /. Am. Chem. Sot. 1956,?8,3874. *Author to whom correspondence should be addressed.
‘H. H. Sister, G. Omietanski and B. Rudner, Chem. Rev. 1957, 51, 1021. 4R. A. Rowe and L. F. Audrieth, J. Am. Chem. Sot. 1956,78, 563. ‘L. F. Audrieth and L. H. Diamond, .r Am. Chem. Sot. 1954.76, 4860. 61.T. Gilson and H. H. Sisler, Inorg. Chem. 1%5,4, 273. ‘G. Omietanski and H. H. Sisler, J. Am. Chem. Sot. 1956,78, 1211. *H. H. Sisler, A. Sarkis, H. Ahuja, R. S. Drag0 and N. L. Smith, .I. Am. Chem. Sot. 1959,81,2982. 9H. Prakash and H. H. Sisler, Allgem. IL prakt. Chem. 1970, 21(4), 123. ‘OR.Appel and A. Hauss, Chem. Ber. 1960,93,405. “C P Haber, P. L. Herring and E. A. Lawton, J. Am. Chem. Sic. ‘1958,80,2116. “H. H. Sisler, H. S. Ahuja and N. L. Smith, Znorg.Chem. 1%2, 1, 84.
Porrkrdmn Vd. 1, No. 6, pp. 537-539, 1962 Printed in Great Britain.
0?77-5387/82/060537-03$03.00/0 Pergunon Press Ltd.
Complex Formation Between Uranium(V1) Ion and some ahninoacids (Received 18 December 1981) Abstract-The formationconstants for the complexation of the VOP by a number of C-substituted glycines have been determined by potentiometric titrations.
INTRODUCTION Although there have been some reports on aminopolycmboxylic acid complexes with uranyl ion using solvent extraction or spectrophotometric methods’ little work has been published on aaminoacid complexes of uranium using a potentiometric titration technique. Typical of the formation constants found are those for uranyl complexes of nitrilotriacetic acid and glycine reported as Log K = 9.56f 0.03 and 7.53 respectively.2” We report here the results of a study for UO,” complexes of a series of a-aminoacids containing sulphur and oxygen atoms, having certain affinity to chelate with metal ions in cooperation with the aminoacid’s nitrogen. The aminoacids studied were a series of C-substituted glycines of general formula R-CH(NH2) CGGH, where R = -CH*OH, -CHzSH, -CHr, -CH2CH2SCHI, (CH&CH-, (CH&CHCHr and C2H$H(CH3 all able to form five membered chelate rings. For comparison, complexes of serine and cysteine were investigated. Concurrently complexes of serine showed greater stability than those of alanine due to the presence of an OH group in serine which also showed an affinity to form an extra bond with the metal ion. In contrast to valine or leucine, methionine must therefore form more stable complexes due to its molecule containing a sulphur atom. The only example of a related investigation so far reported has been by Cefola et al.* who studied complexes of glycine and UOP ions. Studies of mixed ligand complexes of uranyl ion with aminoacids and carboxylic acids or monocarboxylates have been published recently3-’ each suggesting relatively strong bond formation between uranyl ion and oxygen donors. EXPERlMgNTAL Measurement of protonation and complex formation constants; For the hydrogen and metal ions, formation constants were calculated from potentiometric titration curves’ obtained using a combined electrode calibrated in terms of hydrogen ion
concentration, (H] at 25°C. The electrode was also calibrated by 0.05M KHP before and after each titration and agreement was always better than 0.005PH units. Potentials were measured with a Mettler DK31 diaital oH meter. All solutions were made up in a background 07 KNO, (total I = 0.1 M). The aminoacids under experiment were converted to the fully protonated form by adding the calculated amount of standard nitric acid. The titration cell was thermostatically controlled (25?O.l”C). Carbonate free alkali were added by means of a Mettler DV 210 automatic burette dispensing 0.022 0.001ml of alkali at each reading. The titrations were reproducible to 0.007pH unit throughout. Solutions of 1:2 or I:5 metal to ligand concentration were titrated with alkali and at the beginning of each titration total volume of solution in the cell was always kept at 30ml. At higher pH values, when ligands titrated in the presence of a metal ion precipitation occured. In such instances- therefore, data before -precipitation only were considered and used to calculate KML, and KML, values. Throughout this study uranyl nitrate (Analar) was used for the preparation of metal ion solutions. The aminoacids alanine, methionine, serine, leucine, valine, isoleucine and cycteine were obtained from Merck (Germany). The proton and metal complex formatiom constant were calculated using a Fortran program. RESULTSANDDUXUSSION Calculated proton and uranyl complex formation constants are given in Table 1 with those for comparable ligands. The obtained results for proton complex formation constants show the trends expected from inductive effects of the substituents. This effect is more noticable with the protonation of the amino group (LogfiHL) than the carboxyl group(Log ,¶HzL), since the amine nitrogen is closer to the substituent. The replacement of an a-hydrogen atom with an alkyl is seen to produce a small regular increase in proton complex formation
Notes
538
Table 1. Proton and many1 complex formation constants for the amino acids at 25” and I = 0.1 M. Standard deviation (u values) in parentheses
DSerine DCysteine D-Methionine DL-Alanine DL-Vahne L-Leucine L-Isoleucine
log 8 HL 9.16(4) 9.12*
log B H2L 11.509(9)
8.244(4) 8.331 8.921(3) 9.052$ 9.592(3) 9.603(2) 9.621(2) 9.652(4)
10.552(8) 10.50t 11.271(7) 11.203$ 12.067(8) 12.242(9) 12.266(6) 12.350(9)
log
B 1
b-a2
8.66(3)
14.66(5)
5.84(2)
11.85(5)
6.41(l)
13.38(2)
7.33(2) 7.lo(lj 7.13(3) 7.02(3)
14.97(4) 14.72(2) 14.36(7) 14.66(8)
*Ref. [6]. tRef. [7]. *Ref. [8].
constant. This increase is to be expected as a result of the positive inductive effect of the aIky1 groups causing electron repulsion and therefore tending to make the a-carbon atom negatively charged. This effect will be transmitted to both the nitrogen atom and the carboxyl group resulting a decrease in acidity. The amino acids containing a sulphur or oxygen donor atom have lower proton complex formation constants than those of the other amino acids investigated in this work; even a-alanine with a simillar molecular structure forms a stronger proton complex than serine A Log /3HL = 0.43) with cysteine having a (A Log f3HL = 1.35). This is due to the large negative inductive effect of the attached corboxyl or sulphydryl groups. The possibility of a steric effect has been demonstrated by Martell et al? for L-Cysteine with Log PHL = 8.13 and DL-penicillamine Log 8 = 7.88 in 0.1 M potassium nitrate. For methionine the values are higher than those of cysteine due to a lower negative inductive effect of -SMe group. The proton complex formation constants for the series was found to be Dcysteine < Dmethionine < Dserine < DL-a-alanine < DL valine < L-leucine < L-isoleucine, respectively. With UOr*’ all ligands produced a light yellow coloured solution and the solubility decreased as the alkyl chain increased in length. Mixtures of UOz*+and serine were found to be the most soluble of all. The dearee of formation (A) of the amino acid complexes with uranyiion generally approached values 0.2-1.5 and complexes of serine were found to be markedly more stable, with A values approaching 1.85 and formation constants comparable to those previously reported for iminodiacetic acid” (Log f3= 8.93). Above A= 1.5 there was some evidence of the formation of tris-complexes but since the maximum M: L ratio used was 1:3 the formation constants calculated were not sufficiently reliable. A maximum coordination number of 8 has been suggested for uranium ion,” therefore a maximum of three bidentated ligand molecules are sutlicient to coordinate with UOr’+ to reach the specified coordination number. A quantitative comparison of the values for log BHrL-log /32 shows that the affinity of the ligands for protons and uranyl ion is comparable whetherthe ligand contains-extra donor atoms (S or 0) or not. The existing small differences can be explained in terms of steric factors and the fact that formation of a metal
*Author to whom correspondence should be addressed.
complex involves chelate formation of the N-U-O type while the proton complexes involve formation of separate N-H and @H bonds. In comparing the results for serine with those of alanine the proton complexes of serine (IogflHrL) tend to be weaker than those of alanine by 1.26log units while the metal complexes are only marginally weaker (e.g. A Log t3 = 0.31). This can be regarded as an indication of extra bonding in serine. A similar comparison of results for cysteine and those for alanine suggests only limited bonding between the metal ions and the sulphur atom. Methionine forms complexes more stable than cysteine, this could be explained in terms of the difference of inductive effects in the two molecules. Uranyl complex formation constants for other amino-acids, investigated in this work are also given in Table 1 followed by the pattern expected from the varying inductive effects of the substituent groups. Acknowledgement-The authors should like to thank Mrs. A. Elhami for her assistance during the course of the investigation. M. NOURMAND* N. MEISSAMI Nuclear Research Centre, P.O. Box 3327 Tehran Iran
REFRRRNCRS
‘M. J. C. Nastasi and F. A. Lima, I. Radioanalytical Chem. 1972, 35, 289. 2M. Cefola, R. C. Taylor, P. S. Gentile and A. V. Celiano, J. Phys. Chem. 1%2,66,790. 3P. V. Selvaraj and M. Santappa, J. fnorg, Nucl. Chem. 1977,39, 119. 4P. V. Selvaraj and M. Santappa, J. Inorg. Nucl. Chem. 197738, 837. ‘L. Magon, R. Protanova, B. Zarli and A. Bismondo, J. Jnorg. Nucl. Chem. 1972,34, 1971. 6E. V. Raju and M. B. Mathur, J. Inorg. Nucl. Chem. 1968,30, 2181. ‘D. D. Perrin and I. J. Sayce, J. Chem. Sot. (A), 1%8,53. *M. Israeli and L. D. Pet&, J. Inorg. Nucl. them. 1975,37,999. ‘G. R. Lenz and A. E. Martell. Biochem. J. 1964.3.745. ‘OK.S. Rajan and A. E. Mart& J. Jnorg. Nucl. &em. 1964,X 789. “S. H. Ebele, Komplexverbindungen der Acticiden mit organishen liganden. K.F.K 1136,Karlsruher, Oct. (1970).