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Complexation of putrescine with copper(II), zinc(II), lead(II) and magnesium(II) in aqueous solution
Polyhedron Vol. 8. No. 22, pp. 2645-2648, Printed in Great Bntain
1989 0
S3.00f.00 0277-5387/89 1989 Pergamon Press plc
COMPLEXATION OF PUTRESCINE WITH COPPER@), ZINC(II), LEAD(II) AND MAGNESIUM(I1) IN AQUEOUS SOLUTION LECHOSLAW
tOMOZIK*
and ANNA
WOJCIECHOWSKA
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland (Received 23 January 1989 ; accepted 19 May 1989) Abstract-On
the basis of potentiometric and spectral studies the presence of two types of the CuL*(OH)+ complex, where L = putrescine, was detected. In one complex two nitrogen atoms of the first ligand and one nitrogen atom of the second ligand are involved in the coordination, whereas in the other complex both nitrogen atoms of each amine take part in the coordination. At pH ca 7, the formation of the CuHL3+ complex was proved. The stability constant of the CuL,(OH)+ complex was determined.
Putrescine (1 ,Cdiaminobutane), spermidine (4azooctane- 1,8-diamine) and spermine (4,9-diazododecane- 1,12-diamine) are biogenic amines which occur in the cells of living organisms and participate in the genetic information transfer, interacting in the protonated forms (as cations) with negativelycharged fragments of nucleic acid chains. I-7 However, the mechanism of this reaction still remains to be explained, as well as the character of the processes between the metal ions and nucleic acid molecules. Moreover, the possibilities of formation of metal-polyamine complexes and the reactions of such compounds with electron-donor fragments of DNA and RNA should also be considered. In the cases of spermine and spermidine it was found that they formed coordination compounds with metals in aqueous solution,5-7 yet it is still uncertain whether these compounds can also be given by putrescine. Some authors would rather suggest that putrescine does not coordinate metal ions in solution,7’8 though its complex compounds were isolated from absolute ethanol. 9 Results of our studies on complexing properties of this amine with some metal cations contradict such a suggestion. Instead they testify to the fact that under certain conditions putrescine forms coordination compounds in aqueous solution. Results of the studies on complex formation of
*Author to whom correspondence
should be addressed.
putrescine with Cu2+, Zn’+, Pb2+ and Mg2+ are presented in this work. EXPERIMENTAL Putrescine dihydrochloride (Institute of Bioorganic Chemistry of Polish Academy of Sciences in Poznan) was used in these studies. Results of the elementary analysis are in agreement with the theoretical composition of the compound. Copper was used in the form of the perchlorate, the preparation of which was described earlier. ‘O Perchlorates of the other metals used in these studies were obtained likewise. Potentiometric measurements were carried out using a Radiometer TTT85 titrator with an autobiurette ABU80 ; a previously calibrated electrode GK 2401 C ’ ’ was also applied. The concentration of putrescine in all the titration systems was 1.00 x lo- 3 mol dme3. All titrations were performed under an argon atmosphere at ionic strength p = 0.1 (NaClO,), at 293 f 1 K. NaOH (0.0465 mol dme3) free of CO2 was used as the titrant. The pH values at which turbidity appears for the systems with copper( zinc(II), lead(II), magnesium(I1) and putrescine were measured on a spectral calorimeter SPECOLCarl Zeiss Jena fitted with an attachment TK for solutions with composition and concentrations identical to those used in the potentiometric studies.
L. EOMOZIK
2646
and A. WOJCIECHOWSKA
In turbidity measurements, ligand solutions of the same concentration as those of metal-containing systems were used as reference. The turbidity was assumed to appear at a pH value for which the declination of a pointer was observed at a maximum amplification obtainable with this equipment. W-vis measurements were performed on a UVvis SHIMADZU 160 spectrometer. The concentrations of putrescine and Cu2+ in the samples were q,, = 3.00 x 10m3 mol dm- 3, ccu2+= 5.19 x lop4 mol dmp3; cht = 3.00x lop3 mol dm-3, cc-z+ = 1.14x 10-3moldmp3. The model discrimination and determination of complex stability constants were carried out on EMC-RIAD 32 and AMSTRAD PC-l 512 computers using adopted versions of SCOGS’2 and MINIQUAD ’ 3 programs, whereas the form distribution in the system was calculated using the HALTAFALL14 program. RESULTS
AND DISCUSSION
Complexing abilities of putrescine (Put) in comparison with other biogenic amines (spermine, spermidine) are definitely weaker. This is due to the formation of a seven-membered chelate ring, which is very unfavourable because of the spatial arrangement and also energetic reasons. However, it is difficult to agree with some authors who claim that the above facts exclude the formation of coordination compounds of 1,Cdiaminobutane. Let us add that there is still a possibility of this compound forming with the participation of only one nitrogen atom in the complexation. It should be remembered that putrescine compounds may be formed with metal ions in the solid phase.8 Moreover, during the complexation of spermine with Cu2+, sevenmembered rings’ (in addition to six-membered rings) are also formed. The first step in the studies on the putrescinemetal system was to determine the ligand association constants (Table 1). Then mixtures of
(1 .OOx lo- 3 mol dme3) with Cu2+ were titrated. The studies were performed for a few concentrations of copper ions at cation-ligand ratios ranging from 1: 6 to 1 : 8.5. Titration at relatively higher concentrations of metal prevented the application of the potentiometric method due to precipitation at a relatively low pH. In all the experiments performed, this process was observed from pH cu 7.0. On further addition of base the precipitate gradually dissolved (at pH ca 9.0 the solution became clear). For the calculations, points from a titration curve were used with a pH range for which no precipitate was recorded. Limiting values of this range were determined by measuring the turbidity described in the Experimental. The putrescine
initial formation of the precipitate was also determined on the grounds of measurements of the pH.
An inflection on the titration curve corresponds to the time of precipitate formation. The solid state formation is undoubtedly related to the reaction type : M2++xH
2
O--M(OH),“-X+xHt
(the coordination compound can also undergo hydrolysis) which results in an increase in the acidity of the medium. The above observations are consistent with the results of turbidity studies. We distinguished two sets of points on the titration curve. One set included the points within the pH range from 3.0 to 7.0 (“before precipitation”) and the other set included the points for the pH range from 9 to 12 (“after precipitation”). Both sets were treated in the computer analysis as one series of measurements. The formation of only one complex: ML,(OH), logb,_,, = 0.065, was detected in the system (Table 1). The complexation takes place from a pH of about 9.0 (Fig. 1). This complex practically binds all the metal from a pH of about 10.5. A computer analysis proved no complexation at the pH before precipitation, despite the fact that in
Table 1. Stability constants log&,, of putrescine complexes with H+ and Cu2+ for the Cu,H,L, type %
0
Put--cu*+
0 1
2 1 -1
1 1 2
20.51 (0.01) 10.83 (0.01) 0.065 (0.06)
50.
(21 HL
40- (3) L
6
“The value of R obtained for MINIQUAD calculations was 0.024. For the hydroxo complex ML,(OH)+ its stability constant /l = &JKw (in our calculations pK, = 14.167).
7
8
9
IO
II
PH
Fig. 1. Distribution diagram of the Put-Cu*+ system; percentages of the species refer to total metal except for the metal-free forms which refer to total ligand.
Complexation of putrescine
IO-
4L
IO
05
15
2.0
v[crd]
I!?&. 2. Titcatian curves af 551putrescke,:
$21putrescke
with Cu2+.
thi> range the pH value for the ti-tration carve of-the Cu*+-ligand system is lower than for the titration curve of the ligand alone (Fig. 2). Additional information about the system was obtained by electron spectroscopy measurements. At pH 10.0 (significant concentration of ML,(OHl+, Fig. I), the maximum absorption of the system PutCu was found for A = 634.5 nm. ‘Tire calculated value of the molar absorbance a&iSdent of the compSex was E = 195 brn3 rno\Y ’ cm- ’ (Table 2). The maximum corresponds to the case of three coordinated nitrogen atoms. 5,7,’ 5 At pH 11.6 the maximum of absorption is shifted to A = 601 nm (E = 93 dm3 mall’ cm-i). The ML,(OH)+ complex still dominates in the system (Fig. 1) but, as it follows from spectral measurements, its coordination is different. The maximum absorption at A = 601 nm corresponds most likely to tetracoordination by the nitrogen atoms. Thus, we may conclude that putrescine forms the ML,(OH) complex with copper from a pH of ca
2647
9.0. One of the attached ligands forms a sevenmembered ring with the metal and the other coordinates only with one nitrogen atom. At a pH of ca 11.6 two chelate rings are formed in the ML,(OH)+ complex. Electron spectroscopy was also performed for the Put-Cu2+ system at pH ranging from 3.0 to 7.0. From a pH of ca 3 to ca 6.8 the position of the absorption maximum of this system (754 nm) indicates the presence of uncoordinated copper. At pH 7.0 (Table 2) the maximum absorption at I = 705 nm was observed. This testifies to the presence of a complex with only one coordinated nitrogen atom, i.e. a compound of the MHL type without 2 &%W? Z+ls &.%i?& a S-&Wl&r B c’lkr ‘ir &?@BZment with the observed shift of the titration curve of Put-cL12+ with respect to the titration curve of the ligand. It should be emphasized that the UV-vis meaSuerrleritS ofthe PriKYrl~ + %iystern Were carried out together with a study of copper(I1) perchlorate alone under the same conditions as for the mixture with the ligand. The maximum absorption values for the Cu2+ ion (hydrated) are found at wavelengths which are markedly longer than for PutCu” (Table 2). In order to solve the problem of Put-Cu2+ complex formation in solution over precipitate formation, tie Put-W+ sys2em wjti a pmrestinc concentration of 3.00 x 1O-3 mol dmp3 and a Cu2+ concentration of 1.14 x lo- 3 mol drn- 3 (which expanded the range of pH “with precipitate” from 6.4 to 12.0) was subjected to the UV-vis investigation. A solution of pH 11.6 was filtered and UV vis measurements were taken. Then the solution of pH 11.6 was acidified until the required pH was reached (10.4,10.0,8.7,7.0), filtered and the absorption spectra of the clear solutions were recorded (Table 2). Rmaxvalues for the obtained series of results are almost the same as the results obtained
Table 2. Results of UV-vis measurements for the system Put-&‘+. (a) cfit = 3.00 x 10-3, ccu2+= 5.19 x lop4 mol dmm3; (b) cput= 3.00 x 10p3, cCU2+ = 1.14x 10-3moldm-3; (c) cc-~+=5.19x 10-4moldm-3.
PI-I
1max (nm)
7.0 8.7
705.0 647.5
0.010” 0.034
10.0 10.4 11.6
634.5 630.5 601.5
0.044 0.042 0.037
1 A
(dm3 moI”- ’ cm-‘)
Traces of precipitate 105.0 105.0 92.75
2;)
A
maX (nm)
705.0 649.0
0.008 0.022
754.0 752.0
633.5 631.0 603.0
0.044 0.039 0.050
640.5 645.0 638.0
“At pH 7 the presence of the CuHL3+ complex was not proved using a potentiometric method.
2648
L. LOMOZIK
and A. WOJCIECHOWSKA
for a system of lower concentration of Cu’+ (for a smaller pH range of precipitate formation). Hence it should be assumed that the complexing equilibria for both cases examined are analogous, which means that “over the precipitate” the MHL3+ complex and the ML,(OH)+ complex are formed at a pH of cu 7.0 and a pH of ca 9.0, respectively. The MHL3+ formation [as it can be concluded from distribution curves (Fig. l)] proceeds as follows : H,L*+ +Cu2+ _
CUE+
+H+.
It should be added that the results of spectral studies do not exclude the formation of the ML*+ complex (non-chelate with coordination by one nitrogen only). Due to the low concentration of the compound formed at pH ca 7.0 it was not possible to prove the formation of a protonated compound using computer analysis, or to determine its stability constant. Consequently, we could not present it on a distribution curve. Since the value of pK for putrescine is 9.68 it is difficult to assume that a simple complex (at pH = 7.0) may be formed in which putrescine behaves as a monodentate ligand and where the other donor atom is deprotonated. Under the conditions of ML,(OH)+ complex formation both the HL+ (dominates at pH about 10.0) and L (dominates at pH above 11.O) forms may take part in its formation. In the latter case amine is completely deprotonated, which undoubtedly facilitates coordination with all nitrogen atoms (H+ does not compete with a metal cation). These observations testify to the fact that two different forms of the ML,(OH)+ complex may occur (as it follows from spectral data). An attempt to determine the structure of the precipitate formed in the system led us to a conclusion that it contains mainly hydroxo-compounds of copper and a certain amount of the amine examined. However, we have failed to prove unambiguously the composition of the precipitate on the grounds of elementary analysis and spectral investigations. Potentiometric studies on putrescine complexation with Pb2+, Zn2+ and Mg2+ have also been carried out. In the case of the Mg2+ ion, for-
mation of coordination compounds in solution should be excluded (titration curve of Put-Mg2+ system overlaps completely with the titration curve of a ligand in the whole acidity range-pH from 3.0 to ca 11.0). On the other hand, in the case of the Pb” and Zn” ions even at metal-ligand ratios equal to 1 : 9, a precipitate appears in the solution (at a pH of ca 7.8 and 8.2, respectively) and it does not dissolve even after further additions of NaOH. For these two systems it seems more reasonable to conclude that the presence of Pb” and Zn” complexes with putrescine cannot be proved using potentiometric methods, rather than to say that such complexes do not occur in solution. Acknowledgements-This work was financially supported by the Polish Academy of Sciences, Project RPBP 0 1.6. We thank Professor Maciej Wiewiorowski for helpful discussions.
REFERENCES 1. U. Bachrach, Function of Naturally Occurring Polyamines. Academic Press, New York (1973). 2. S. S. Cohen, Introduction to the Polyamines. PrenticeHall, Englewood Cliffs, New Jersey (1971). 3. C. W. Tabor and H. Tabor, Ann. Rev. Biochem. 1984, 53, 749. 4. B. N. Palmer and H. K. I. Powell, J. Chem. Sot., Dalton Trans. 1974,2086. 5. B. N. Palmer and H. K. I. Powell, J. Chem. Sot., Dalton Trans. 1974, 2089. 6. A. Anichinii, L. Fabrizzi, R. Barbucci and A. Mastroiani, J. Chem. Sot., Dalton Trans. 1977,2224. 7. D. M. Templeton and B. Sarkar, Can. J. Chem. 1985, 63, 3112. 8. C. R. Bertsch, W. C. Fernelius and B. P. Block, J. Phys. Chem. 1958,62,444. 9. P. Pfeiffer, Naturwiss. 1948, 35, 190. 10. L. Lomozik, Monatsh. Chem. 1984,115,261. 11. H. M. Irving, M. G. Miles and L. D. Pettit, Anal. Chim. Acta 1967,38,475. 12. I. G. Sayce, Talanta 1968, 15, 1397. 13. A. Sabatini, A. Vacca and P. Gans, Talanta 1974, 21, 53. 14. N. Ingri, W. Kakolowicz, L. G. Sillen and B. Warnqvist, Talanta 1967, 14, 1261. 15. R. Boggs and J. Donohue, Acta Cryst. 1975, B31, 320.
1989 0
S3.00f.00 0277-5387/89 1989 Pergamon Press plc
COMPLEXATION OF PUTRESCINE WITH COPPER@), ZINC(II), LEAD(II) AND MAGNESIUM(I1) IN AQUEOUS SOLUTION LECHOSLAW
tOMOZIK*
and ANNA
WOJCIECHOWSKA
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland (Received 23 January 1989 ; accepted 19 May 1989) Abstract-On
the basis of potentiometric and spectral studies the presence of two types of the CuL*(OH)+ complex, where L = putrescine, was detected. In one complex two nitrogen atoms of the first ligand and one nitrogen atom of the second ligand are involved in the coordination, whereas in the other complex both nitrogen atoms of each amine take part in the coordination. At pH ca 7, the formation of the CuHL3+ complex was proved. The stability constant of the CuL,(OH)+ complex was determined.
Putrescine (1 ,Cdiaminobutane), spermidine (4azooctane- 1,8-diamine) and spermine (4,9-diazododecane- 1,12-diamine) are biogenic amines which occur in the cells of living organisms and participate in the genetic information transfer, interacting in the protonated forms (as cations) with negativelycharged fragments of nucleic acid chains. I-7 However, the mechanism of this reaction still remains to be explained, as well as the character of the processes between the metal ions and nucleic acid molecules. Moreover, the possibilities of formation of metal-polyamine complexes and the reactions of such compounds with electron-donor fragments of DNA and RNA should also be considered. In the cases of spermine and spermidine it was found that they formed coordination compounds with metals in aqueous solution,5-7 yet it is still uncertain whether these compounds can also be given by putrescine. Some authors would rather suggest that putrescine does not coordinate metal ions in solution,7’8 though its complex compounds were isolated from absolute ethanol. 9 Results of our studies on complexing properties of this amine with some metal cations contradict such a suggestion. Instead they testify to the fact that under certain conditions putrescine forms coordination compounds in aqueous solution. Results of the studies on complex formation of
*Author to whom correspondence
should be addressed.
putrescine with Cu2+, Zn’+, Pb2+ and Mg2+ are presented in this work. EXPERIMENTAL Putrescine dihydrochloride (Institute of Bioorganic Chemistry of Polish Academy of Sciences in Poznan) was used in these studies. Results of the elementary analysis are in agreement with the theoretical composition of the compound. Copper was used in the form of the perchlorate, the preparation of which was described earlier. ‘O Perchlorates of the other metals used in these studies were obtained likewise. Potentiometric measurements were carried out using a Radiometer TTT85 titrator with an autobiurette ABU80 ; a previously calibrated electrode GK 2401 C ’ ’ was also applied. The concentration of putrescine in all the titration systems was 1.00 x lo- 3 mol dme3. All titrations were performed under an argon atmosphere at ionic strength p = 0.1 (NaClO,), at 293 f 1 K. NaOH (0.0465 mol dme3) free of CO2 was used as the titrant. The pH values at which turbidity appears for the systems with copper( zinc(II), lead(II), magnesium(I1) and putrescine were measured on a spectral calorimeter SPECOLCarl Zeiss Jena fitted with an attachment TK for solutions with composition and concentrations identical to those used in the potentiometric studies.
L. EOMOZIK
2646
and A. WOJCIECHOWSKA
In turbidity measurements, ligand solutions of the same concentration as those of metal-containing systems were used as reference. The turbidity was assumed to appear at a pH value for which the declination of a pointer was observed at a maximum amplification obtainable with this equipment. W-vis measurements were performed on a UVvis SHIMADZU 160 spectrometer. The concentrations of putrescine and Cu2+ in the samples were q,, = 3.00 x 10m3 mol dm- 3, ccu2+= 5.19 x lop4 mol dmp3; cht = 3.00x lop3 mol dm-3, cc-z+ = 1.14x 10-3moldmp3. The model discrimination and determination of complex stability constants were carried out on EMC-RIAD 32 and AMSTRAD PC-l 512 computers using adopted versions of SCOGS’2 and MINIQUAD ’ 3 programs, whereas the form distribution in the system was calculated using the HALTAFALL14 program. RESULTS
AND DISCUSSION
Complexing abilities of putrescine (Put) in comparison with other biogenic amines (spermine, spermidine) are definitely weaker. This is due to the formation of a seven-membered chelate ring, which is very unfavourable because of the spatial arrangement and also energetic reasons. However, it is difficult to agree with some authors who claim that the above facts exclude the formation of coordination compounds of 1,Cdiaminobutane. Let us add that there is still a possibility of this compound forming with the participation of only one nitrogen atom in the complexation. It should be remembered that putrescine compounds may be formed with metal ions in the solid phase.8 Moreover, during the complexation of spermine with Cu2+, sevenmembered rings’ (in addition to six-membered rings) are also formed. The first step in the studies on the putrescinemetal system was to determine the ligand association constants (Table 1). Then mixtures of
(1 .OOx lo- 3 mol dme3) with Cu2+ were titrated. The studies were performed for a few concentrations of copper ions at cation-ligand ratios ranging from 1: 6 to 1 : 8.5. Titration at relatively higher concentrations of metal prevented the application of the potentiometric method due to precipitation at a relatively low pH. In all the experiments performed, this process was observed from pH cu 7.0. On further addition of base the precipitate gradually dissolved (at pH ca 9.0 the solution became clear). For the calculations, points from a titration curve were used with a pH range for which no precipitate was recorded. Limiting values of this range were determined by measuring the turbidity described in the Experimental. The putrescine
initial formation of the precipitate was also determined on the grounds of measurements of the pH.
An inflection on the titration curve corresponds to the time of precipitate formation. The solid state formation is undoubtedly related to the reaction type : M2++xH
2
O--M(OH),“-X+xHt
(the coordination compound can also undergo hydrolysis) which results in an increase in the acidity of the medium. The above observations are consistent with the results of turbidity studies. We distinguished two sets of points on the titration curve. One set included the points within the pH range from 3.0 to 7.0 (“before precipitation”) and the other set included the points for the pH range from 9 to 12 (“after precipitation”). Both sets were treated in the computer analysis as one series of measurements. The formation of only one complex: ML,(OH), logb,_,, = 0.065, was detected in the system (Table 1). The complexation takes place from a pH of about 9.0 (Fig. 1). This complex practically binds all the metal from a pH of about 10.5. A computer analysis proved no complexation at the pH before precipitation, despite the fact that in
Table 1. Stability constants log&,, of putrescine complexes with H+ and Cu2+ for the Cu,H,L, type %
0
Put--cu*+
0 1
2 1 -1
1 1 2
20.51 (0.01) 10.83 (0.01) 0.065 (0.06)
50.
(21 HL
40- (3) L
6
“The value of R obtained for MINIQUAD calculations was 0.024. For the hydroxo complex ML,(OH)+ its stability constant /l = &JKw (in our calculations pK, = 14.167).
7
8
9
IO
II
PH
Fig. 1. Distribution diagram of the Put-Cu*+ system; percentages of the species refer to total metal except for the metal-free forms which refer to total ligand.
Complexation of putrescine
IO-
4L
IO
05
15
2.0
v[crd]
I!?&. 2. Titcatian curves af 551putrescke,:
$21putrescke
with Cu2+.
thi> range the pH value for the ti-tration carve of-the Cu*+-ligand system is lower than for the titration curve of the ligand alone (Fig. 2). Additional information about the system was obtained by electron spectroscopy measurements. At pH 10.0 (significant concentration of ML,(OHl+, Fig. I), the maximum absorption of the system PutCu was found for A = 634.5 nm. ‘Tire calculated value of the molar absorbance a&iSdent of the compSex was E = 195 brn3 rno\Y ’ cm- ’ (Table 2). The maximum corresponds to the case of three coordinated nitrogen atoms. 5,7,’ 5 At pH 11.6 the maximum of absorption is shifted to A = 601 nm (E = 93 dm3 mall’ cm-i). The ML,(OH)+ complex still dominates in the system (Fig. 1) but, as it follows from spectral measurements, its coordination is different. The maximum absorption at A = 601 nm corresponds most likely to tetracoordination by the nitrogen atoms. Thus, we may conclude that putrescine forms the ML,(OH) complex with copper from a pH of ca
2647
9.0. One of the attached ligands forms a sevenmembered ring with the metal and the other coordinates only with one nitrogen atom. At a pH of ca 11.6 two chelate rings are formed in the ML,(OH)+ complex. Electron spectroscopy was also performed for the Put-Cu2+ system at pH ranging from 3.0 to 7.0. From a pH of ca 3 to ca 6.8 the position of the absorption maximum of this system (754 nm) indicates the presence of uncoordinated copper. At pH 7.0 (Table 2) the maximum absorption at I = 705 nm was observed. This testifies to the presence of a complex with only one coordinated nitrogen atom, i.e. a compound of the MHL type without 2 &%W? Z+ls &.%i?& a S-&Wl&r B c’lkr ‘ir &?@BZment with the observed shift of the titration curve of Put-cL12+ with respect to the titration curve of the ligand. It should be emphasized that the UV-vis meaSuerrleritS ofthe PriKYrl~ + %iystern Were carried out together with a study of copper(I1) perchlorate alone under the same conditions as for the mixture with the ligand. The maximum absorption values for the Cu2+ ion (hydrated) are found at wavelengths which are markedly longer than for PutCu” (Table 2). In order to solve the problem of Put-Cu2+ complex formation in solution over precipitate formation, tie Put-W+ sys2em wjti a pmrestinc concentration of 3.00 x 1O-3 mol dmp3 and a Cu2+ concentration of 1.14 x lo- 3 mol drn- 3 (which expanded the range of pH “with precipitate” from 6.4 to 12.0) was subjected to the UV-vis investigation. A solution of pH 11.6 was filtered and UV vis measurements were taken. Then the solution of pH 11.6 was acidified until the required pH was reached (10.4,10.0,8.7,7.0), filtered and the absorption spectra of the clear solutions were recorded (Table 2). Rmaxvalues for the obtained series of results are almost the same as the results obtained
Table 2. Results of UV-vis measurements for the system Put-&‘+. (a) cfit = 3.00 x 10-3, ccu2+= 5.19 x lop4 mol dmm3; (b) cput= 3.00 x 10p3, cCU2+ = 1.14x 10-3moldm-3; (c) cc-~+=5.19x 10-4moldm-3.
PI-I
1max (nm)
7.0 8.7
705.0 647.5
0.010” 0.034
10.0 10.4 11.6
634.5 630.5 601.5
0.044 0.042 0.037
1 A
(dm3 moI”- ’ cm-‘)
Traces of precipitate 105.0 105.0 92.75
2;)
A
maX (nm)
705.0 649.0
0.008 0.022
754.0 752.0
633.5 631.0 603.0
0.044 0.039 0.050
640.5 645.0 638.0
“At pH 7 the presence of the CuHL3+ complex was not proved using a potentiometric method.
2648
L. LOMOZIK
and A. WOJCIECHOWSKA
for a system of lower concentration of Cu’+ (for a smaller pH range of precipitate formation). Hence it should be assumed that the complexing equilibria for both cases examined are analogous, which means that “over the precipitate” the MHL3+ complex and the ML,(OH)+ complex are formed at a pH of cu 7.0 and a pH of ca 9.0, respectively. The MHL3+ formation [as it can be concluded from distribution curves (Fig. l)] proceeds as follows : H,L*+ +Cu2+ _
CUE+
+H+.
It should be added that the results of spectral studies do not exclude the formation of the ML*+ complex (non-chelate with coordination by one nitrogen only). Due to the low concentration of the compound formed at pH ca 7.0 it was not possible to prove the formation of a protonated compound using computer analysis, or to determine its stability constant. Consequently, we could not present it on a distribution curve. Since the value of pK for putrescine is 9.68 it is difficult to assume that a simple complex (at pH = 7.0) may be formed in which putrescine behaves as a monodentate ligand and where the other donor atom is deprotonated. Under the conditions of ML,(OH)+ complex formation both the HL+ (dominates at pH about 10.0) and L (dominates at pH above 11.O) forms may take part in its formation. In the latter case amine is completely deprotonated, which undoubtedly facilitates coordination with all nitrogen atoms (H+ does not compete with a metal cation). These observations testify to the fact that two different forms of the ML,(OH)+ complex may occur (as it follows from spectral data). An attempt to determine the structure of the precipitate formed in the system led us to a conclusion that it contains mainly hydroxo-compounds of copper and a certain amount of the amine examined. However, we have failed to prove unambiguously the composition of the precipitate on the grounds of elementary analysis and spectral investigations. Potentiometric studies on putrescine complexation with Pb2+, Zn2+ and Mg2+ have also been carried out. In the case of the Mg2+ ion, for-
mation of coordination compounds in solution should be excluded (titration curve of Put-Mg2+ system overlaps completely with the titration curve of a ligand in the whole acidity range-pH from 3.0 to ca 11.0). On the other hand, in the case of the Pb” and Zn” ions even at metal-ligand ratios equal to 1 : 9, a precipitate appears in the solution (at a pH of ca 7.8 and 8.2, respectively) and it does not dissolve even after further additions of NaOH. For these two systems it seems more reasonable to conclude that the presence of Pb” and Zn” complexes with putrescine cannot be proved using potentiometric methods, rather than to say that such complexes do not occur in solution. Acknowledgements-This work was financially supported by the Polish Academy of Sciences, Project RPBP 0 1.6. We thank Professor Maciej Wiewiorowski for helpful discussions.
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