Stereoselectivity of N-carboxymethyl-amino acid complexes of copper(II) toward optically active amino acids

J. inorg, nucl. Chem., 1973, Vol. 35, pp. 523-535.

Pergamon Press.

Printed in Great Britain

STEREOSELECTIVITY OF N-CARBOXYMETHYLAMINO ACID COMPLEXES OF COPPER(II) TOWARD OPTICALLY ACTIVE AMINO ACIDS R I C H A R D V. SNYDER* and ROBERT J. A N G E L I C I t Department of Chemistry, Iowa State University of Science and Technology, Ames, Iowa 50010 (Received 25 M a y 1972)

Abstract-The stability constants for a series of mixed, labile copper(ll) complexes of amino acids and amino acid derivatives are reported. These constants indicate that Cu(ll) complexes of Ncarboxymethyl-L-valine, -L-isoleucine, or -L-serine or Cu(II) complexes of N-benzyl-N-carboxymethyI-L-alanine or -L-leucine coordinate L-valine, L-leucine, or L-threonine more strongly than their D isomers. N-carboxymethyI-L-aspartic acid, -L-glutamic acid and -D-valine show opposite preferences. These results are discussed in terms of amine proton interactions between the ligands bound to the copper. INTRODUCTION

reported that the complex N-carboxymethyl-L-valinato-copper (!I), (N-Cm-L-Val)Cu, generally formed more stable complexes with L-amino acidates than with the D enantiomers [1]. That is, the stability constants, Kx, for the reaction (N-Cm-L-Val)Cu + A- = (N-Cm-L-Val)Cu(A)- were larger for the L-isomers of the amino acidates (A-). This preferential coordination by the complexes was later utilized to partially resolve racemi¢ mixtures of amino acids using column chromatography by binding the complex to a styrene-divinylbenzene ¢opolymer[2]. While these are not the first, there are few reports of stereoselectivity involving amino acids and labile metal ions. Wellman e t al. [3] concluded that little interaction existed between the two amino acids in his(amino a¢idato)copper(II) complexes. G i l l e d et al.[4, 5] demonstrated that no stereoselectivity existed for a series of bidentate, bis(amino acidato)copper(ll) complexes. Bennett[B] reported that the asparagine complexes, (D-AspNH2).,Cu and (L-AspNH2)zCu, were favored over (D-AspNH2) (L-AspNH2)Cu, but a subsequent report by Wiegers et al. concluded there was little, if any, stereoselectivity [7]. In a study of several metal ions and d, 1- or m e s o diaminobutanetetraaceti¢ acid by Sharpe and Irving[8], the complex of the d,l PREVIOUSLY w e

* National Science Foundation Trainee, 1968-69. tAlfred P. Sloan Fellow, 1970-72. 1. 2. 3. 4. 5. 6. 7. 8.

B. E. Leach and R. J. Angelici, J. A rn. chem. Soc. 91, 6296 (1969). R. V. Snyder, R. J. Angelici and R. E. M e c k , J . A m . chem. Soc. 94, 2660 (1972). K. M. Wellman, T. G. Mecca, W. Mungall and C. R. H u e , J. A rn. chem. Soc. 89, 3646 (1967). R. D. Gillard, H. M. Irving, R. M. Parkins, N. C. Payne and L. D. Pettit, J. chem. Soc. (A), 1159 (1966). R. D. Gillard, H. M. Irving and L. D. Pettit, J. chem. Soc. (N) 673 (1968). W. E. Bennett, J. A rn. chem. Soc. 81,246 ( 1959). G. A. Wiegers, F. Jellinek andJ. H. Ritsma, Rec. Tray. Chirn. 84, 1577 (1965). K. Sharpe and H. M. Irving, J. inorg, nucl. C h e m . 33, 203 (1971). 523

524

R I C H A R D V. SNYDER and ROBERT J. ANGELIC1

iigand was shown to be significantly more stable that that of the m e s o isomer. However, these complexes contain one ligand and two chiral centers rather than two ligands with one chiral center each. Histidine (Hist) has provided the most consistent source of stereoselective reactions of the common amino acids. In 1963, McDonald and Phillips showed by NMR methods [9] that Co(L-Hist)(D-Hist) was favored over [Co(D- or LHist)2]. Ritsma et al., confirmed this using potentiometric methods[10]. The work [ 11] of Barnes and Pettit in 1970 showed that [ ( D - H i s t ) ( L - H i s t ) M ] was preferred for M = Zn or Ni, but [(L- or D-Hist)2M] was preferred for M --- Cu. Similar conclusions were reached by other workers [ 12, 13]. Hix and Jones [ 14] concluded that there was no preference by D- or L-histidinatonickel(II) for the methyl ester of either D- or L-histidine, but hydrolysis of the ester occurred faster when the ester and the acid were both of the same chirality. Because stereoselectivity in labile metal systems had been fairly rare, and because we had observed it in our chromatographic work, it was decided to determine potentiometric stability constants for a series of N-carboxymethyl-amino acidato copper(II) complexes with a variety of optically active amino acids. We report here the results of these potentiometric studies and offer some explanations for the observations in terms of the stereochemistry of the copper complexes formed. EXPERIMENTAL Materials All materials were reagent grade, unless otherwise noted. Practical grade benzaldehyde was purified by distillation under N2 at atmospheric pressure and was stored in an air-tight bottle at 0°C. Potassium nitrate and potassium hydrogen phthalate were dried at 100°C before use. Nuclear magnetic resonance spectra were measured on a Varian Associates A-60 or Hitachi Perkin-Elmer R20B spectrometer. Computer programs were written in Fortran 1V for an 1BM 360. Preparation o f N-carboxymethyI-D- or L-amino acids Adapting a procedure originally given by Leach[15], 50mmoles of bromoacetic acid and 50 mmoles of L-aspartic acid (L-Asp) were separately neutralized with 7M NaOH to a pH of 11 (the bromoacetic acid at 0°C). These solutions were then stirred together at 50°C, and more NaOH was added as needed to maintain a pH of 11. When the reaction ceased (after about 90 rain and no further NaOH was required), enough 4M HCI was added to bring the pH to 2. The solution was then loaded onto a 2.5 × 81 cm column of Dowex 50W-X4 (100-200 mesh) cation exchange resin in the H + form and eluted with 240 ml of water (one column volume) followed by 0.50M NH,. A flow rate of about 1 ml/min was used, and 15 ml fractions were collected. Anions (mostly C1- from added HCI) passed directly through the column, cations (mostly Na + from NaOH) adhered to the resin displacing H +, and the desired product (N-carboxymethyl-L-aspartic acid, N-Cm-L-Asp)[16] was displaced from the column by ammonia. Fractions containing the purified product had a pH between 2 and 4, and these were saved. They were mixed together and reduced in volume at 50°C under water aspirator 9. 10. 11. 12. 13. 14. 15. 16.

C. C. McDonald and W. D. Phillips, J. Am. chem. Soc. 85, 3736 (1963). J. H. Ritsma, J. C. Van De Grampel and F. Jellinek, Rec. Tray. Chim. 88, 411 (1969). D.S. Barnes and L. D. Pettit, Chem. Comm. 1000 (1970). P.J. Morris and R. B. Martin,J. inorg, nucl. Chem. 32, 2891 (1970). D. S. Barnes and L. D. Pettit, J. inorg, nucl. Chem. 33, 2177 (1971). J. E. Hix, Jr. and M. M. Jones, J. Am. chem. Soc. 90, 1723 (1968). B. E. Leach and R. J. Angelici, lnorg. Chem. 8, 907 (I 969). Abbreviation of the N-carboxymethyl group will be N-Cm instead o f - M A (for monoacetate) as in earlier publications (Refs.[1,2 and 15].

N-carboxymethyl-amino acid complexes

525

vacuum until crystals began to form. Addition of ethanol equal to three times the solution volume induced further precipitation. After filtering, the precipitate was washed with alcohol and ether and dried under vacuum. Addition of ether to the filtrate from the first filtration produced more product. Total yield was about 80%. The analogous derivatives of D-valine (D-VaIL L-serine (L-Ser), L-glutamic acid (L-GIu), L-valine (L-Val) and L-isoleucine (L-lieu) were prepared in a similar manner. In the purification of the less soluble amino acid derivative N-Cm-L-Glu the NH3 concentration was reduced to 0-25 M, while for N-Cm-L-Val, N-Cm-L-Ser and N-Cm-L-lleu, 0.75M NH3 could be used. Characterization of the products was accomplished by N M R and elemental analysis (see Tables 1 and 2). To increase the solubilities to a convenient level for nmr observation, enough NaOD was added to the D..,O solutions to neutralize all acid protons. Table 1. Analytical data for ligands Compound N-Cm-L-Val N-Cm-D-Val N-Cm-L-Asp N-Cm-L-GIu N-Cm-L-Ser N-Cm-L-lleu

N-Bz-N-Cm-L-AIa. ½H20 N-Bz-N-Cm-L-Leu. ½H20

%C 47"88 47.88 37.66 40.98 36.61 50.23 58.52 62.48

Calculated %H %N

%C

Found %H %N

7.46 7-46 4-74 5-40 5-53 7-90 6.14 7.34

47'67 47.90 37.59 40.84 36-47 50.74 58.68 62-54

7.36 7.54 4.84 5-48 5.73 7.65 6.34 7.29

7-98 7.98 7.32 6.83 8.54 7-32 5.69 4.86

8.21 8-00 7.15 6-86 9.16 7,21 5.40 5.02

Preparation of N-benzyl-N-carboxymethyl-L-alanine (N-Bz-N-Cm-L-AIa) and N-benzyl-N-carboxyrnethyl-L-leucine (N-Bz-N-Cm-L-Leu) Following a procedure given by Volger et al. [17] 5.3g (50 m-moles) of benzaldehyde was added to a stirred solution of 6'55g (50 m-moles) of L-Leu and 25 ml of 2M NaOH. After 15 min the solution had become homogeneous and was cooled on an ice bath. While keeping the temperature below 15°C, 0'57g (15 m-moles) of NaBH4 was added in small portions followed by 30 rain of stirring. The additions of benzaldehyde and NaBH4 were repeated with the same quantities as before followed by 2 more hrs of stirring. At this time 4M HCI was added until the pH reached 7. The product, Nbenzyl-L-leucine (N-Bz-L-Leu) precipitated from solution, was washed with a small amount of cold water, and was dried under vacuum overnight. The yield based on leucine was 91%. To complete the preparation, 10.41g (49.1 m-moles) of N-Bz-L-Leu was dissolved in water by adding IM NaOH until the pH reached 11. To this, 6.83g (49.1 m-moles) of bromoacetic acid previously dissolved and neutralized to a pH of 11 at 0°C was added with constant stirring at 60°C. After the addition of one equivalent of base to maintain constant pH (about 2.75 hr), 4M HCI was added until the pH reached 2. The product (N-Bz-N-Cm-L-Leu.½H20) precipitated and was allowed to stand for 3 hr in the pH 2 solution. The product was then filtered, washed with H20 and dried under vacuum overnight. Addition of ethanol to the filtrate precipitated additional product. Total yield was 94%. N-Bz-N-Cm-L-AIa.½ H20 was prepared similarly. Both products were characterized by analysis and NMR.

Titrations Titrations were performed at 25°C under constant stirring and nitrogen flush in a 70 ml waterjacketed container. Carbonate-free base was added in equal increments of 0.05 or 0.10 ml from a 5-00 ml buret calibrated in divisions of 0.01 ml. The pH of the solutions was measured with a Beckman Zeromatic SS-3 pH meter and associated glass and calomel electrodes. The meter was standardized vs two standard buffers (either 4.01 and 7.00 or 7.00 and 10.00) before each titration. Titration of a known concentration of strong acid with strong base of known concentration showed less than 0-03 pH units variance between measured and calculated pH, so it was assumed that the meter read concentration directly between pH 2 and 10. 17. P. Quirt, J. Hellerbach and K. Volger, Hel. Chim. Acta 46; 327 (1963).

526

RICHARD V, SNYDER and ROBERT J. ANGELICI

Table 2. Proton NMR data Compound B A CH(CH3)2 NHC CO2H

I

Proton

Chemical shift*

Multiplicity

A B C D

1.67 2.18 3.51 3.61

doublet multiplet doublet singlet

B C D

2.35 3.30 3.09

three peaks quartet singlet

A B C

2"35 2.35 3.70 3.63

multiplet multiplet triplet singlet

3.85 3.97

singlet obscured singlet

D E F

1.01 2.00 ca. 3.85 3-80 1.41 0.97

doublet multiplet obscured singlet multiplet triplet

B C D G H

1-23 3.37 3.15 3"70 7.33

doublet multiplet singlet singlet singlet

B C D E F G H

1.48 3.28 3-13 1.48 0.85 3-82 7-28

multiplet multiplet doublet multiplet doublet doublet singlet

c

CH2CO2H D B

CH2CO2H NH HCO2H ~HzCo, H D B

A

CH~CH2COzH I NHCHCO2H C H2CO2H D B CH2OH J N H ~ H C O~H C ~H2CO2H D B A E F HC(CH3)CH2CH3

~

D B C D

A B

I

C

NHCHCO2H C H~CO ~H D H

G

CH3~ CoH5CH2 NtLI4CO2H ~H2CO2 H D B

E

F

H G CH~CH(CHah| Ca H5CH2N~HCO~H ] C CH2CO2 H D

ca. 3.96

*Relative to (CH3hSiCH2CH2CH2SOa Na in D20/NaOD solution.

N-carboxymethyl-amino acid complexes

527

The solutions to be titrated were prepared by adding equal molar quantities of 0.1M Cu(NOa)2 and optically pure samples of either N-carboxymethyl- or N-benzyl-N-carboxymethyl-amino acids (either as 0.1M solutions or as the solid) and sufficient 0.2OOM KNOa and water to make a total volume of 40.00 ml and an ionic strength of 0.10M based on Cu z+, NOa- and K + ions assuming no initial complex formation. Copper and amino acid derivative concentrations were typically about 7 × 10-aM. After titrating with NaOH (about 0.16M) to the neutralization point, one equivalent of either the L or D isomer of an amino acid was added as a 0-1M solution, and then the titration was continued. Titrations to determine proton stability constants were performed in a similar manner except that the Cu(NOa)z was eliminated. In all cases data from four separate titrations were used to calculate each constant. The concentration of the solutions used to prepare the titration solutions was either calculated from the weight of the material used (as in the case of amino acids or amino acid derivatives) or was determined by titrametric standardization (as in the case of NaOH using KHP or Cu(NO3)2 using EDTA)[18]. Due to low solubility, N-Bz-N-Cm-L-Leu and N-Cm-L-GIu were each weighed directly before each titration, and solutions of N-Bz-N-Cm-L-AIa and N-Cm-L-Asp were prepared at less than 0.1M concentrations.

Data treatment Data from the titrations of amino acids (HA) or their N-carboxymethyl derivatives (H.,AD) were used to calculate a series of proton stability constants K for the following type of reaction: H + H,,AD (or A) = H.+tAD (or HA), where charges have been omitted for convenience. Up to four such equilibria might be considered for one compound, but in all cases the values of the K's differed sufficiently so that some constants could be considered independently, while others required only two simultaneous equilibria. If one defines the step-wise proton stability constants (totaling m in number) as K . = [AH.]/ [AH._I][H], where A H . - t is the conjugate base of the acid A H . , and At is defined as total amino acid concentration present in solution (At = ~,~"=0AH.), and Ht is designated as total titratable hydrogen concentration present in solution ( H t - - - H + ~ = 0 n A H . ) , then by simplifying the At and H, equations so all terms in A appear separately on one side, dividing one equation by the other, and n then simplifying further, the following results: E.m=ofln[nAt-(Ht-H)]H~= 0, where ft. ~ , / T ~=l K~. Each titration produced a series of independent linear equations (one for each point in the titrationl which were solved in groups of m for the/3's by Gaussian elimination. These were then used to calculate the K values (K. = fl./fl._j ). In dealing with the complex formations, it was possible to consider only one equilibria at a time. either ( l or 2). Cu + AD ~' Cu(AD)

(1

Cu(AD) + A ~ Cu(AD)(A).

(2)

Using the same notations as above plus Cut equaling total copper present, two equations for the average number of A's per Cu (which were derived from expressions of At and Cut) were set equal to each other and then solved for either K s or Kx. The final expression took the form K = [At-AE~_0fl,,H~]/A(Cut-- At + AE~= 0/3,,Hn). A can be calculated from a consideration of At, the acid stability constants, and the pH. Each point in the titration yielded one value for K, and the average of all K's was reported by the computer. In practice the first and last 20% of the titrations were discarded in all calculations. In each case concentrations were corrected for volume changes. When titrating beyond a pH of 7, base was required in order to raise the pH of the solution irrespective of what was dissolved in it. To correct for this use of base, 40.00 ml of 0.100M KNO3 was titrated with base in 0.01 ml intervals, and these pH and volume readings were used in other titrations above pH 7 to compensate for base needed to raise the pH of the water. This was accomplished by adding extra base at each pH in amounts determined from the KNO3 titration. RESULTS AND DISCUSSION

The preparation of the N-carboxymethyl-amino acid derivatives was accomplished as previously described [15], but the isolation employed ion exchange 18. H. A. Flasehka, In EDTA Titrations: An Introduction to Theory and Practice. Pergamon Press, Oxford (1959).

528

R I C H A R D V. S N Y D E R and ROBERT J. A N G E L I C I

chromatography instead of fractional crystallization. The product binds to the resin in the cationic form (HOzCCHzNH~CH+(R)COzH), but it is easily displaced by the more basic ammonia. The amount of the product displaced per volume of eluant is determined by the concentration of the ammonia. At high concentration the quantity of the product displaced may exceed its solubility in H20 and precipitate on the column. Thus, lower ammonia concentrations (0-25M) were used for the less soluble products (see Experimental). The progress of the product on the column was followed by measuring the pH of the fractions collected. The pH of the first few column volumes was very acid (pH of 0.5-1) as the cations added with the base during the preparation displaced hydrogen ions from the resin. Once these displaced protons were eluted, the pH rose to around 5 which ideally would be 7, but a small fraction (less than 5%) of the product eluted from the column with water as eluant. As the majority of the product eluted from the column ahead of the ammonia, the pH dropped sharply to about 2-4, after which it rose rapidly as the NHz eluted. Any fraction with a pH above 4 was discarded or recycled as it contained ammonia. Titrations of the products showed that up to 5% of the product could be in the sodium salt form instead of the acid form, but this could be reduced to less than 1% with a second pass through the column. The column was regenerated by washing with 4 M HCI. The preparation of the N-benzyl-N-carboxymethyl-amino acid derivatives was accomplished in two steps: first, addition of benzaldehyde to form the Schitf base and reduction by sodium borohydride; second, reaction of the Nbenzyl-amino acid intermediate with bromoacetic acid in basic solution. This method succeeded for L-alanine, L-leucine, and L-phenylalanine, but the second step failed for L-valine. A reasonable explanation for this involves steric crowding by the alpha-side chain which prevents nucleophilic attack by the nitrogen. In a study of Bowman [ 19] of reactions between amino acids and aldehydes which involves nucleophilic attack at carbon, it was shown that valine was less reactive than glycine, leucine or phenylglycine. From our work, two moles of benzaldehyde reacted with glycine to form the N,N-dibenzylglycine, but only one mole reacted with alanine, leucine, valine or phenylalanine to form the N-benzylamino acids even with benzaldehyde in a 2:1 excess. Both these studies indicate that the size of the alpha-side chain on the amino acid affects its reactivity when nucleophilic attack by nitrogen is involved.

pKa results Calculated pKa values for the new ligands together with literature values for related ligands are listed in Table 3. With the possible exception of N-benzyl I M D A [CnH~CHsN(CH~COsH)2] the literature values and ours agree. The standard deviations in pK,1 and pK~2 are generally --+0.03 log units, but the standard deviations of some pKaa and pKa4 values run as high as -+ 0.20 log units. This is understandable since these constants were calculated from titration data obtained near a pH of 1.9 where the electrodes are less dependable. Each reported result is an average of values from four separate titrations, and the errors are given as standard deviations. 19. R. E. Bowman, J. chem. Soc. 1346 (1950).

N-carboxymethyl-amino acid complexes

529

Table 3. pK,, values* Compound

pK~1

pK,2

Glycine IMDA N-Bz-IMDA N-Bz-IMDA N-Cm-L-Val N-Cm-L-Asp N-Cm-L-GIu N-Cm-L-Ser N-Cm-L-11eu N-Bz-N-Cm-L-AIa N-Bz-N-Cm-L-Leu L-Val L-Val L-Thr L-Thr

9.73 9.33 8.90 9-03 9.34 9.82 9.28 8.89 9.34 9.57 9.09 9-63 9.62 9.09 9.03 9.67 9-60

2.34 2.58 2.24 2.29 2.53 3"80 4.16 2-54 2.54 2.07 2.14 2,39 2.32 2,31 2.29 2.46 2,36

L-Leu L-l.eu

pK,z

pK,~4

Ref. [201+ 1211 I22]

1 "36 1 "65

2"44 2 "49 1 "70 1"63 1 "70

1"65 1 "56

1 "70

[241~ 1231§ [2415

*Ionic strength=0.10M KNO:~, T = 25.0°C unless otherwise noted. ?lonic strength = 0M, T = 20°C. 1:lonic strength = 0-01M. §Ionic strength = 0.2M. Stability constants

I t w a s e v i d e n t e a r l y t h a t t h e t w o e q u i l i b r i a d e f i n e d b y r e a c t i o n s (1 a n d 2) ( s e e e x p e r i m e n t a l ) c o u l d b e c o n s i d e r e d i n d e p e n d e n t l y . T h e e v i d e n c e f o r this is s h o w n in Fig. 1, a g r a p h o f p H vs v o l u m e o f N a O H . T h e l o w e r c u r v e in t h e r a n g e f r o m 2 to 3 e q u i v a l e n t s o f b a s e a d d e d s h o w s t h a t L - V a l a d d s to t h e p r e v i o u s l y f o r m e d [ ( N - C m - L - V a l ) C u ] c o m p l e x at a p H w e l l a b o v e t h e p H w h e r e C u 2÷ a n d N - C m - L - V a l c o m b i n e . I n t h e s a m e r a n g e , t h e u p p e r c u r v e i n d i c a t e s h y d r o x o - c o m p l e x f o r m a t i o n , b u t this o c c u r s at a p H a b o v e t h e r a n g e w h e r e t h e L - V a l a d d s to t h e [ ( N - C m - L - V a l ) C u ] c o m p l e x . F o r this r e a s o n it w a s n o t n e c e s s a r y to c o n s i d e r h y d r o x o c o m p l e x e s in t h e c a l c u l a t i o n s o f K~, or gf.

Values for the stability constants Ks for the reaction of either N-Cm- or N-Bz-N-Cm-amino acids and Cu 2÷ are listed in Table 4. Some literature values are also listed for comparison. The Kr values for the N-Cm-amino acid ligands are very similar, but smaller than the N-Bz derivatives. Generally N-benzyl derivatives exhibit lower stabilities toward metals than the parent ligand. For 20. 2 I. 22. 23. 24. 25. 26.

R. M. lzatt, J. J. Christensen and V. Kothari, lnorg. Chem. 3, 1565 (1964). L. C. Thompson, lnorg. Chem. 1,490 (1962). T. Ando, Bull. chem. Soc. Japan 35, 1395 (1962). E. V. Raju and H. B. Mathur, J. inorg, nucl. Chem. 311,2181 (1968). L. E, Maley and D. P. MeUor, A ustr. J. Sci. Res. 2(A), 579 (1949). G. Anderegg, Helv. Chim. Acta 47, 1801 (1964). P. Souchey, N. lsraily and P. Oouzerk, Bull. Soc. chim. Fr. 12, 3917 (1966): R. Herring, J. prakt. Chem. 34, 69 (1966).

530

R I C H A R D V. S N Y D E R and ROBERT J. A N G E L I C I 10.80 I0-00 9"20 8.40

/&

7.60

,'v

6.80 6.00 5.20 4.40 3.60 2.80 2.0£

I I

I 2 Equivalents

I 3

bose

Fig. 1. Titration curves of Cu 2++ N-Cm-L-Val and Cu(N-Cm-L-Val)+ L-Val.

instance, log Ke for alanylglycine and Cu(II) is 5.44[27], but only 4.51 for Nbenzylalanylglycine [28]. And the log K I for Ni plus aspartic acid or N-benzylaspartic acid is 7-1417] and 6-84[29] respectively. There is no obvious explanation for the reverse trend for the ligands reported here. It is interesting to note that both N-Cm-L-GIu and N-Cm-L-Asp form protonated tridentate complexes with copper. Only by treating the data as representing a complex between Cu 2÷ and protonated N-Cm-L-Glu could consistent stability constants be obtained from the calculations. The pKa for the protonated complex was calculated to be 4.54 _-+_-0.09. The proton is believed to reside on the carboxylate group of the side chain since it is the most basic of the three acid functions (pKa = 4.16), and it would be least likely to coordinate to the copper due to the formation of a seven-membered ring. In the case of N-Cm-L-Asp the situation is more complex since neither treating the ligand as tridentate nor tetradentate gave consistent stability constants. It is thus thought that both protonated and unprotonated copper complexes exist in solution in simultaneous equilibrium for N-Cm-L-Asp. Similar behavior was noted by Jellinek et al. in the Cu 2+ complexes of L-Asp and L-Glu [7]. The stability constants K~ [see Eqn (2)] for the reactions of either [Cu(N-CmD or L-amino acid)] or [Cu(N-Bz-N-Cm-L-amino acid)] and various D- or Lamino acids are listed in Table 5. The conclusions of this study are in part based on the difference between two stability constants whose standard deviations are 27. G. F. Bryce, J. M. H. Pinkerton, L. K. Steinrauf and R. N. Gurd, J. Biol. Chem. 240, 3829 (1965). 28. U. I. Salakhutdinov, A. P. Borisona, Y. V. Granovskii, I. A. Savich and V. I. Spitsyn, Proc. .4cad. Sci. USSR 177, 1039 (365) (1967). 29. G. Shtacher, J. inorg, nucl. Chem. 28, 845 (1966).

N-carboxymethyl-amino acid complexes

531

Table 4 Log K[ values* Compound

Log Kf

Ref.

IMDA N-Bz-IMDA N-Bz-IMDA N-Cm-L-lleu N-Cm-L-Ser N-Cm-L-GIu N-Cm-L-Val N-Bz-N-Cm-L-Leu N-Bz-N-Cm-L-AIa

10.63 9.88, 10.29 10.73(3) 10.68(6) 10-93 (6) 10.56(21) 10.97(6) 10.96(10) 12.37(26)

[25]t [2615

*Ionic strength=0.10M KNO3, unless otherwise noted. t T = 20°C. Ionic strength = 0.1M KCI. S.D. given in parentheses.

T = 25.0°C

Table 5. Log Kx values* Compound

N-Cm-D-Val N-Cm-L-Ileu N-Cm-L-Ser N-Cm-L-Asp N-Cm-L-Val N-Cm-L-Glu N-Bz-N-Cm-L-AIa N-Bz-N-Cm-L-Leu

Valine L

D

5.51(3) 5.32(0) 5-46(3) 4.24(2) 5-34(5) 5.08(5) 5.53(1) 5.42(4)

5.57(2) 5-31(1) 5.39(2) 4.39(3) 5.30(2) 5.37(2) 5.45(3) 5.30(5)

L

Threonine D

5.30(1) 5-27(4)

5-17(2) 5.25(6)

5.14(6) 5-53(2) 5-28(4)

L

Leucine D

5.07(7)

5.46(3) 5-43(0) 4.23(2) 5-44(1)

5.35(1) 5,41(1) 4,37(2) 5.35(3)

5-28(2) 5.09(4)

5.63(3) 5.43(2)

5.63(2) 5.23(4)

* Ionic strength = 0" 10M KNOa, T = 25-0°C and the S. D. are given in parentheses.

about the same as the observed differences. The validity of the conclusions based on one pair of these constants would be questionable. However, the consistency in the preference of the L isomer over the D by the L derivatives of the neutral amino acids in both the questionable N-carboxymethyl derivatives and the more certain N-benzyl-N-carboxymethyl derivatives increases the reliability of the conclusions. Thus, the body of data acquires a reliability that an individual datum would not. The constants in Table 5 indicate that when the parent amino acid of the N-Cm or N-Bz-N-Cm derivative has a noncoordinating side chain (as is the case for L-Val, D-Val, L-lieu, L-Ser, L-AIa, or L-Leu), then the complex formed by an amino acid and an N-Cm-amino acid with copper is slightly more stable when the ligands have the same chirality than when they are opposite. This is in general agreement with equilibrium data reported by Leach and Angelici[ 1]; it indicated that [Cu(N-Cm-L-Val)] complexed more strongly to L-isomers than D-isomers for Ser, Phe, Ala, and Leu. The opposite was reported for Val. They

532

RICHARD

V. S N Y D E R

and R O B E R T J. A N G E L I C I

also reported differences in log Kx for D,L-amino acid enantiomers complexing to [Cu(N-Cm-L-Val)] as being on the order of 0.41-0.81. Our data support the general conclusions, but we find the stability constants to be much closer together, and that L-Val not D-Val forms the more stable complex. In our earlier column chromatographic work[2], it was also concluded that L isomers form more stable complexes with N-Cm-L-Val than do the D isomers since the D isomers eluted ahead of the L isomers. The observed stereoselectivity must be explained in terms of the stereochemistry of the complexes. The most likely conformation of the amino acid derivatives bound to copper will be meridional rather than facial due to the planar nature of the Jahn-Teller-distorted Cu 2÷ ion. If one makes a model of this structure, an amino acid coordinating to the remaining sites will experience an essentially symmetrical environment since the two asymmetric centers (C and N) are not close enough to the amino acid for any interaction. However, if one of the carboxylate groups were to move to an axial position, then a coordination site in the square plane which is cis and closer to the asymmetric nitrogen would be available. Justification for this rearrangement can be made on grounds other than accounting for the observed results. The difference between the nonrearranged structure (I) and the rearranged structure (II) is whether the N-carboxymethylamino acid ring will prefer to be facial or meridional. Legg and Cooke concluded after a study of the isomers of [Co(dien)(IMDA) 1+] that I M D A prefers facial over meridional coordination by 2 to 1 due to strain in the I M D A ring[30]. Thus in the copper case the facial isomer could be favored once another bidentate ligand has coordinated in the plane.

°

I

II

There are no data available to indicate whether it is the carboxylate group of the amino acid or of the N-carboxymethyl group that changes to the axial position. However, whichever it is, it would prefer to move only one way; that is, the carboxylate which moves will prefer to move to only one of the two available axial positions. This is a result of the nitrogen being asymmetric. Newman projections looking from the nitrogen toward the asymmetric carbon are shown in (III) and (IV). Structure (IV) offers less steric crowding and should be preferred. Thus, the nitrogen has a preferred conformation, and because of this the carboxylate groups move to only one of the two axial positions. The amino groups of the amino acid will coordinate either cis or trans to the nitrogen of the N-carboxymethyl-amino acid. Support for the cis structure comes 30. J. 1. Legg and D. W. Cooke, lnorg. Chem. 5, 594 (1966).

N-carboxymethyl-amino acid complexes

H7C3._~H -02CHIC ~ "'"~'Cu

co~

533

HTC3._~-OH2CH2C H" ~-"~'Cu

co~

117

rcr

from several bis(amino acidato)copper(II) crystal structures[31-36]. From a carparison of circular dichroism spectra of [Cu(L-Ileu)2], [Cu(L-Val)2], and [Cu (L-Tyr)2] in solution and in the solid state, Gillard concluded[37] that since the two spectra were essentially the same, the structures were the same. He cites a trans structure for [Cu(L-AIa)2] in the article[36], but a study by Weeks[33] on Cu(Ileu)2 showed cis nitrogens and casts doubt on Gillard's conclusions. The majority of evidence indicates a cis structure for bis(amino acidato)copper(lI) complexes, and this will be assumed true for the system under consideration. The proposed structures for [Cu(N-Cm-L-amino acid)(L-amino acid) 1-] are (V) and (VI). H20

_V-'x., X o~:=..:-~--~..-Z-c~O ~7.~,_~-...C,'J~'C~ H N~ ~ R ~ / / ~ . ~ t . . s ~ ' . . ~ m C .-.~ .

/I H20

.

.

.

H ,,~,-- . . . . ~ PIH H--C~' R I

c,,O

.

H ~

r'N"-" H ~ r~

// 0

X7"F

The proposed site of interaction between the two ligands is the amine hydrogens. When the chiralities of the alpha-carbons of the coordinated ligands are the same, then the amine hydrogens are staggered. When the chiralities are opposite, then the amino acid chelate ring will pucker to the opposite side of the Cu plane due to the alpha-group being equatorial, and one of its amine hydrogens will move to eclipse the amine hydrogen on the C-carboxymethyl derivative. Thus, there is more steric interference and subsequently a less stable complex when the chiralities are opposite than when they are the same. The differences in K~ for the two enantiomers for any one amino acid listed in Table 5 is larger for the N-benzyl-N-carboxymethyl derivative than for the N-carboxymethyl derivatives. This observation can be explained by concepts which were developed for the N-carboxymethyl derivatives. The single amine 31. 32. 33. 34. 35.

H. C. Freeman,Adv. Prot. Chem. 22, 257 (1967). D. van der Helm and W. A. Franks,Acta Crystallogr. 2511, 451 (1969). C. M. Weeks, A. Cooper and D. A. Norton, Acta Crystallogr. 25B, 443 (1969). H.C. Freeman and M. R. Snow,Acta Crystallogr. 17, 1463 (1964). H. C. Freeman, J. M. Guss, M. J. Healy, R. P, Martin and C. E. Nockolds, Chem. Comm. 225 (1969). 36. R. D. Gillard, R. Mason, N. C. Payne and G. B. Robertson, J. Chem. Soc. (A) 1864 (1969). 37. R. D. Gillard and S. H. Laurie, Chem. Comm. 489 (1969), 38. A. Dijkstra, Acta Crystallogr. 20, 588 (1966).

534

R I C H A R D V. S N Y D E R and R O B E R T J. A N G E L I C I

proton of the derivatives has been replaced by a benzyl group, and it is the methylene of the benzyl group which makes a closer approach to the amine protons of the amino acid. It is apparent from the results that the benzyl group enhances the effects previously described. The N-benzyl derivatives are important because the Cu(N-Cm-L-Val) complex used in the partial resolution of amino acids was bound by a benzyl linkage at the ligand nitrogen atom. The data here imply that the polystyrene resin matrix does more than simply bind the amino acid derivative; it also enhances the stereoselectivity of the complex. This was also concluded by Davankov[39] in his work with L-proline substituted chloromethylated polystyrene. The two derivatives not discussed so far are N-Cm-L-Asp and N-Cm-L-GIu. These differ from the others because they have potentially coordinating side chains, and as a result of this show a reversed stereoselectivity; that is, their copper complexes bind D-amino acids more strongly than the L-amino acids. Following the same basic approach as before, the tetradentate ligand will coordinate three groups in the plane and the side chain carboxylate to an axial position. Coordination of an amino acid forces the N-carboxymethyl group to an axial position. The coordination of the side chain changes the position of the amine proton relative to the previously discussed amino acid derivatives. An amino acid coordinating cis to the derivative with the same chirality as the alphacarbon of the derivative will have one of its amine protons eclipsed by an amine proton of the derivative [see Structure (VII)]. Opposite amino acid chirality will produce the opposite result (staggered protons). Thus, opposite chiralities of the N-carboxymethyl derivatives and amino acid should be preferred over identical chiralities. o

.-q I-b~C

of-.

.-

/ "c / " "o xTrt

The values of K~ for the reactions of [Cu(N-Cm-L-Asp) 1-] with amino acids are smaller by a factor of ten compared to complexes such as [Cu(N-Cm-LVal)]. The N-Cm-L-Asp complex carries a negative charge instead of being neutral, thus offering electrostatic repulsion to the entering anionic amino acid. This same effect is not observed for [Cu(N-Cm-L-GIu)I-]; it has stability constants typical of neutral complexes. The weakly coordinating nature of the side chain carboxylate has already been mentioned concerning the tridentate nature of N-Cm-L-GIu. However, there must be some interaction (perhaps hydrogen 39. V. A. Davankov and S. V. Rogozhin, J.

Chromatog. 60, 280 (1971).

N-carboxymethyl-amine acid complexes

535

bonding to a coordinated water) between the carboxylate and the axial position of the metal to explain its stereoselectivity which is similar to that of N-Cm-LAsp. Acknowledgement-We wish to acknowledge partial support of this research by the U.S. Public Health Service through grant GM-12626 of the National I nstitute of General Medical Services.