6096
The Formation of Peptide Bonds in the Coordination Sphere of Cobalt (111) James P. Collman3 and Eiichi Kimura Contribution from Venable Laboratory, Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514. Received June 9, 1967 Abstract: Peptide formation in the coordination sphere of cobalt(II1) is observed in the reaction of cis-[Co(trien)C1~1C1with glycine esters to form ci~-P-[Co(trien)(glyglyOR)]~+. Similar peptide and glycinamide complexes were prepared from cis-[Co(trien)Clz1C1and glycylglycine esters or glycinamides. Other peptide complexes, [Co(trien)(glyglyNHz)13+,[C~(trien)(glyglyglyOR)]~+, and [Co(trien)(glyalaOR)I3+,were prepared from the intermediate, cis-P-[Co(trien)(glyOCzH~)Cl]*+.The peptide complexes are shown to be intermediates in the previously
described4peptide hydrolysis using [Co(trien)(OH)(OH,)]*+.
T
etramine-cobalt(II1) complexes with two reactive each of these examples a single chloride ion in the cocoordination sites react with peptides in aqueous ordination sphere is replaced by an amino group. solution in such a way that the N-terminal amino acid Reaction of cis-[Co(trien)C12]+(111) with a series of residue is hydrolytically cleaved from the peptide chain glycine esters in D M F or DMSO did not afford the to form a chelate ring with the complex ion.: We expected cis-[C~(trien)(glyOR)Cl]~+ but instead gave have been studying this reaction because of its relecomplexes ~is-[Co(trien)(glyglyOR]~+ (V) in which the vance to mechanisms of reactions of coordinated ligands glycylglycine ester acts as a chelate bonded to cobalt and its potential as a selective method of peptide modithrough the terminal amino group and the amide carfication. bonyl oxygen (Figure 2). During the course of our Complexes such as cis-[Co(en)~(0H)(OH~)]~+,~ cisinvestigation certain of these peptide complexes were /3-[Co(trien)(OH)(OHz)]2+,4 ~is-[Co(tren)(OH)(OH~)]~+,7independently prepared by Buckingham, et al. 1 2 , 1 3 and [Co(cyclen)(OH)(OH2)]2+ have been found to be Herein are described the synthesis and characterization effective in this stoichiometric peptide hydro1ysis.O of these novel complexes as well as implications of these We now report related reactions whereby metal ions results to the mechanism of the peptide hydrolysis reacpromote the formation of peptide bonds through cotions. ordination of ester carbonyl groups. This discovery emanated from our attempt to prepare Discussion cis-chlorotriethylenetetramine-glycine ester complexes, When an aqueous suspension of the sparingly soluble ci~-[Co(trien)(glyOR)Cl]~+. Alexander and Busch'O violet cis-p-[Co(trien)+ClZ](111) was treated with methyl had previously reported the reaction of aliphatic amines or ethyl glycinate at room temperature, the violet crystals and glycine esters with trans-[Co(en)2Cln]+ to yield dissolved to form an orange solution. Addition of cis-[C~(en)~(RNH~)Cl]~+ and cis-[C~(en)~(glyOR)Cl]~+. acetone resulted in the precipitation of a methanol We found the tren analog, [Co(tren)Cl2]+(I), also forms soluble, hygroscopic orange solid which was then con[Co(tren)(glyOR)CI]*+(11) upon treatment with methyl verted into a crystalline perchlorate salt of V. The and ethyl glycinates (Figure 1). In the trien series same dipeptide complexes were obtained in higher yields Pearson, Boston, and Basolo" had prepared cisusing D M F or DMSO as the reaction medium. In [Co(trien)(NH3)C1I2+ from a similar reaction with DMSO the reaction is almost instantaneous at 50". ammonia. We have found that n-butylamine forms Perchlorate salts of these dipeptide complexes are cis-[C~(trien)(n-C~HgN")Cl]~+ (IV) (Figure 1). In sufficiently stable to be recrystallized from hot water. (1) This work was supported by the National Institutes of Health The structures which are assigned to the dipeptide under Grant GM08350. Portions of this work were presented at the ester complexes (Va-c) are based on their syntheses, 153rd National Meeting of the American Chemical Society, Miami, chemical reactions, molar conductivities, and infrared, Fla., April 1967, Abstract L 139. (2) Abstracted from the Ph.D. dissertation of E. Kimura, University visible, and proton magnetic resonance spectra, and of North Carolina, 1967. have gained support from an X-ray diffraction study. l 4 (3) Department of Chemistry, Stanford University, Stanford, Calif. (4) J. P. Collman and D. A. Buckingham, J . A m . Chem. SOC.,85, The dipeptide complexes V were prepared inde3039 (1963). pendently from the corresponding glycylglycine esters ( 5 ) D. A. Buckingham, J. P. Collman, A. Happer, and L. G. Marzilli, under similar conditions. That the glycine esters are ibid., 89, 1082 (1967). (6) D. A. Buckingham and J. P. Collman, Inorg. Chem., in press. not converted into glycylglycine esters or diketopiper(7) S. L. Young, Ph.D. dissertation, University of North Carolina, 1967. (8) J. P. Collman and P. W. Schneider, Inorg. Chem., 5 , 1380 (1966). (9) The following abbreviations are used throughout this paper: en for ethylenediamine, trien for triethylenetetramine, tren for 4-(2aminoethyl)diethylenetriamine, cyclen for 1,4,7,10-tetraazacyclododecane, gly for glycine, ala for alanine, glyglyOR for glycylglycine esters, glyNRz for glycinamides, and glyOR for glycine esters. (10) (a) M. D. Alexander and D. A. Busch, J . A m . Chem. SOC.,88, 1130 (1966); (b) Inorg. Chem., 5, 602 (1966). (11) R.G.Pearson, C. R. Boston, and F. Basolo, J . Phys. Chem., 59, 305 (1955).
Journal of the American Chemical Society
89:24
(12) D. A. Buckingham, L. G. Marzilli, and A. M. Sargeson, J . A m . Chem. SOC.,89, 2772 (1967). (13) We are indebted to D. A. Buckingham for making his results available to us prior to publication. (14) Unpublished results of M. Fehlmann, H. Freeman, D. A. Buckingham, and A. M. Sargeson communicated to us through D. A. Buckingham. The fi structure of V and related complexes is thus correct as shown in Figure 2. The structure of the glycinato complex follows from this and is the one dipicted in Figure 2. The same structure was suggested in our earlier paper.6 The structure of the tren complex I1 is uncertain. One of the two possible isomers is depicted in Figure 1 .
Noaember 22, 1967
6097
k1
I
Cl
I
0.
C1
I1
---?
' I
NHCHzCOzR Va, R = CH3 b, R = C2H5 C, R = i-C3H7
I11
61
c1
IV
I11 Figure 1.
azine in the absence of the metal complex but under otherwise identical reaction conditions was demonstrated by thin layer chromatographic analyses. Thus the complex metal ion is promoting the formation of a peptide bond. Alkaline aqueous hydrolysis of the dipeptide complexes V (a, b, or c) at 50" affords the glycinato complex VI, which is identical with the complex prepared from treating ~is-p-[Co(trien)(OH)(OH~)]~f (VII) with glycine, glycylglycine, or glycylalanine, the latter being examples of the peptide cleavage reaction (Figure 2).6 Related glycinamide complexes VI11 have been prepared by the reaction of the dichlorotrien complex I11 with a series of glycinamides in DMF. The characterization of these bidentate amide complexes VI11 is also discussed below. Alkaline hydrolysis of the coordinated glycinamide bonds in VI11 yielded the glycinato complex VI ; however, hydrolysis of the coordinated N,N-dialkylamides VI11 was markedly slower than hydrolysis of the glycylglycine complexes V. It is not clear whether this kinetic difference is due to electronic or steric effects. Molar conductivities of the glycylglycine and glycinamide complexes V and VI11 are presented in Table I along with data for other representative examples for comparison. The conductance data support the repreTable I. Conductivity Values for Dipeptide and Related Complexes
Complex
Molar conductance,a ohms-'
[Co(trien)(glyglyOMe)](ClO4)3(Va) [Co(trien)(glyglyOEt)](ClOl)a(Vb) [Co(trien)(glyglyO-i-Pr)](ClO4)~(Vc) [Co(trien)(glyNHMe)]C1(C104b(VIIIa) [Co(trien)(glyNH-i-Pr)lCl(ClO~ (VIIIb) )~ [Co(trien)(glyNMe2)](C104)3 (VIIIc) [Co(trien)(glyNEtz)l(ClOl)s(VIIId) [Co(trien)(gly)](CIO4)~(VI) B-[Co(trien)(n-BuNH~)l](ClO4b (IV) a-[Co(trien)(NOz)*](C104bb cis-[Co(en)~(glyOR)Cl]Cl~~
340 335 34 3 395 (28") 375 (28") 390 (27') 398 (27") 205 232 98 220
a Measured in 10-8 M concentrations in HzO at 25" otherwise noted. *Prepared according to the method of A. M. Sargeson and G. H. Searle, Inorg. Chem., 6, 787 (1967). Prepared according to the method of Alexander.1Ob
Collman, Kimura
0 VI
i)H VII Figure 2.
sentation of these complex salts as unitrivalent electrolytes. l 5 Certain of the glycinamide complexes VI11 were isolated as mixed chloride perchlorate salts. Qualitative experiments demonstrated the presence of ionic chloride. Elemental analyses of the perchlorate derivatives V show the absence of ionic chloride. Visible spectra of the dipeptide and glycinamide complexes are summarized in Table 11. The spectra of other relevant cobalt(II1) complexes are listed for comparison. The detailed spectra of four of these complexes are presented in Figure 3. Two bands corresponding to the two spin-allowed d-d transitions for octahedral cobalt(II1) are observed in the visible spectrum of each complex.16 The similarity between the spectra of glycylglycine, glycinamide, and glycinato complexes V, VIII, and VI is particularly evident. This suggests analogous ligand fields about the central metal due to one oxygen and five nitrogen atoms. Alexander and Busch'O report that chelated amino acid esters, ~is-[Co(en)~(glyOR)], 3+ exhibit spectra (A 487 mp (e 80) and 344 mp ( E 92)) very similar to that of the corresponding glycinato complex, cis-[Co(en)~(gly)]~+ (X 487 mp (e 98) and 346 mp (e 107)). The contrast with the spectra of complexes containing chloropentamine ligands is illustrated in Figure 3 and Table 11. The near congruence (Figure 3) (15) M. M. Jones, "Elementary Coordination Chemistry," PrenticeHall, Inc., Englewood Cliffs, N. J., 1964, p 254. (16) C. J. Ballhausen, "Introduction to Ligand Field Theory," McGraw-Hill Book Co., Inc., New York, N. Y., 1962, p 259.
1 Peptide Formation in the Coordination Sphere of
Cobalt(III)
6098 160
Table II. Visible Spectral Data for Dipeptide and Related Complexes in HzOSolution
rI
Complex
[C~(trienXglyglyOMe)](ClO~)~ (Va) [C0(trienXglyglyOEt)](ClO4)~ (Vb)
I
I
450
500
I 3 50
400
I
550
Wave length m p
Figure 3. Visible absorption spectra of (A) [Co(trienXglygly0R)l3+, (B) [Co(trienXgly)]2+, (C) cis-@-[Co(trienXBuNH~)Cl] z+, ( D) cis-[Co(en)2(BuNHz)CI1lf.
of the spectra of the dipeptide complexes and the glycinato complex resulted in our failure to detect such peptide complexes as intermediates in our earlier spec-
[Co(trienXglyglyO-i-Pr)](ClOa)a(Vc) [Co(trien)(glyNHMe)]CI(ClO4)~ (VIIIa) [Co(trienXalyNH-i-Pr)1C1(ClOah(VIIIb) [Co(trienj&glyNH~)]Cl(ClOa); (XI) ' [C0(trienXglyNMe2)](ClO~)~ (VIIIc) [Co(trienXglyNEt~)I(ClO4)3(VIIId) [Co(trienXglyglyglyOEt)]Cls (XIII) [Co(trienXglyalaOEt)]CI3(XII) [Co(trienXgly)l(CIO4)~ (VI) @-[Co(trien)(n-BuNH~)Cl](ClO4)~ (IV) @-[Co(trien)(glyOEt)CIlCl~ (X) ~t-[Co(trien)(glyOEt)Cl](ClO~(XV) )2 cis-[Co(en)&lyOEt)Cl]Cl~~
--Xmax, 347 (137) 356 (140) 346 (137) 346 (153) 347 (164) 346 ii38j 346 (154) 346 (185) 346 (133) 346 (166) 348 (135) 368 (96) 372 (103) 369 (96) 367 (82)
mfi ( €
max)--
479 (128) 478 (129) 478 (126) 478 (132) 479 (1 38) 478 (iz7j 479 (132) 480 (167) 480 (125) 480 ( 152) 480 (127) 483 (99) 487 (99) 510 (107) 525 (77)
Reference 10.
(1618 cm-l) lower than that of the free ligand (1665 cm-l). An earlier study of the infrared spectra of both N and 0 bound urea complexes had demonstrated that coordination of N causes the carbonyl frequency to
1
0.0
(broken line indicates partially deuterated sample). Figure 4. Infrared spectrum of [Co(trienXglyglyOCH~)](ClO~)3
tral study of the kinetics of the peptide hydrolysis reaction.6 This point is elaborated below. The infrared spectrum of the glycylglycine methyl ester complex Va is presented in Figure 4 along with the spectrum of the partially deuterated complex. The intense band at 1740 cm-l and another at 1240 cm-l are assigned to the normal stretching modes of the uncoordinated ester group, indicating little or no interaction with the metal ion. The strong band at 1625 cm-l is assigned to coordinated amide. The carbonyl frequency of the amide group in uncoordinated glycylglycine methyl ester hydrochloride is at 1675 cm- l. The infrared spectra of the glycinamide complexes show similar features. This spectral shift upon coordination is similar to that of the chromium(I11) nicotinamide complex l8 whose spectrum in DzO shows a band at a frequency (17) M. P. Springer and C. Curran, Inorg. Chem.. 2,270 (1963). (18) F. R. Nordmeyer and H. Taube, J . Am. Chem. Soc., 88. 4295 (1 966).
Journal of the American Chemical Society J 89:24
increase whereas 0 coordination has the opposite effect.lg Recrystallization of the dipeptide complexes V from hot D20resulted in the deuteration of all N-H groups as shown by nmr spectra. The infrared spectra of the deuterated complexes show N-D stretching bands -2400 cm-'. The perturbed amide I band at 1625 cm-' is not affected by deuteration, but the absorption at 1575 cm-l is replaced by a new band at 1510 cm-' as shown in Figure 4. If the band at 1575 cm-l were due to an N-H asymmetric deformation, deuteration would be expected to shift it to -1150 cm-1.20 The small isotopic shift (NH/ND of 1.04) is consistentz1 with an amide I1 band which usually occurs in this region as a mixed vibration of C-N stretching and N-H deformation. z 2 (19) R. B. Penland, S. Mizushima, C. Curran, and J. V. Quagliano, ibid., 79, 1575 (1957). (20) A. Rosenberg, Acta Chem. Scand., 11, 1390 (1957). (21) T. Miyazawa, T. Shimanouchi, and S. Mizushima, J . Chem. Phys., 24, 408 (1956).
November 22, 1967
6099 Table 111. Proton Magnetic Resonance Spectra of Peptide and Related Complexes a-CH2
+NHaCHzCONHCHzCOzMeCI[Co(trienXglyglyO-i-Pr)] [Co(trienXgly)l*+ cis-[Co(en)dglyOMe)Cl]*+ +NH3CHzCO2MeC1cis-[Co(en)2(gIyO-i-Pr)Cl] 2+ +NH3CH3C02-i-PrCIICo(en)~(gl~)l~+ [Co(trienXglyNHMe)la+ +NH3CHzCONHMeC1[Co(trien)(glyNH-i-Pr)13+ +NH3CH2CONH-i-Pr-Cl[Co(trien)(glyNMe~)]~+ +NH3CHzCONMezC1[Co(trienXglyNEt~)l 3+
4.31c 4.30 4.07 4.20 3.70 3.47 4.0 3.46 3.92 3.65 4.02 3.83 4.00 3.78 4.15 4.01 4.15
+NHaCHzCONEtzCI-
4.01
[Co(trien)(glyglyOMe)13+
-
Band positionso
r
Compound
B-CHz
4.48c 4.35 4.10 4.30
Ester or amide alkyl groupb
3.92 S (CH3)C 3.83 S (CH3) 3.98 S (CHz) 5.12 Qi (CH), 1.27 D (gem-CHs) 3.75 S (CH3) 3.85 S (CH3) 5.13 Qi (CH), 1.28 D (gem-CHa) 5.16Qi (CH), 1.30D (gem-CHa) 2.94 S (CH3) 2.80 S (CHI) -4.1 (CH), 1.20 D (gem-CHI) -4.0 (CH), 1.15 D (gem-CHa) 3.13 S (2CHa) 2.95 S (CHa), 3.0S (CHa) 3.43 Q (2CH2), 1.17 T (CHa), 1.24 T (CH3) 1.11 T (CH3), 1.18T (CH3)
0 Sodium 3-(trirnethylsilyl)-l-propanesulfonate was used as an internal standard in DzO solutions. Values (ppm) are downfield from the standard. b Line splittings: S = singlet, D = doublet, T = triplet, Q = quartet, Qi = quintet. Measured in acidic DzO. Prepared according to the method of Alexander.lo
