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Structure and Conformation of Photosynthetic Pigments and Related Compounds, 8 [1]Molecular Structure of an Iron(III) Chlorophyll Derivative - Chloro(phytochlorinato methyl ester)iron(III)Mathias O. Senge3 *, Karin Ruhlandt-Sengeb, Kevin M. Smith3

a Department of Chemistry, University of California, Davis, CA 95616, USA b Department of Chemistry, Syracuse University, Syracuse, NY 13244, USAZ. Naturforsch. 50b, 139-146 (1995); received April 13, 1994Chloro(phytochlorinato methyl ester)iron(III), Chlorophyll Derivatives, Crystal Structure

The crystal and molecular structure of chloro(methyl phytochlorinato)iron(III), 4, have been determined by single crystal X-ray crystallography to obtain further information on the conformation of metallochlorins related to chlorophyll. The compound crystallized with two independent molecules mainly distinguished by the orientation of the axial ligand. The macrocycles show significant deviations from planarity larger than those observed in corre­sponding magnesium(II) complexes. The overall type of distortion is similar to those found in chlorojphyllides. Compound 4 crystallized in the monoclinic space group P 2 t (MoKa,A = 0.71063 A) with unit cell dimensions a = 12.035(6) A, b = 13.396(8) A, c = 19.04(2), b = 97.51(2) A, Z = 4, V = 3043(4) A3. The structure was refined to an /?-value of 0.075 on the basis of 3974 reflections with I > 3.0a(I) (130 K).

Introduction

The investigation of tetrapyrroles is a long established field in structural chemistry [2]. How­ever, while a recent survey of the Cambridge Crys- tallographic data base [3] listed over 700 structures of porphyrins, structural data on reduced porphy­rin systems and especially chlorophylls (Chi), e. g.1, are much more limited. A fter the initial studies on the gross chemical and absolute structures [4-9] and the aggregation of chlorophyll deriva­tives [1 0 ] recently interest in the structural chemis­try of reduced systems has been renewed with re­gard to studies on the conformational flexibility of the tetrapyrrole macrocycle [1 1 ] and related to studies on e. g., the green hemes and other related hydroporphyrins [1 2 ].

The few structurally characterized metallohydro- porphyrins include mainly methyl (2 ) and ethyl esters of Mg(II) complexes which are related to the natural Chls [6 - 8 ]. Much attention has been

* Reprint requests to Dr. M. O. Senge.New address: Institut für Organische Chemie (WE 02), Freie Universität, Takustraße 3, D-14195 Berlin, Germany.

given to Ni(II) complexes of synthetic model com­pounds, which were used to elucidate the basic structural chemistry of chlorins [13, 14], bacterio- chlorins [15], isobacteriochlorins [16], and hexa- hydroporphyrins [15,17], In addition, a small num­ber of other synthetic hydroporphyrins with Cu(II) [18], Fe(II) [19], Fe(III) [12, 20], and Zn(II) [21] as central metal have been charac­terized by X-ray crystallography. With the excep­tion of the Mg(II) structures [6 - 8 ] the tetrapyrrole ligands used for these studies all are related to 5,10,15,20-tetraphenylporphyrin (TPP) or 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) [2]. While these compounds served very well to estab­lish the basic characteristics of reduced porphyrins[2 ], they are only to a limited extent suitable as model compounds for the analysis of the macro­cycle conformation found in Chi with its fused iso- cyclic pentanone ring V. Thus, effects of the metal coordination on the conformation in metallo Chi derivatives need to be studied in phytochlorin de­rivatives.

Recently we reported the structure of (methyl 132-demethoxycarbonylpheophorbidato a)nickel- (II), 3, a Chi a derivative with a four-coordinate metal center [22]. Due to the small nickel(II) ion this structure showed a highly nonplanar macro-

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140 M. O. Senge et al. • Photosyntetic Pigments and Related Compounds

1 2

cycle conformation and demonstrated that nickel-(Il)chlorins are only of limited use as model com­pounds of Chi in spectroscopic studies. These re­sults were in agreement with a comparative spec­troscopic investigation of nickel(II) and iron(III) Chi derivatives by Anderson et al. [23], who pointed out that iron(III) derivatives with their five coordinated metal centers are much more suitable model compounds than the nickel(II) de­rivatives of Chi a. In order to obtain more infor­mation on the conformation and coordination geometry in five-co-ordinated metallo Chi deriva­tives we present here the crystal structure determi­nation of chloro(methyl phytochlorinato)iron(III) [formerly: chloro(methyl meso-pyropheophorbid- ato a)iron(III) or Fe(III)Cl methyl mesopyropheo- phorbide a] and compare the structural results ob­tained with those of closely related species.

Results and Discussion

The molecular structure of Fe(III)Cl methyl phytochlorin, 4 and the numbering scheme are given in Fig. 2. Atomic coordinates are listed in Table I, while selected bond lengths and bond angles of the macrocycle atoms are compiled in Table II [24]. The compound crystallises in the noncentrosymmetric space group P2!. This assign­ment was confirmed by successful refinement, the

Table I. Atomic coordinates [xlO4] and isotropic dis­placement coefficients [Ä xlO 3] for 4*.

Atom X y z ueqFe(l) 7784(1) 8791 456(1) 26(1)ci(i) 6674(2) 7618(2) 819(2) 34(1)N(21) 7020(7) 9200(7) - 522(4) 31(3)N(22) 7214(6) 10086(6) 869(4) 29(3)N(23) 9076(7) 8946(7) 1251(4) 32(3)N(24) 8888(6) 7870(6) - 73(4) 23(3)C (l) 7101(7) 8736(7) -1160(4) 28(3)C(2) 6248(7) 9108(7) -1698(5) 31(3)C(2 A) 6106(10) 8726(8) -2459(6) 35(4)C(3) 5665(8) 9826(7) -1374(4) 27(3)C(3 A) 4720(10) 10412(11) -1742(7) 47(4)C(3 B) 5171(12) 11323(12) -2050(9) 61(6)C(4) 6143(8) 9869(7) - 641(4) 35(4)C(5) 5731(8) 10455(8) - 138(4) 32(4)C(6) 6237(7) 10545(8) 584(4) 36(4)C(7) 5836(8) 11195(7) 1095(4) 29(3)C(7 A) 4783(9) 11784(9) 991(7) 38(4)C(8) 6604(7) 11132(7) 1700(4) 24(3)C(8 A) 6557(8) 11718(9) 2395(7) 34(4)C(8B) 7025(11) 12787(11) 2308(8) 50(5)C(9) 7447(7) 10430(8) 1554(4) 33(4)C(10) 8345(7) 10139(8) 2039(5) 27(3)C (ll) 9157(8) 9451(7) 1887(5) 33(4)C(12) 10145(8) 9166(8) 2349(5) 28(3)C(12 A) 10499(10) 9493(9) 3087(7) 37(4)C(13) 10636(7) 8432(8) 1981(6) 25(3)C(13 A) 11526(9) 7673(9) 2033(7) 39(4)C(13B) 11332(10) 7065(11) 1326(6) 40(4)C(14) 9967(8) 8296(9) 1315(7) 43(4)C(15) 10283(10) 7564(10) 893(6) 35(4)C(16) 9748(8) 7285(8) 255(6) 29(3)C(17) 10023(9) 6467(9) - 234(6) 31(4)C(17 A) 9497(10) 5493(9) - 15(7) 37(4)C(17B) 8219(11) 5510(10) - 90(9) 51(5)C(17C) 7972(15) 4434(13) - 20(17) 446(62)C(17D) 6347(16) 3423(13) 110(11) 92(8)C(18) 9561(10) 6867(9) - 966(6) 31(4)C(18 A) 10451(10) 7432(11) -1301(7) 42(4)C(19) 8717(8) 7582(9) - 772(4) 29(3)C(20) 7879(8) 8019(8) -1280(6) 33(4)0(1) 12263(7) 7541(8) 2521(5) 47(3)0(2) 7730(19) 4513(15) 568(18) 330(31)0(3) 6863(14) 4336(14) - 97(11) 178(10)Fe(2) 7203(1) - 51(1) 4459(1) 26(1)Cl(2) 8362(2) 1111(2) 4143(2) 36(1)N(25) 7854(6) - 494(5) 5445(4) 25(3)N(26) 7795(6) -1324(6) 4045(4) 29(3)N(27) 5950(6) - 192(6) 3659(4) 26(3)N(28) 6127(7) 913(7) 4956(4) 33(3)C(21) 7854(7) 38(7) 6063(4) 34(3)C(22) 8619(7) - 371(7) 6634(5) 26(3)C(22 A) 8866(10) - 10(11) 7369(7) 48(4)C(23) 9171(8) -1137(8) 6343(4) 40(4)C(23 A) 10067(8) -1822(8) 6689(6) 23(3)C(23B) 9587(10) -2848(9) 6862(7) 38(4)C(24) 8679(7) -1202(7) 5615(4) 30(3)C(25) 9028(8) -1842(7) 5121(4) 34(4)C(26) 8666(7) -1893(7) 4372(4) 27(3)C(27) 9084(8) -2555(8) 3873(5) 37(4)C(27 A) 10080(11) -3218(12) 4010(7) 47(5)

C02Me3

C02Me4

M. O. Senge et al. • Photosyntetic Pigments and Related Compounds 141

Table I. (continued).

Atom X y z ueqC(28) 8413(7) -2407(8) 3235(5) 33(4)C(28 A) 8480(11) -3001(9) 2577(7) 43(4)C(28B) 8066(11) -4033(10) 2589(7) 46(5)C(29) 7579(7) -1685(6) 3362(4) 23(3)C(30) 6672(7) -1382(8) 2887(5) 26(3)C(31) 5850(7) - 714(7) 3029(4) 27(3)C(32) 4887(8) - 455(9) 2533(6) 32(3)C(32 A) 4505(9) - 890(9) 1817(6) 33(3)C(33) 4362(10) 250(9) 2908(7) 38(4)C(33 A) 3385(10) 939(9) 2920(6) 35(4)C(33B) 3517(8) 1506(9) 3606(6) 32(4)C(34) 5010(7) 395(7) 3591(6) 27(3)C(35) 4640(9) 1159(9) 4016(7) 35(4)C(36) 5149(8) 1388(9) 4683(7) 43(4)C(37) 4713(9) 2095(9) 5202(7) 38(4)C(37 A) 3699(9) 1628(9) 5482(7) 37(4)C(37B) 3883(9) 551(10) 5747(7) 44(4)C(37C) 2850(8) - 53(8) 5741(5) 41(4)C(37D) 1115(12) - 438(13) 5094(9) 66(6)C(38) 5735(8) 2194(9) 5780(6) 29(3)C(38 A) 6416(11) 3127(10) 5681(8) 44(4)C(39) 6438(9) 1259(10) 5634(5) 36(4)C(40) 7221(9) 879(7) 6155(6) 29(3)0(4) 2652(7) 1110(7) 2413(5) 46(3)0(5) 2137(7) 128(8) 5159(5) 61(4)0(6 ) 2676(9) - 700(8) 6143(6) 68(4)

* Equivalent isotropic U defined as one third of the trace of the orthogonalized Ujj tensor.

inability of coordinate transfer to the centrosym- metric space group and by the different confor­mations observed for the side chains and ring V. The chlorin crystallized with two crystallograph­ically independent, but chemically equivalent, molecules in the asymmetric unit. The refinement was impeded by poor crystal quality resulting in a weak intensity data set and a high number of unobserved reflections and relatively high R - \al- ues. The structure suffers from disorder in the pro­pionic acid ester sider chain of molecule 1. All at­tempts to model the disorder by the utilization of split positions failed, the side chain was refined with only one position for each atom. This resulted however in large thermal parameters for C(17B), C(17C), and 0 (3 ), showing the strong libration vibration (see Fig. 1). Besides differences in the conformation, which will be discussed below, the main structural difference between both molecules is the orientation of the F e -C l vector which is oriented below the chlorin plane in the macrocycle with Fe(l), and can thus be regarded as the a- chloro form; above and below the molecular plane

Table II. Selected bond lengths [A] and angles [°] for the two crystallographically independent molecules of 4.

Atom Molecule 1 Molecule 2

F e-C l 2.230(4) 2.226(4)C t-N (21) 1.998(8) 1.968(8)C t-N (22) 2.033(8) 2.035(8)Ct-N (23) 1.935(8) 1.904(8)Ct-N (24) 2.095(8) 2.084(8)Fe-N(21) 2.040(8) 2.026(8)Fe-N(22) 2.059(8) 2.045(8)Fe-N(23) 2.035(8) 2.007(7)Fe-N(24) 2.157(8) 2.135(9)N (21)-C (l) 1.380(11) 1.376(11)N(21)-C(4) 1.381(12) 1.380(11)N(22)-C(6) 1.375(12) 1.378(11)N(22)-C(9) 1.378(11) 1.380(10)N (2 3 )-C (ll) 1.379(12) 1.379(12)N(23)-C(14) 1.373(14) 1.370(11)N(24)-C(16) 1.380(12) 1.379(13)N(24)-C(19) 1.376(11) 1.377(13)C (l)-C (2 ) 1.440(11) 1.438(12)C (l)-C (20) 1.381(14) 1.384(13)C(2)-C (3) 1.383(13) 1.378(14)C(3)-C (4) 1.438(11) 1.436(11)C(4)-C (5) 1.381(13) 1.379(12)C(5)-C (6) 1.434(11) 1.437(10)C(6)-C (7) 1.435(13) 1.437(13)C(7)-C (8) 1.382(11) 1.382(12)C(8)-C (9) 1.436(13) 1.437(13)C(9)-C(10) 1.383(12) 1.385(11)C (1 0 )-C (ll) 1.400(14) 1.386(13)C (ll)-C (1 2 ) 1.436(13) 1.439(12)C(13)-C(13 A) 1.471(15) 1.498(17)C(13 A )-C (13B ) 1.565(18) 1.500(17)C(13B)-C(15) 1.565(16) 1.543(15)C(13)-C(14) 1.423(16) 1.439(16)C(14)-C(15) 1.354(18) 1.412(16)C(15)-C(16) 1.351(15) 1.370(18)C(16)-C(17) 1.503(16) 1.513(18)C(17)-C(18) 1.528(15) 1.546(15)C(18)-C(19) 1.478(16) 1.557(17)C(19)-C(20) 1.427(13) 1.374(14)

C l-F e-N (21) 104.8(3) 106.0(2)C l-F e-N (22) 103.4(2) 102.5(2)C l-F e-N (23) 105.7(3) 107.2(3)C l-F e-N (24) 100.0(2) 97.8(3)N (21)-Fe-N (22) 89.5(3) 90.0(3)N (22)-Fe-N (23) 83.7(3) 83.9(3)N (23)-Fe-N (24) 86.9(3) 87.5(3)N (24)-Fe-N (21) 87.7(3) 87.1(3)N (21)-Fe-N (23) 149.5(3) 146.8(3)N (22)-Fe-N (24) 156.4(3) 159.5(3)F e -N (2 1 )-C (l) 127.6(6) 126.5(6)F e-N (21)-C (4) 124.6(6) 126.7(6)F e-N (22)-C (6) 122.5(6) 124.8(5)F e-N (22)-C (9) 127.5(6) 128.4(6)F e -N (2 3 )-C (ll) 131.4(7) 132.8(6)Fe-N (23)-C (14) 120.9(7) 123.0(6)Fe-N (24)-C (16) 125.8(6) 130.3(8)Fe-N (24)-C (19) 126.0(6) 120.9(7)C (l)-N (2 1 )-C (4 ) 106.7(7) 103.6(7)C (6)-N (22)-C (9) 106.3(8) 105.7(7)

142 M. O. Senge et al. ■ Photosyntetic Pigments and Related Compounds

Table II. (continued).

Atom Molecule 1 Molecule 2

C (ll)-N (2 3 )-C (1 4 ) 105.7(8) 103.5(7)C (16)-N (24)-C (19) 106.7(8) 108.2(10)N (2 1 )-C (l)-C (2 ) 110.2(8) 112.2(8)N (21 )-C (l)-C (2 0 ) 125.8(8) 126.3(8)C (2 )-C (l)-C (2 0 ) 124.0(8) 121.5(8)C (l) -C (2 )-C (3 ) 106.2(7) 105.9(8)C (2 )-C (3 )-C (4 ) 107.3(8) 105.8(8)N (21)-C (4)-C (3) 109.5(8) 112.3(8)N (21)-C (4)-C (5) 126.3(7) 122.7(7)C (3)-C (4 )-C (5 ) 124.1(8) 124.9(8)C (4 )-C (5 )-C (6 ) 124.6(8) 128.9(8)N (22)-C (6)-C (5) 125.1(9) 122.3(8)N (22)-C (6)-C (7) 110.5(7) 110.8(7)C (5 )-C (6 )-C (7 ) 124.2(8) 126.8(8)C (6 )-C (7 )-C (8 ) 106.3(8) 106.1(8)C (7)-C (8 )-C (9 ) 106.9(8) 106.8(8)N (22)-C (9)-C (8) 109.9(7) 110.2(7)N (22)-C (9)-C (10) 125.6(9) 123.2(8)C (8)-C (9)-C (10) 124.5(8) 126.6(8)C (9 )-C (1 0 )-C (ll) 123.6(9) 125.7(8)N (2 3 )-C (ll)-C (1 0 ) 122.3(8) 120.9(7)N (2 3 )-C (ll)-C (1 2 ) 111.4(9) 114.8(8)C (1 0 )-C (ll)-C (1 2 ) 126.3(8) 124.2(8)C (ll)-C (1 2 )-C (1 3 ) 104.6(8) 102.9(9)C(12)-C (13)-C (13 A) 144.3(10) 147.2(11)C (12)-C (13)-C (14) 108.2(9) 109.2(9)C(13 A )-C (13)-C (14) 107.1(10) 103.6(10)C(13)-C(13 A )-C (13B ) 106.0(9) 109.4(9)C(13 A )-C (13B )-C (15 ) 104.5(10) 105.9(9)N (23)-C (14)-C (13) 110.0(10) 110.5(9)N (23)-C (14)-C (15) 133.1(11) 132.8(9)C (13)-C (14)-C (15) 116.8(10) 116.2(9)C (13B )-C (15)-C (14) 105.5(10) 104.4(9)C (13B )-C (15)-C (16) 127.8(11) 131.1(11)C (14)-C (15)-C (16) 126.6(11) 123.8(10)N (24)-C (16)-C (15) 119.4(10) 120.2(11)N (24)-C (16)-C (17) 110.3(8) 113.2(10)C (15)-C (16)-C (17) 129.9(10) 126.5(9)C (16)-C (17)-C (18) 103.1(9) 102.0(9)C (17)-C (18)-C (19) 100.4(9) 101.8(9)N (24)-C (19)-C (18) 113.7(8) 110.6(9)N (24)-C (19)-C (20) 123.0(10) 129.0(11)C (18)-C (19)-C (20) 123.0(9) 120.4(9)C (l)-C (2 0 )-C (1 9 ) 127.4(10) 123.1(9)

refer to drawing the molecule in the conventional scheme shown in formula 4. Formula 4 shows the /3-chloro form with displacement of the F e -C l unit above the molecular form, which is found in the crystal structure for the macrocycle containing Fe(2). Differences in the conformation of the pro­pionic methyl ester at C(17) are also observed.

The core of the macrocycle is characterized by the five-coordinated iron(III) center which is lig­ated by the four pyrrole nitrogens and a chloride atom in axial position. The axial F e-C l bond

Fig. 1. Computer generated plot and numbering scheme of the two independent molecules of 4. Ellipsoids are drawn for 40% occupancy. Both molecules have been adjusted arbitrarily with respect to each other.

lengths are 2.230(4) and 2.226(4) A in the two in­dependent molecules, respectively. These lengths agree well with data found in a related iron(III)- bacteriochlorin and are slightly longer than those found in porphyrins [2, 3]. The F e -C l vector is not orthogonal to the molecular plane, the C l-F e -N angles range from 97.8 to 107.2(3)°. The general similarity of the core geometry in porphyrins and hydroporphyrins for pentaco-ordinated high spin iron(III) was already noted by Strauss et al. in their comparison of [Fe(III)TPP]20 and

M. O. Senge et al. • Photosyntetic Pigments and Related Compounds 143

Fig. 2. Deviations of the macrocycle atoms from the least-squares plane of the four nitrogen atoms [AxlO 2]. Molecule 1 (top), molecule 2 (bottom).

[Fe(III)TPC]20 [20]. The averaged bond lengths for the F e -N pyrrole bonds are 2.045(8) A in mol­ecule 1 and 2.026(8) A in molecule 2, respectively. In comparison, the F e -N bonds to the reduced pyrrole ring are considerably longer with 2.157(8) A in molecule 1 and 2.135(9) A in mol­ecule 2. A similar situation is observed for the C t-N distances. The overall bond lengths are con­sistent with data obtained for other iron por­phyrins (average F e -N bond lengths of 2.065 A for 5-co-ordinated high-spin iron) [2], the differ­ence between the Fe-N (pyrrole) and F e - ^ p y r ­rolidine) bonds is a well documented fact in struc­tural porphyrin/chlorin chemistry [2]. The iron(III) is displaced towards the axial chloride from the mean plane of the macrocycle by 0.48 A. These displacements support values described for the related Fe(III)octaethylisobacteriochlorin(Cl) [11]

and the //-oxo-dimer [Fe(III)tetraphenylchlorin]20 [19]. These displacements are in the range ob­served for high-spin, five-coordinated iron(III) porphyrins [2]. In general, the core geometry of the present iron(III) chlorin is similar to those found in other iron(III) hydroporphyrins and to those of other high-spin iron(III) porphyrins.

Besides being characterized by the inequiva­lency of the F e -N bond lengths the chlorin (hydroporphyrin) character of the macrocycle is clearly evidenced by the elongated C(17)-C(18) bond lengths; which are 1.528(15) A for molecule1 and 1.546(15) A for molecule 2, respectively. The meso character of the compound, i. e. the presence of an ethyl instead of a vinyl group at C(3), is evi­denced by the elongated bond lengths of 1.48(2) and 1.50(2) Ä for the C(3 A )-C (3 B ) bond in the two molecules. These data agree well with struc­tural reports of the corresponding free base methyl phytochlorin [25] and other related hydro­porphyrins.

Fig. 3 A and B show views of the conformation of the two molecules in 4. Both macrocycles show significant deviations from planarity (Fig. 2). The average deviation from the least-squares plane of the 24 core atoms is 0.13 A for molecule 1 and 0.16 A for molecule 2, respectively. The largest in­dividual deviations are observed for the /3-pyrrole atoms and for the outer atoms of the isocyclic ring. Overall the conformation in both macrocycles is rather similar with respect to the FeCl vector, the only major difference being that the zig-zag con­formation of the pyrrolidine ring in inverted with regard to the C(17)-C(18) axis. Generally, the conformations might be described as saddle type (as defined by Scheidt and Lee [2]), indicated by the alternating displacement of individual pyrrole rings above and below the mean plane. The angles formed between individual pyrroles and the 4N- plane are, in the order of ring I, II, III, and IV: 5.8, 5.7, 5.2, 11.6° in molecule 1, and 4.4, 3.2, 8 .8 , 10.7° in molecule 2. Nevertheless, significant devi­ations are also observed for the meso-carbons and thus the conformation contains a significant de­gree of ruffling. In molecule 1 they are displaced on average by 0.07 A from their least-squares plane, while molecule 2 diviations of 0.104 A are observed. While these deviations from planarity are about twice as large as those observed in re­lated Mg(II) chlorophyll derivatives (see Fig. 3D ),

144 M. O. Senge et al. • Photosyntetic Pigments and Related Compounds

*311

I c

D

Fig. 3. Linear display of the skeletal deviations for the 26 macrocycle atoms from the least-squares plane of the four nitrogen atoms for (A) molecule 1 (B) and mol­ecule 2 of 4 (the arrows indicate the direction of the F e-C l vector), and for comparison (C) methyl phyto- chlorin,23 and (D) methyl me5,o-132-demethoxycarbonyl- chlorophyllide a.7b

they are only half as large as those observed in the related Ni(II) derivative 3. Additionally, the degree of out-of-plane rotation observed for the pyrrole rings is much less in the Fe(III)Cl structure 4 than observed in 3. Besides having a more non- planar macrocycle conformation, a second distinc­tion of the Fe(III)Cl structure 4 with regard to the related Mg(II) structure lies in the relative orien­tation of rings I and II. In the related free base (Fig. 3C) and the corresponding chlorophyllide (Fig. 3D) rings I and II are slightly displaced

towards one side of the macrocycle plane. In the present Fe(III)Cl derivative they show alternating displacement above and below the macrocycle plane. For comparison, the conformation of [Fe(III)TPC)20 was described as a mixture of doming and S4-ruffling [20], while in the related Fe(III)octaethylisobacteriochlorin(Cl) structure a domed conformation with most atoms being dis­placed below the macrocycle plane was observed[12]. Therefore, the conformation in the present Fe(III)chlorin is considerably different from that of the isobacteriochlorin and resembles more the situation found in the ^-oxo-dimer. However, the presence of the fused isocyclic pentanone ring makes a direct comparison difficult.

Fig. 4 shows the molecular packing in the unit cell. The molecules pack by formation of parallel running layers. Neighboring molecules in a given layer are characterized by the orientation of the F e-C l vector in opposite directions. The mol­ecules in neighboring layers are almost orthogonal to each other. The 4N-planes of the two Fe(l) con­taining molecules are tilted by 93.3° against each other while those containing Fe(2) form an angle of 84.5°. The only short intermolecular non-bond-

Fig. 4. View of the molecular packing in the unit cell.

M. O. Senge et al. • Photosyntetic Pigments and Related Compounds 145

ing contacts found are H(8 A A )-C l(2 ) = 2.904 Ä and H (4 0 ) -0 ( l) 2.446 Ä. No other evidence for intermolecular contacts <4 A was found. Similarly the crystal structure revealed no evidence for jr-interactions between adjacent molecules.

The results of the conformational analysis show clearly the differences between the Ni(II) (3), Mg(II) (2), and Fe(III)Cl (4) complexes. The prob­lems associated with using S4-ruffled Ni(II) com­plexes as models for spectroscopic studies in chlorophylls has been pointed out by us earlier[22]. Under the assumption, that no major differ­ences occur between the solid state and solution structure, the better correlation between spectro­scopic data of the Mg(II) and Fe(III)Cl derivatives in such studies becomes clear. While the present iron chlorin still shows a higher degree of con­formational distortion than the related Mg(II) chlorins, the type of conformation in the Fe(III) and Mg(II) molecules is similar. However, the higher degree of conformational distortion in 4 and differences in the conformation of ring I andII indicate that direct correlations of spectroscopic studies on iron(III) chlorins with those of Mg(II) chlorophylls should be interpreted with some caution.

Recently, an interesting specific biological rec­ognition of the co-ordination number in metallo chlorins was found. Chlorophyll synthetase, the enzyme catalyzing the esterification of chloro- phyllide a was found to accept only metallopheo- phorbides having a coordination number of 5 (Zn, Mg, Cd). Those with a square-planar metal coordi­nation (Cu, Ni) were not accepted as substrates in the reaction [26]. It might be interesting to specu­late if this distinction involves only the recognition of a five-co-ordinated metal center of if general differences in the macrocycle conformation of five- and four-co-ordinated metallochlorins occur which are recognized by the enzyme.

Experimental

Compound 4 was prepared as described in Ref.[23]. Crystals were immersed in light hydrocarbonoil and inspected under a microscope. Several crystal batches were prepared with different sol­vent combinations. The crystals were all found to

be twinned and/or of poor quality but a suitable fragment could be cut from a large crystal, grown by slow diffusion of a concentrated solution of the chlorin in methylene chloride into /7 -hexane. The crystal was mounted on a glass fiber, and placed in the low-temperature nitrogen stream of a Siemens R3m /V diffractometer [27].

Compound 4 [C34H 33C\FeN40 3; M = 636.9] crystallized in the monoclinic space group P 2 ] with unit cell parameters: a = 12.035(7) A, b = 13.396(8)oA, c = 19.04(2) Ä, ß = 97.51(2)°, U = 3043(3) A 3 (by least-squares refinement on dif­fractometer angles for 23 automatically centred re­flections with 2 0 ° < 26 < 29°, X = 0.710(69) A), Z = 4, 2 independent molecules; Dc = 1.390 Mg/m3. F(000) = 1328; crystal dimensions (green parallel­epiped): 0.4x0.31 x0.13 mm; //(M o -K a) = 0.625 mm-1. Intensity data were collected at 130 K using a Siemens R3m /V diffractometer (graphite monochromated M o -K a radiation) in the to mode with a a> scan width of 2 .0 ° and a scan speed of 8.08 °/min_1. 7275 independent reflections were collected; index range: 0° < 20/55.0°, 0 ^ h < 15,0 < k < 17, -24 < / < 24; 3974 reflections were considered observed with I > 3.0a(I). 2 Check re­flections were measured every 198 reflections and showed only small variations (< 1.5%) which were corrected during processing. A Lorentz and polar­ization correction was applied. An absorption cor­rection was carried out using the program XABS [28], extinction effects were disregarded. The structure was solved via a Patterson synthesis fol­lowed by subsequent structure expansion. Struc­ture solution and refinement were carried out using the SHELXTL PLUS program system; scat­tering factors were used as supplied with SHELXTL PLUS [29]. The refinement was car­ried out by full-matrix least-squares on I FI using the same program system. The function minimized was Iw (F 0 - F c)2. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were included at calculated posi­tions using a riding model (C -H distance 0.96 A; Uiso(H) = 0.04 A 3). Calculations were performed on a Vaxstation 3200. The data-to-parameter ratio was 5.1:1. The weighting scheme used was w - 1 = ct2 (F) + 0.0133 F2. The final difference Fourier syn­thesis gave largest and mean A/o of 0.098, 0.006; the largest difference peak was 1 . 2 1 eA -3, located in a disordered propionic acid ester region (see remarks in result section). The final cycle of re­finement on I FI included 780 variable parameters and converged with R = 0.075, wR = 0.099, S - 0.82.

A ckn o w led g em en t National Institutes of Health (HL-22252) and theFinancial support to M. O. S. from the Deutsche National Science Foundation (CHE-93-05577) is

Forschungsgemeinschaft and to K. M. S. from the gratefullV acknowledged.

146______________________________________ M. O. Senge et al. • Photosyntetic Pigments and Related Compounds

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[24] Further details of the crystal structure determi­nation are available on request from the Fachinfor- mationszentrum Karlsruhe, Gesellschaft für wissen­schaftlich-technische Information mbH, D-76344 Eggenstein-Leopoldshafen, Germany, on quoting the depository number CSD 58365, the names of the authors and the journal citation.

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