The crystal struture of monoclinic ribonuclease-S at six angstroms resolution.

An independent structure analysis has been made of ribonuclease-S crystallized in a monoclinic space group C2 at 6 Å resolution. The conformations of the two crystallographically independent molecules (molecule ZA and ZB) were compared with that of a chemically identical molecule (molecule Y) crystallized in a trigonal space group P3₁21, the structure of which has been solved to 2.0 Å resolution by Wyckoff et al. (1970). The N-terminal tail of the S-protein of molecule ZA assumes a unique conformation somewhat resembling that of ribonuclease-A, while the corresponding part of molecule ZB assumes about a similar conformation to that of molecule Y. Apart from the solvated terminal region, the overall arrangements of various features of the three structures are very similar, although possibilities of local conformational differences are not considered at this stage of the analysis. The environments of the three molecules in the crystal lattice are compared in detail. Two of the four molecules in the primitive cell are related to each other by a crystallographic 2-fold axis very similar to a 2-fold relationship found in the Y-form. All other relationships are quite different.     

Human plasminogen kringle 4. Crystallization and preliminary diffraction data of two different crystal forms

Human plasminogen kringle 4 has been crystallized in two different crystal forms: monoclinic, a = 32.78(3), b = 49.17(2), c = 46.27(3) A, beta = 100.67 degrees, space group P2(1), four molecules/unit cell, two molecules/asymmetric unit; orthorhombic, a = 32.09(7), b = 49.14(6), c = 49.47(9) A, space group P2(1)2(1)2, four molecules/unit cell. Both crystal forms have a large protein fraction (66% for monoclinic and 62% for orthorhombic) and diffract x-rays to 2.0 A resolution. A self-rotation function has been calculated with monoclinic data indicating a non-crystallographic 2-fold rotation approximately parallel to a* (peak height of 14.3 x sigma). Cross-rotation function calculations are in progress utilizing the coordinates of the conserved structure of kringle 1 of prothrombin and plasminogen kringle 4.     

Intercalation of water molecules between nucleic acid bases in the crystal structures of 6-azathymine hemihydrate and 5-amino-2-thiocytosine dihydrochloride dihydrate

The crystal structure of 6-azathymine hemihydrate (6AzTH) exhibits a novel intercalation of water molecules interposed half-way between the modified bases 6.3 to 6.7 A apart. The crystal contains four molecules of 6-azathymine (6AzT) and two water molecules as the independent repeating unit. These two water molecules together with the four bases form two separate water sandwiches. In the crystal structure these sandwiches form two sets of local clusters. The anhydrous crystalline form of 6AzT, on the other hand, is stabilized by base stacking interactions. Both the water molecules in 6AzTH that are involved in sandwich formation have trigonal coordination around them. A reexamination of the crystal structure of 5-amino-2-thiocytosine (5A2TC) revealed that one of the water molecules in this structure also forms a water sandwich and has trigonal coordination whereas the other water molecule with tetrahedral coordination does not form a sandwich. The environment and the characteristics of the intercalated water molecule in these structures suggest a possible role for such water intercalations in the dynamics of DNA. Crystals of 6AzTH are monoclinic, space group P21/n, with unit cell parameters a = 8.861 (1), b = 13.177 (3), c = 20.662 (2) A, beta = 93.35 (1) degrees, and Z = 16. From diffractometer data (2503 reflections, greater than or equal to 3 sigma), the crystal structure was solved and refined to an R of 0.056.     

Refinement of the crystal structure of ribonuclease S. Comparison with and between the various ribonuclease A structures.

Ribonuclease S (RNase-S) is a complex that consists of two proteolytic fragments of bovine pancreatic ribonuclease A (RNase-A): the S-peptide (residues 1-20) and S-protein (residues 21-124). We have refined the crystal structures of three RNase-S complexes. The first two contain the full-length 20-residue S-peptide and were studied at pHs of 4.75 and 5.5. The third one consists of a truncated form of S-peptide (residues 1-15) and was studied at pH 4.75 as the reference structure for a series of mutant peptide complexes to be reported separately. Excluding residues 16-23 which are either missing (in the S15 complex) or disordered (in both S20 complexes), all three structures refined at 1.6-A resolution are identical within the estimated errors in the coordinates (0.048 A for the backbone atoms). The R-values, residual error, range from 17.4% to 18.6%. The final model of S20, pH 4.75, includes 1 sulfate and 84 water molecules. The side chains of 11 residues were modeled in two discrete conformations. The final structures were independent of the particular RNase-A or RNase-S used as a starting model. An extensive comparison with refined crystal structures of RNase-A reveals that the core of the molecule which is held together with extensive hydrogen bonds is in identical pattern in all cases. However, the loop regions vary from one structure to another and are often characterized by high B-factors. The pattern of thermal parameters appears to be dependent on crystal packing and correlates well with the accessibility calculated in the crystal. Gln60 is a conserved residue in all sequences known to date for this class of ribonucleases. However, it is the only residue that is clearly defined in an unfavorable position (phi = -100 degrees, psi = -130 degrees) on the Ramachandran plot. The origin of the substantial differences between RNase-A and RNase-S in stability to both acid and temperature denaturation and in susceptibility to proteolysis at neutral pH is not obvious in our visual comparison of these two structures.     

The structures of RNase A complexed with 3'-CMP and d(CpA): active site conformation and conserved water molecules.

The interactions of RNase A with cytidine 3'-monophosphate (3'-CMP) and deoxycytidyl-3',5'-deoxyadenosine (d(CpA)) were analyzed by X-ray crystallography. The 3'-CMP complex and the native structure were determined from trigonal crystals, and the d(CpA) complex from monoclinic crystals. The differences between the overall structures are concentrated in loop regions and are relatively small. The protein-inhibitor contacts are interpreted in terms of the catalytic mechanism. The general base His 12 interacts with the 2' oxygen, as does the electrostatic catalyst Lys 41. The general acid His 119 has 2 conformations (A and B) in the native structure and is found in, respectively, the A and the B conformation in the d(CpA) and the 3'-CMP complex. From the present structures and from a comparison with RNase T1, we propose that His 119 is active in the A conformation. The structure of the d(CpA) complex permits a detailed analysis of the downstream binding site, which includes His 119 and Asn 71. The comparison of the present RNase A structures with an inhibitor complex of RNase T1 shows that there are important similarities in the active sites of these 2 enzymes, despite the absence of any sequence homology. The water molecules were analyzed in order to identify conserved water sites. Seventeen water sites were found to be conserved in RNase A structures from 5 different space groups. It is proposed that 7 of those water molecules play a role in the binding of the N-terminal helix to the rest of the protein and in the stabilization of the active site.     

Structure of a hinge-bending bacteriophage T4 lysozyme mutant, Ile3-->Pro.

The mutant T4 phage lysozyme in which isoleucine 3 is replaced by proline (I3P) crystallizes in an orthorhombic form with two independent molecules in the asymmetric unit. Relative to wild-type lysozyme, which crystallizes in a trigonal form, the two I3P molecules undergo large hinge-bending displacements with the alignments of the amino-terminal and carboxy-terminal domains changed by 28.9 degrees and 32.9 degrees, respectively. The introduction of the mutation, together with the hinge-bending displacement, is associated with repacking of the side-chains of Phe4, Phe67 and Phe104. These aromatic residues are clustered close to the site of the mutation and are at the junction between the amino and carboxyl-terminal domains. As a result of this structural rearrangement the side-chain of Phe4 moves from a relatively solvent-exposed conformation to one that is largely buried. Mutant I3P also crystallizes in the same trigonal form as wild-type and, in this case, the observed structural changes are restricted to the immediate vicinity of the replacement. The main change is a shift of 0.3 to 0.5 A in the backbone of residues 1 to 5. The ability to crystallize I3P under similar conditions but in substantially different conformations suggests that the molecule undergoes large-scale hinge-bending displacements in solution. It is also likely that these conformational excursions are associated with repacking at the junction of the N-terminal and C-terminal domains. On the other hand, the analysis is complicated by possible effects of crystal packing. The different I3P crystal structures show substantial differences in the binding of solvent, both at the site of the Ile3-->Pro replacement and at other internal sites.     

Crystallization studies of glycosylated and unglycosylated human recombinant interleukin-2.

Glycosylated interleukin-2 (glyIL-2) has been crystallized in two crystal forms, and unglycosylated interleukin-2 (uIL-2) has been crystallized in three forms. The glycosylated form of the human recombinant IL-2 has been crystallized from 1.9 M ammonium sulfate, pH 6.5 to 7.0 in the hexagonal space group P6(2)22 or its enantiomorph. The crystals diffract to 2.8 A and contain two or three molecules per asymmetric unit. A second crystal form grows from 1.4 to 1.5 M ammonium sulfate in 0.2 M ammonium acetate, pH 5.0-5.5, as polycrystalline rosettes which are not suitable for even a preliminary crystallographic analysis. The uIL-2 crystallizes from 1.0 to 1.7 M ammonium sulfate, 0.2 M ammonium acetate, pH 4.5-5.6 in the monoclinic space group P2(1), and less frequently in the orthorhombic space group P2(1)2(1)2(1) from 2.5 M ammonium sulfate, pH 4.5 to 5.7. Cross-seeding uIL-2 with seeds from hexagonal crystals of glyIL-2 promotes nucleation of trigonal crystals of unglycosylated IL-2. These trigonal crystals belong to the space group P3(1)21 or its enantiomorph, with similar cell dimensions to the glyIL-2 hexagonal crystals.     

Directional cloning of cDNA using a selectable SfiI cassette

To increase the efficiency of directionally cloning cDNA, we have constructed a pair of vectors and devised a cDNA cloning strategy that improves upon previously published methods. The vectors, pLIB: AZ and pLIB: ZA, have two unique (distinct religation specificities; GGCCN/NNNNGGCC) SfiI sites (SfiI.A and SfiI.B) flanking a stuffer fragment which contains the tetracycline-resistance element. These vectors permit the directional cloning of cDNA in both sense (pLIB: AZ) and antisense (pLIB: ZA) orientations relative to the promoter for phage T3 RNA polymerase. cDNA that was synthesized using a primer with a 5' sequence of a SfiI.B site followed by an oligo(dT)16 3' tail was then ligated to an adaptor with the sequence of a SfiI.A site produced directional molecules that could be cloned into the pLIB vectors. Complex libraries with 10(7) members were produced from as few as 6 x 10(5) cells. The SfiI sites and stuffer can be subcloned as a cassette to permit directional cloning in other vectors, as there are several restriction enzyme sites flanking this region to the 5' and 3'.     

Structure of oxidized bacteriophage T4 glutaredoxin (thioredoxin). Refinement of native and mutant proteins.

The structure of wild-type bacteriophage T4 glutaredoxin (earlier called thioredoxin) in its oxidized form has been refined in a monoclinic crystal form at 2.0 A resolution to a crystallographic R-factor of 0.209. A mutant T4 glutaredoxin gives orthorhombic crystals of better quality. The structure of this mutant has been solved by molecular replacement methods and refined at 1.45 A to an R-value of 0.175. In this mutant glutaredoxin, the active site residues Val15 and Tyr16 have been substituted by Gly and Pro, respectively, to mimic that of Escherichia coli thioredoxin. The main-chain conformation of the wild-type protein is similar in the two independently determined molecules in the asymmetric unit of the monoclinic crystals. On the other hand, side-chain conformations differ considerably between the two molecules due to heterologous packing interactions in the crystals. The structure of the mutant protein is very similar to the wild-type protein, except at mutated positions and at parts involved in crystal contacts. The active site disulfide bridge between Cys14 and Cys17 is located at the first turn of helix alpha 1. The torsion angles of these residues are similar to those of Escherichia coli thioredoxin. The torsion angle around the S-S bond is smaller than that normally observed for disulfides: 58 degrees, 67 degrees and 67 degrees for wild-type glutaredoxin molecule A and B and mutant glutaredoxin, respectively. Each sulfur atom of the disulfide cysteines in T4 glutaredoxin forms a hydrogen bond to one main-chain nitrogen atom. The active site is shielded from solvent on one side by the beta-carbon atoms of the cysteine residues plus side-chains of residues 7, 9, 21 and 33. From the opposite side, there is a cleft where the sulfur atom of Cys14 is accessible and can be attacked by a nucleophilic thiolate ion in the initial step of the reduction reaction.     

Sugar interaction with metal ion: crystal structure and spectroscopic study of SrCl2.galactitol.4H2O

The crystal structure of SrCl(2).galactitol.4H(2)O has been determined. It belongs to monoclinic system, C2/c space group with unit cell dimensions: a=13.9849(3), b=14.1601(5), c=8.3026(3) A, beta=104.621(2) degrees, V=1590.9(9) A(3) and Z=4. Each Sr(2+) ion in the unit cell binds to two molecules of galactitol through O2 and O3 in one alditol and O2' and O3' in the other, as well as to four water molecules. Sr-O distances in SrCl(2).galactitol.4H(2)O complex range from 2.5420 to 2.6359 A. FT-IR, Raman and far-IR spectra of SrCl(2).galactitol.4H(2)O all show that SrCl(2) coordinates with galactitol through OH groups of the sugar molecule to form the new complex.     

Calculation of site affinity constants and cooperativity coefficients for binding of ligands and/or protons to macromolecules. I. Generation of partition functions and mass balance equations

The thermodynamics of binding of a ligand A and/or proton H to a macromolecule M is treated by the partition function method. In complex systems, the representation of the equilibria by means of cumulative constants beta PQR used as coefficients in partition functions ZM, ZA, and ZH is ill-suited to least-squares refinement procedures because the cumulative constants are interrelated by common cooperativity functions gamma j(i) and common site affinity constants kappa j. There is therefore the need to express ZM, ZA, ZH as functions of site constants kappa j and cooperativity coefficients bj. This is done by developing an algebra of partition functions based on the following concepts: (i) factorability of partition functions; (ii) binary generating function Jj = (1 + kappa j[Y])i tau for each class j of sites, represented by column (Jj) and row (Jj) vectors; (iii) cooperativity between sites of one class described by functions gamma j(i), represented by diagonal matrices gamma j; (iv) probability of finding microspecies represented by elements of tensor product matrix Ll = (J1)[J2]; (v) statistical factors mij obtained from Newton polynomials, Jj; (vi) power operators Oi', O(i-l)', and O(i tau-l)', transforming vectors Jj; and (vii) operators Oi or O(i-l) indicating tensor products of i or (i-l) vectors Jj. Vectors Jj combined in tensors Ll give rise to both an affinity/cooperativity space and a parallel index space. The partition functions ZM, ZA, and ZH and the total amounts TM, TA, and TH can be obtained as an appropriate sum of elements of matrices Ll, each of which is represented in an index space by a combination p1, p2,...q1, q2,...r1, r2,... of indices ij. From these indices the contribution of that element to partition function ZM, ZA, or ZH and to total amount TM, TA, or TH is calculated in the affinity/cooperativity space as product of factors: [i tau !/i !(i tau-i)!]kappa ij(exp[bj (i-1)i])[X]i, i being any index p, q, r and X any component M, A, or H. Future applications of this algorithm to practical problems of macromolecule-ligand-proton equilibria are outlined.     

The crystal structure of the 1:1 complex of beta-cyclodextrin with trans-cinnamic acid

The crystal structure of the 1:1 complex of beta-cyclodextrin (cyclomaltoheptaose) with trans-cinnamic acid was studied by X-ray diffraction. Two beta-cyclodextrin molecules related by a twofold crystal axis form dimers in the hydrophobic cavity of which, two guest molecules are entirely buried. The complex crystallizes in the monoclinic C2 space group with channel-type molecular packing. The oxygen atoms of the carboxylate group of the trans-cinnamic acid molecule form strong hydrogen bonds with two water molecules lying in the interdimeric space of the hydrophobic channel.     

High resolution structures of the 4-4-20 Fab-fluorescein complex in two solvent systems: effects of solvent on structure and antigen-binding affinity.

Three-dimensional structures were determined for three crystal forms of the antigen binding fragment (Fab) of anti-fluorescein antibody 4-4-20 in complex with fluorescein. These included 1) a triclinic (P1) form crystallized in 47% (v/v) 2-methyl-2,4-pentanediol (MPD); 2) a triclinic (P1) form crystallized in 16% (w/v) poly(ethylene glycol), molecular weight 3350 (PEG); and 3) a monoclinic (P21) form crystallized in 16% PEG. Solvent molecules were added to the three models and the structures were refined to their diffraction limits (1.75-A, 1.78-A, and 2.49-A resolution for the MPD, triclinic PEG, and monoclinic PEG forms, respectively). Comparisons of these structures were interesting because 4-4-20 exhibited a lower antigen-binding affinity in 47% MPD (Ka = 1.3 x 10(8) M-1) than in either 16% PEG (Ka = 2.9 x 10(9) M-1) or phosphate-buffered saline (Ka = 1.8 x 10(10) M-1). Even though the solution behavior of the antibody was significantly different in MPD and PEG, the crystal structures were remarkably similar. In all three structures, the fluorescein-combining site was an aromatic slot formed by tyrosines L32, H96, and H97 and tryptophans L96 and H33. In addition, several active site constituents formed an electrostatic network with the ligand. These included a salt link between arginine L34 and one of fluorescein's enolate oxygen atoms, a hydrogen bond between histidine L27d and the second enolic group, a hydrogen bond between tyrosine L32 and the phenylcarboxylate group, and two medium range (approximately 5 A) electrostatic interactions with lysine L50 and arginine H52. The only major difference between the triclinic MPD and PEG structures was the degree of hydration of the antigen-combining site. Three water molecules participated in the above electrostatic network in the MPD structure, while eight were involved in the PEG structure. Based on this observation, we believe that 4-4-20 exhibits a lower affinity in MPD due to the depletion of the hydration shell of the antigen-combining site.     

High-resolution crystal structures of ribonuclease A complexed with adenylic and uridylic nucleotide inhibitors. Implications for structure-based design of ribonucleolytic inhibitors

The crystal structures of bovine pancreatic ribonuclease A (RNase A) in complex with 3',5'-ADP, 2',5'-ADP, 5'-ADP, U-2'-p and U-3'-p have been determined at high resolution. The structures reveal that each inhibitor binds differently in the RNase A active site by anchoring a phosphate group in subsite P1. The most potent inhibitor of all five, 5'-ADP (Ki = 1.2 microM), adopts a syn conformation (in contrast to 3',5'-ADP and 2',5'-ADP, which adopt an anti), and it is the beta- rather than the alpha-phosphate group that binds to P1. 3',5'-ADP binds with the 5'-phosphate group in P1 and the adenosine in the B2 pocket. Two different binding modes are observed in the two RNase A molecules of the asymmetric unit for 2',5'-ADP. This inhibitor binds with either the 3' or the 5' phosphate groups in subsite P1, and in each case, the adenosine binds in two different positions within the B2 subsite. The two uridilyl inhibitors bind similarly with the uridine moiety in the B1 subsite but the placement of a different phosphate group in P1 (2' versus 3') has significant implications on their potency against RNase A. Comparative structural analysis of the RNase A, eosinophil-derived neurotoxin (EDN), eosinophil cationic protein (ECP), and human angiogenin (Ang) complexes with these and other phosphonucleotide inhibitors provides a wealth of information for structure-based design of inhibitors specific for each RNase. These inhibitors could be developed to therapeutic agents that could control the biological activities of EDN, ECP, and ANG, which play key roles in human pathologies.     

Three-dimensional structure of a light chain dimer crystallized in water. Conformational flexibility of a molecule in two crystal forms

The three-dimensional structure of an immunoglobulin light chain dimer (Mcg) crystallized in deionized water (orthorhombic form) was determined at 2.0 A resolution by phase extension and crystallographic refinement. This structure was refined side-by-side with that of the same molecule crystallized in ammonium sulfate (trigonal form). The dimer adopted markedly different structures in the two solvents. "Elbow bend" angles between pseudo 2-fold axes of rotation relating pairs of "variable" (V) and "constant" (C) domains were found to be 132 degrees in the orthorhombic form and 115 degrees in the trigonal form. Modes of association of the V domains and, to a lesser extent, the pairing interactions of the C domains were different in the two structures. Alterations in the V domain pairing were reflected in the shapes of the binding regions and in the orientations of the side-chains lining the walls of the binding sites. In the trigonal form, for instance, the V domain interface was compartmentalized into a main binding cavity and a deep pocket, whereas these spaces were continuous in the orthorhombic structure. Patterns of ordered water molecules were quite distinct in the two crystal types. In some cases, the solvent structures could be correlated with conformational changes in the proteins. For example, close contacts between V and C domains of monomer 1 of the trigonal form were not retained in orthorhombic crystals. Ordered water molecules filled the space created when the two domains moved apart.     

Cyclic diguanylic acid behaves as a host molecule for planar intercalators

Cyclic ribodiguanylic acid, c-(GpGp), is the endogenous effector regulator of cellulose synthase. Its three-dimensional structure from two different crystal forms (tetragonal and trigonal) has been determined by X-ray diffraction analysis at 1 A resolution. In both crystal forms, two independent c-(GpGp) molecules associate with each other to form a self-intercalated dimer. A hydrated cobalt ion is found to coordinate to two N7 atoms of adjacent guanines, forcing these two guanines to destack with a large dihedral angle (32 degrees), in the dimer of the tetragonal form. This metal coordination mechanism may be relevant to that of the anticancer drug cisplatin. Moreover, c-(GpGp) exhibits unusual spectral properties not seen in any other cyclic dinucleotide. It interacts with planar organic intercalator molecules in ways similar to double helical DNA. We propose a cage-like model consisting of a tetrameric c-(GpGp) aggregate in which a large cavity ('host') is generated to afford a binding site for certain planar intercalators ('guests').     

Crystallization and subunit structure of canine myeloperoxidase

Three crystal forms of canine myeloperoxidase are described. An orthorhombic form in space group P2(1)2(1)2(1) has unit cell dimensions: a = 108.3 A (1 A = 0.1 nm) b = 205.9 A and c = 139.9 A. A trigonal form in space group P3(1)21 or P3(2)21 has unit cell dimensions: a = b = 138.9 A and c = 145.2 A. A monoclinic form in space group C2 has unit cell dimensions: a = 117.2 A, b = 96.9 A, c = 131.4 A and beta = 116.3 degrees. Unusual features in the diffraction patterns of the monoclinic form place restrictions on the molecular packing in the crystal. The proposed model for the molecular packing requires that the myeloperoxidase molecule consist of two identical or near-identical halves. In the intact molecule these halves may be related either by a crystallographic dyad axis or by an approximate dyad axis in which one subunit is translated relative to the other by 3.2 A along the symmetry axis. The trigonal crystal form appears most suitable for high-resolution X-ray structural analysis.     

Complex of NS3 protease and NS4A peptide of BK strain hepatitis C virus: a 2.2 A resolution structure in a hexagonal crystal form.

The crystal structure of the NS3 protease of the hepatitis C virus (BK strain) has been determined in the space group P6(3)22 to a resolution of 2.2 A. This protease is bound with a 14-mer peptide representing the central region of the NS4A protein. There are two molecules of the NS3(1-180)-NS4A(21'-34') complex per asymmetric unit. Each displays a familiar chymotrypsin-like fold that includes two beta-barrel domains and four short alpha-helices. The catalytic triad (Ser-139, His-57, and Asp-81) is located in the crevice between the beta-barrel domains. The NS4A peptide forms an almost completely enclosed peptide surface association with the protease. In contrast to the reported H strain complex of NS3 protease-NS4A peptide in a trigonal crystal form (Kim JL et al., 1996, Cell 87:343-355), the N-terminus of the NS3 protease is well-ordered in both molecules in the asymmetric unit of our hexagonal crystal form. The folding of the N-terminal region of the NS3 protease is due to the formation of a three-helix bundle as a result of crystal packing. When compared with the unbound structure (Love RA et al., 1996, Cell 87:331-342), the binding of the NS4A peptide leads to the ordering of the N-terminal 28 residues of the NS3 protease into a beta-strand and an alpha-helix and also causes local rearrangements important for a catalytically favorable conformation at the active site. Our analysis provides experimental support for the proposal that binding of an NS4A-mimicking peptide, which increases catalytic rates, is necessary but not sufficient for formation of a well-ordered, compact and, hence, highly active protease molecule.     

X-ray structure analysis of 3,11,17-trioxoandrostane-5 alpha-carbonitrile and of 17 alpha-ethoxycarbonyloxy-3-oxoandrost-4-ene-17-carbonitrile.

The crystal and molecular structures of the title compounds were determined by x-ray diffractometric analysis. Torsion angles and puckering parameters are reported for both compounds. In 1 the 5 alpha-cyano group influences the A-ring conformation. The carbonate ester 3 crystallizes in the monoclinic P2(1) space group with two molecules (I and II) in the asymmetric unit. The D-ring conformation is to some extent different between I and II.     

The X-ray investigation of 16 alpha,17 alpha-thiazolidine derivatives of delta 4- and delta 5-pregnanes and conformational analysis of their oxathiolane analog

An X-ray study of 3,20-dioxo-4-pregnene-[16 alpha,17 alpha-d]--2',2'-dimethylthiazolidine (I) and 3 beta-hydroxy-20-oxo-5--pregnene-[16 alpha,17 alpha-d]-2',2'- dimethylthiazolidine (II) has been carried out. Two independent molecules in crystal II have significantly different conformations of the D and E rings, although according to the atom-atom potential calculations the energy of interaction of these molecules with their neighbors in crystal is the same. The calculation of conformational energy of 3,20-dioxo-4-pregnene-[17 alpha,16 alpha-d]-2',2'--dimethyloxathiolane (III) by the molecular mechanics method (MMM) indicates a possibility of existence of two similar conformers also for this molecule. The MMM calculation shows also that the conformation of molecule III (as well as progesterone) with the 17 beta-acetyl group torsion angle C(16)C(17)C(20)0(20) close to -120 degrees is possible.