Single diastereomers of unsymmetrical tris-spirocyclic cyclotriphosphazenes based on 1,1'-bi-2-naphthol--synthesis and structures

Diastereoselective synthesis and characterization of chiral unsymmetrical tris-spirocyclic cyclotriphosphazenes based on chiral 1,1'-bi-2-naphthol (BINOL) are reported. Specifically, the chiral compounds (-)N(3)P(3)[1,1'-O(2)(C(10)H(6))(2)](O-2,2'C(6)H(4)-C(6)H(4)O)Cl(2) [(-)-4] and (-)N(3)P(3)[1,1'-O(2)(C(10)H(6))(2)](OCH(2)CH(2)NMe)(2) [(-)-5] are prepared by starting with the chiral mono-spiro compound (-)N(3)P(3)[1,1'-O(2)(C(10)H(6))(2)]Cl(4) [(-)-3]. Synthesis of four other chiral spirocyclics, N(3)P(3)[1,1'-O(2)(C(10)H(6))(2)](OCH(2)CH(2) NMe)(O-2,2'C(6)H(4)-C(6)H(4)O)[(-)-6 and (+)-6], N(3)P(3)[1,1'-O(2)(C(10)H(6))(2)](NMe(2))(4) [(-)-7], N(3)P(3)[1,1'-O(2)(C(10)H(6))(2)](O-2,2'C(6)H(4)-C(6)H(4)O)(NMeCH(2)CH(2)OH)(2) [(-)-8 and (+)-8], and N(3)P(3)[1,1'-O(2)(C(10)H(6))(2)](O-2,2'C(6)H(4)-C(6)H(4)O)[NHCH(2)CH(2)CH(2)Si(OEt)(3)](2) (9) is also reported herein. Compounds 4-6 are obtained in the solid state diastereoselectively and their X-ray structures have been determined and discussed. The diastereoselectivity is also shown by structural characterization of two distinct isomers in the case of 6 [(-)-6 and (+)-6, respectively] by starting with precursor of 3 having (R) or (S)-BINOL residue. The (1)H NMR spectra of 7 and 8 exhibit doublets with virtual coupling for the methyl protons, consistent with the chiral nature of the binaphthoxy residue. The potential of 9, which hydrolyzes readily in CDCl(3) solution, as a useful precursor for chiral polymer applications is highlighted.     

Stereochemical specificity of organophosphorus acid anhydrolase toward p-nitrophenyl analogs of soman and sarin

Organophosphorus acid anhydrolase (OPAA) catalyzes the hydrolysis of p-nitrophenyl analogs of the organophosphonate nerve agents, sarin and soman. The enzyme is stereoselective toward the chiral phosphorus center by displaying a preference for the R(P)-configuration of these analogs. OPAA also exhibits an additional preference for the stereochemical configuration at the chiral carbon center of the soman analog. The preferred configuration of the chiral carbon center is dependent upon the configuration at the phosphorus center. The enzyme displays a two- to four-fold preference for the R(P)-enantiomer of the sarin analog. The k(cat)/K(m) of the R(P)-enantiomer is 250 M(-1) s(-1), while that of the S(P)-enantiomer is 110 M(-1) s(-1). The order of preference for the stereoisomers of the soman analog is R(P)S(C) > R(P)R(C) > S(P)R(C) > S(P)S(C). The k(cat)/K(m) values are 36,300 M(-1)s(-1), 1250 M(-1) s(-1), 80 M(-1) s(-1) and 5 M(-1) s(-1), respectively. The R(P)S(C)-isomer of the soman analog is therefore preferred by a factor of 7000 over the S(P)S(C)-isomer.     

Stereoselective formation and hydration of 12-methylbenz[a]anthracene 5,6-epoxide enantiomers by rat liver microsomal enzymes.

The K-region trans-5,6-dihydrodiols formed in the metabolism of 12-methylbenz[a]anthracene (12-MBA) by liver microsomal preparations from untreated, phenobarbital-treated and 3-methylcholanthrene-treated male Sprague-Dawley rats were found by chiral stationary-phase h.p.l.c. (c.s.p.-h.p.l.c.) analyses to contain (5S,6S)/(5R,6R) enantiomer ratios of 93:7, 88:12 and 97:3 respectively. The absolute stereochemistry of a 12-MBA trans-5,6-dihydrodiol enantiomer was elucidated by the exciton-chirality c.d. method. The 5,6-epoxides formed in the metabolism of 12-MBA by liver microsomal preparations from untreated, phenobarbital-treated and 3-methylcholanthrene-treated male Sprague-Dawley rats in the presence of the epoxide hydrolase inhibitor 3,3,3-trichloropropylene 1,2-oxide were isolated from a mixture of metabolites by normal-phase h.p.l.c., and their (5S,6R)/(5R,6S) enantiomer ratios were found by c.s.p.-h.p.l.c. analyses to be 73:27, 78:22 and 99:1 respectively. The absolute configurations of 12-MBA 5,6-epoxide enantiomers, resolved by c.s.p.-h.p.l.c., were determined via high-resolution (500 MHz) proton-n.m.r. and c.d. spectral analyses of the two isomeric methoxylation products derived from each of the 12-MBA 5,6-epoxide enantiomers. Enantiomeric pairs of the two methoxylation products were resolved by c.s.p.-h.p.l.c. The results indicate that enantiomeric 5S,6R-epoxide and 5S,6S-dihydrodiol were the major enantiomers preferentially formed in the metabolism at the K-region 5,6-double bond of 12-MBA by all three rat liver microsomal preparations. Optically pure 12-MBA 5S,6R-epoxide was hydrated predominantly at the C(6) position (R centre) to form 12-MBA trans-5,6-dihydrodiol with a (5S,6S)/(5R,6R) enantiomer ratio of 97:3. However, optically pure 12-MBA 5R,6S-epoxide was hydrated nearly equally at both C(5) and C(6) positions to form 12-MBA trans-5,6-dihydrodiol with a (5S,6S)/(5R,6R) enantiomer ratio of 57:43.     

Sequence- and stereospecific conformational rearrangement of styrene oxide adducts located at A x C mismatched base pairs

The solution structures of R- and S-alpha-(N(6)-adenyl)-styrene oxide adducts mismatched with cytosine at position X(7) in d(CGGACAXGAAG) x d(CTTCCTGTCCG), incorporating codons 60, 61 (underlined), and 62 of the human N-ras protooncogene, were determined. These were the R- and S(61,3)C adducts. The structures for these mismatched adducts differed from the sequence isomeric R- and S(61,2)C adducts [Painter, S. L., Zegar, I. S., Tamura, P. J., Bluhm, S., Harris, C. M., Harris, T. M., and Stone, M. P. (1999) Biochemistry 38, 8635-8646]. The results reveal that the structural consequences of cytosine mispairing opposite the R- and S-alpha-SO adducts differ as a function of DNA sequence. The thermodynamic stability of both the R- and S(61,3)C mismatched adducts was dependent upon pH. At neutral pH, the R- and S(61,3)C adducts exhibited significant structural perturbation and had lower T(m) values, as compared to the R- and S(61,2)C adducts. In both instances, this was attributed to reorientation about the C6-N(6) bond, such that the N(6)H proton faced away from the Watson-Crick face of the purine base and into the major groove. The conformation about the N(6)-C(alpha)-C(beta)-O torsion angle was predicted from rMD calculations to be stabilized by a N/O gauche-type interaction between the styrenyl hydroxyl moiety and adenine N(6) at the lesion site. For the R(61,3)C adduct, the styrenyl moiety remained oriented in the major groove and faced in the 3'-direction. In the properly base-paired R(61,3) adduct, it had faced in the 5' direction. For the S(61,3)C adduct, the styrene ring was inserted into the duplex, approximately perpendicular to the helical axis of the DNA. It faced in the 5'-direction. In the properly base-paired S(61,3) adduct, it had faced in the 3'-direction. The results were correlated with site-specific mutagenesis experiments in vivo. The latter revealed that the R- and S(61,3)-alpha-styrene oxide adducts were nonmutagenic. This may be a consequence of the greater structural perturbation associated with formation of the cytosine mismatch at neutral pH for the R- and S(61,3) adducts as compared to the S(61,2) adduct that exhibited low levels of A --> G mutations.     

The crystal and molecular structure of 2'-deoxy-2'-fluoroinosine monohydrate.

The structure of the hydrate of 2'-deoxy-2'-fluoroinosine has been determined by single-crystal x-ray diffraction. The nucleoside crystallizes in space group P2(1)2(1)2(1) with unit cell dimensions, a = 33.291, b = 10. 871, c = 6.897A. There are two nucleosides and two water molecules in the asymmetric unit. The structure was solved by direct methods and refined to a residual R = 0.095. The two independent nucleosides in the asymmetric unit show different conformations about the glycosidic bond, while other structural details are similar. The base orientation to the sugar is syn in molecule A, whereas anti in molecule B. The exocyclic C(4')-C(5') bond conformation defined with respect to C(3')-C(4')-C(5')-O(5') is gauche+ in both molecules A and B. The sugar ring pucker defined by the pseudorotation phase angle P is a twisted conformation in both, C(3')-endo-C(4')-exo with P = 29 degrees in molecule A and C(4')-exo-C(3')-endo with P = 41 degrees in molecule B. It is shown by comparison with x-ray results of other 2'-fluoronucleosides and unmodified nucleosides including inosines that, in addition to a strong preference of the C(3')-endo type pucker, twisted conformations involving C(4')-exo puckering may be one of characteristic features of 2'-fluoronucleosides.     

Book Reviews

Book Reviewed in this article:
T he L aboratory D iagnosis of V enereal D isease. Public Health Laboratory Service Monograph Series. H.M.S.O. &0.50.
V irus -C ell I nteractions and V iral A ntimetabolites , 1972. Edited by D. Shugar. Federation of European Biochemical Societies.
W orld D irectory of C ollections of C ultures of M icro -O rganisms (1972).     

Discrimination in resolving systems. V. Pseudoephedrine - fluoromandelic acid diastereomers

Resolution of the isomeric 2'-, 3'-, and 4'-fluoromandelic acids with (+)-(1S;2S)-pseudoephedrine in 95% ethanol produces both well and poorly discriminating, hydrated and unsolvated binary salts. Seven observed diastereomeric phases are represented by five crystal structure types including three of the four types observed in the pseudoephedrine mandelates. Type a: monoclinic hemihydrate less-soluble (L) (R)-3'-fluoromandelate and more-soluble (M) (R)-4'-fluoromandelate (I); type b: orthorhombic unsolvated M (S)-2'-fluoromandelate; type c: orthorhombic unsolvated L (R)-2'-fluoromandelate; type d: orthorhombic dihydrate M (S)-3'-fluoromandelate and L (S)-4'-fluoromandelate; type e: monoclinic unsolvated M (R)-4'-fluoromandelate (II). Largest (15-fold) discriminating solubilities in 95% ethanol are found between the diastereomers with 2'-fluoromandelic acid, 50% more than in the corresponding ephedrine system. Principle interionic interactions are hydrogen-bonds between protonated secondary ammonium ions and carboxylates. Infinite chains of these are found in type c, with a four-atom repeating unit H-N(+)-H.O(-C(-)-O) [C(2)(1)(4)], and in types b and d, with a six-atom repeating unit H-N(+)-H.O-C(-)-O [C(2)(2)(6)]. Water of crystallization intervenes in the chains of type a but not of type d hydrated salts, according with higher average dehydration temperatures in the former. Hydrated salts in general are excessively soluble in 95% ethanol.     

Improvements of enzyme activity and enantioselectivity via combined substrate engineering and covalent immobilization

Esterases, lipases, and serine proteases have been applied as versatile biocatalysts for preparing a variety of chiral compounds in industry via the kinetic resolution of their racemates. In order to meet this requirement, three approaches of enzyme engineering, medium engineering, and substrate engineering are exploited to improve the enzyme activity and enantioselectivity. With the hydrolysis of (R,S)-mandelates in biphasic media consisting of isooctane and pH 6 buffer at 55 degrees C as the model system, the strategy of combined substrate engineering and covalent immobilization leads to an increase of enzyme activity and enantioselectivity from V(S)/(E(t)) = 1.62 mmol/h g and V(S)/V(R) = 43.6 of (R,S)-ethyl mandelate (1) for a Klebsiella oxytoca esterase (named as SNSM-87 from the producer) to 16.7 mmol/h g and 867 of (R,S)-2-methoxyethyl mandelate (4) for the enzyme immobilized on Eupergit C 250L. The analysis is then extended to other (R,S)-2-hydroxycarboxylic acid esters, giving improvements of the enzyme performance from V(S)/(E(t)) = 1.56 mmol/h g and V(S)/V(R) = 41.9 of (R,S)-ethyl 3-chloromandelate (9) for the free esterase to 39.4 mmol/h g and 401 of (R,S)-2-methoxyethyl 3-chloromandelate (16) for the immobilized enzyme, V(S)/(E(t)) = 5.46 mmol/h g and V(S)/V(R) = 8.27 of (R,S)-ethyl 4-chloromandelate (10) for free SNSM-87 to 33.5 mmol/h g and 123 of (R,S)-methyl 4-chloromandelate (14) for the immobilized enzyme, as well as V(S)/(E(t)) = 3.0 mmol/h g and V(S)/V(R) = 7.94 of (R,S)-ethyl 3-phenyllactate (11) for the free esterase to 40.7 mmol/h g and 158 of (R,S)-2-methoxyethyl 3-phenyllactate (18) for the immobilized enzyme. The great enantioselectivty enhancement is rationalized from the alteration of ionization constants of imidazolium moiety of catalytic histidine for both enantiomers and conformation distortion of active site after the covalent immobilization, as well as the selection of leaving alcohol moiety via substrate engineering approach.     

DFT studies of the disaccharide, alpha-maltose: relaxed isopotential maps

The disaccharide, alpha-maltose, forms the molecular basis for the analysis of the structure of starch, and determining the conformational energy landscape as the molecule oscillates around the glycosidic bonds is of importance. Thus, it is of interest to determine, using density functionals and a medium size basis set, a relaxed isopotential contour map plotted as a function of the phi(H) and psi(H) dihedral angles. The technical aspects include the method of choosing the starting conformations, the choice of scanning step size, the method of constraining the specific dihedral angles, and the fitting of data to obtain well defined contour maps. Maps were calculated at the B3LYP/6-31+G( *) level of theory in 5 degrees intervals around the (phi(H),psi(H))=(0 degrees ,0 degrees ) position, out to approximately +/-30 degrees or greater, for gg-gg'-c, gg-gg'-r, gt-gt'-c, gt-gt'-r, tg-tg'-c, and tg-tg'-r conformers, as well as one-split gg(c)-gg'(r) conformer. The results show that the preferred conformation of alpha-maltose in vacuo depends strongly upon the hydroxyl group orientations ('c'/'r'), but the energy landscape moving away from the minimum-energy position is generally shallow and transitions between conformational positions can occur without the addition of significant energy. Mapped deviations of selected parameters such as the dipole moment; the C1-O1-C4', H1-C1-O1, and H4'-C4'-O1 bond angles; and deviations in hydroxymethyl rotamers, O5-C5-C6-O6, O5'-C5'-C6'-O6', C5-C6-O6-H, and C5'-C6'-O6'-H', are presented. These allow visualization of the structural and energetic changes that occur upon rotation about the glycosidic bonds. Interactions across the bridge are visualized by deviations in H(O2)...O3', H(O3')...O2, and H1...H4' distances and the H(O2)-O2-C2-C1 and H'(O3')-O3'-C3'-C4' hydroxyl dihedral angles.     

Book reviews

Book reviewed in this article:
C oncepts in V iral P athogenesis III (1989). Edited by A.L. Notkins & M.B.A. Old-stone.
A C olour A tlas of M eat I nspection (1990). By J. Infante Gil & J. Costa Durao.
P romiscuous P lasmids of G ram -N egative B acteria (1989). Edited by Christopher M. Thomas.
S hort P rotocols in M olecular B iology (1989). Edited by F.M. Ausubel et al.
G enetics of B acterial D iversity (1989). Edited by D.A. Hopwood & K.F. Chater.
Y east G enetics . A M anual of M ethods (1989). By J.F.T. Spencer, D.M. Spencer & I.J. Burce.
M etals and M icro -O rganisms (1989). By M.N. Hughes & R.K. Poole.
S eed -B orne D isease and S eed H ealth T esting of R ice (1989). By P. C. Agarwal, C.N. Mortensen & S.B. Mathur.     

Refinement of the solution structure of a branched DNA three-way junction.

We have refined the structure of the DNA Three-Way Junction complex, TWJ-TC, described in the companion paper by quantitative analysis of two 2D NOESY spectra (mixing times 60 and 200 ms) obtained in D2O solution. NOESY crosspeak intensities extracted from these spectra were used in two kinds of refinement procedure: 1) distance-restrained energy minimization (EM) and molecular dynamics (MD) and 2) full relaxation matrix back calculation refinement. The global geometry of the refined model is very similar to that of a published, preliminary model (Leontis, 1993). Two of the helical arms of the junction are stacked. These are Helix 1, defined by basepairs S1-G1/S3-C12 through S1-C5/S3-G8 and Helix 2, which comprises basepairs S1-C6/S2-G5 through S1-G10/S2-G1. The third helical arm (Helix 3), comprised of basepairs S2-C6/S3-G5 through S2-C10/S3-G1 extends almost perpendicularly from the axis defined by Helices 1 and 2. The bases S1-C5 and S1-C6 of Strand 1 are continuously stacked across the junction region. The conformation of this strand is close to that of B-form DNA along its entire length, including the S1-C5 to S1-C6 dinucleotide step at the junction. The two unpaired bases S3-T6 and S3-C7 lie outside of the junction along the minor groove of Helix 1 and largely exposed to solvent. Analysis of the refined structure reveals that the glycosidic bond of S3-T6 exists in the syn conformation, allowing the methyl group of this residue to contact the hydrophobic surface of the minor groove of Helix 1, at S3-G11.(ABSTRACT TRUNCATED AT 250 WORDS)     

Book reviews

Book reviewed in this article:
P arasitology in F ocus : F acts and T rends (1988). Edited by H. Mehlman.
C olour A tlas for the D iagnosis of B acterial P athogens in A nimals (1988). Edited by W. Bisping & G. Amtsberg.
R ole of G ut F lora in T oxicity and C ancer (1988). By I.R. Rowland.
A naerobes T oday (1988). Edited by J.M. Hardie & S.P. Bordello.
T he R elease of G enetically -E ngineered M icro -O rganisms (1988). Edited by M. Sussman, C.H. Collins, F.A. Skinner & D.E. Stewart-Tull.
M icrobial T echnology in the D eveloping W orld (1988). Edited by E.J. Dasilva, Y.R. Dommergues, E.J. Nyns & C. Ratledge.
M icrobial L ipids Volume 1 (1988). Edited by C. Ratledge & S.G. Wilkinson.     

Three-dimensional structural analysis of the group B polysaccharide of Neisseria meningitidis 6275 by two-dimensional NMR: the polysaccharide is suggested to exist in helical conformations in solution

The solution conformations of the group B polysaccharide of Neisseria meningitidis were analyzed by DQF-COSY and pure absorption 2D NOE NMR with three mixing times. The pyranose ring of the sialic acid residue was found to be in the 2C5 conformation. The DQF-COSY analysis indicated that the orientations of H6 and H7 and of H7 and H8 are both gauche. In order to overcome the difficulties in analyzing the NOE data due to the two sets of proton overlaps, molecular modeling of alpha-2,8-linked sialic acid oligomers was carried out to investigate possible conformers, and theoretical NOE calculations were performed by using CORMA (complete relaxation matrix analysis). Our analysis suggests that the polysaccharide adopts helical structures for which the phi (defined by O6-C2-O8-C8) and psi (C2-O8-C8-C7) angles are in the following ranges: phi -60 to 0 degrees, psi 115-175 degrees or phi 90-120 degrees, psi 55-175 degrees. The weak affinity of anti-B antibodies for smaller alpha-2,8-linked oligosaccharides may be due to the fact that such oligomers are more flexible and may not form an ordered structure as the poly(sialic acid) does.     

Structure of the YSPTSPS repeat containing two SPXX motifs in the CTD of RNA polymerase II: NMR studies of cyclic model peptides reveal that the SPTS turn is more stable than SPSY in water.

The carboxyl-terminal domain of RNA polymerase II, which is rich in phosphorylation sites, contains 17--52 tandem repeats with the consensus sequence of the heptapeptide, YSPTSPS. The repeat unit of the heptapeptide has two SPXX motifs showing potential beta-turns, SPTS and SPSY. NMR studies were performed in water at pH 4.0 for two cyclic peptides containing one and two repeat units, cyclo-[C(1)R(2)D(3)Y(4)S(5)P(6)T(7)S(8)P(9)S(10)Y(11)S(12)R(13)D(14)C(15)] (peptide 1) and cyclo-[C(1)R(2)D(3)Y(4)S(5)P(6)T(7)S(8)P(9)S(10)Y(11)S(12)P(13)T(14)S(15)P(16)N(17)Y(18)S(19)R(20)D(21)C(22)] (peptide 2), which are cyclized with a disulfide bridge of two Cys residues at the N- and C-termini. SP in 1 and 2 are predominantly in trans form. The following NMR parameters were detected: (1) lower temperature coefficients of amide proton chemical shifts of T7 and S8 in 1, and Tx (T7 or T14), Sx (S8 or S15), Tz (T14 or T7) and Sz (S15 or S8) in 2, (2) significantly large deviation of H(alpha) chemical shifts from its random coil value (Delta H(alpha)) of Pro preceding the Thr (P6 in 1, and Px and Pz in 2), (3) relatively large (3)J(HNH alpha) coupling constants (>8.7 Hz) of T7 in 1 and Tx and Tz in 2, and (4) NOE (d(NN) (i, i+1)) connectivities between the amide protons of T7-S8 and S10-Y11 in 1, and Tx-Sx, S10-Y11, Tz-Sz, and N17-Y18 in 2, although two Pro-Thr-Ser segments in 2 (each of these are annotated by 'x' and 'z') in the first and second repeat units were not distinguishable. Comparison of the NMR parameters between the cyclic peptides and the corresponding linear peptides indicates that cyclization promotes structural stabilization in water. The present NMR data were consistent with the presence of a beta-turn at both SPTS and SPSY: S(5)P(6)T(7)S(8) and S(8)P(9)S(10)Y(11) in 1, and SPxTxSx, SPzTzSz, SP(9)S(10)Y(11), SP(16)N(17)Y(18) in 2. However, the structure of the SPTS segment is more stable than that of the SPSY segment. Conformations consistent with NMR parameters including NOE distances were obtained through molecular dynamics and energy minimization methods. These calculations yielded two stable conformers for the SPTS segment. One of the two corresponds to a type I beta-turn.     

Reviews

Book Reviewed in this article:
Phloem Transport in Plants. By A. S. C rafts and C. H. C risp .
Introduction to the Fine Structure of Plant Cells. By M yron C. L edbetter and K eith R. P orter .
Symposia of the Society for Experimental Biology No. 25. Control Mechanisms of Growth and Differentiation. Ed. by D. D. D avies and M. B alls .
Plant Lipid Biochemistry. By C. H itchcock and B. W. N ichols .
Cell Respiration. By W. O. J ames .
Prediction and Measurement of Photosynthetic Productivity.
Fortschritte der Botanik , Vol. 32. Ed. by H. E llenberg , K. E sser , H. M erxmüller , P. S itte and H. Z iegler .
Stoffwechselphysiologie der Pflanzen. By G. R ichter .
Pflanzenphysiologie. By D. H ess .
Moleculare Grundlagen der Entwicklung. By H. M ohr and P. S itte .
The Diversity of Green Plants. By P eter B ell and C hristopher W oodcock .
Anatomy of the Monocotyledons. Ed. by C. R. M etcalfe . Vol. V. Cyperaceae. By C. R. M etcalfe .
Soil Micro-organisms. By T. R. G. G ray and S. T. W illiams .
Des Ovules aux Graines. By M. F avre -D uchartre .
Ecology of Leaf Surface Micro-organisms. Ed. by T. F. P reece and C. H. D ickinson .
Fungal Genetics. By J. R. S. F incham and P. R. D ay .
Étude du Mode de Reproduction: Apomixie Facultative du Point de Vue de la Génétique des Populations. (Study of Facultative Apomixis from the View-point of Population Genetics). By J. P erèns .
Drawings of British Plants ; Part XXVIII: Hydrocharitaceae, Orchidaceae. By S tella R oss -C raig .
Plants of Hong Kong. By S. L. T hrower .
An Atlas of Plant Structure , Vol. 1. By B rian B racegirdle and P atricia H. M iles .
Bioiléctricité, Quelques Problèmes. By S. M eylan .
Studying the Past by Pollen Analysis. By R. G. W est .
The Mysterious Origin of Flowering Plants. By K enneth R. S porne .     

Crystal and molecular structure of a DNA duplex containing the carcinogenic lesion O6-methylguanine

The crystal and molecular structure of the first DNA duplex containing the carcinogenic lesion O6MeG has been determined to a resolution of 1.9 A and refined to an R factor of 19%. (d[CGC-(O6Me)GCG])2 crystallizes in the left-handed Z DNA form and has crystal parameters and conformational features similar to those of the parent sequence [d(CG)3]2. The methyl groups on O6 of G4 and G10 have C5-C6-O6-O6Me torsion angles of 73 degrees and 56 degrees, respectively, and protrude onto the major groove surface. The base-pairing conformation for the methylated G.C base pairs is of the Watson-Crick type as opposed to a wobble-type conformation that had been proposed in a B DNA fragment. As in other Z DNA structures, a spine of hydration is seen in the minor groove.     

Crystal structure of the promutagen O4-methylthymidine: importance of the anti conformation of the O(4) methoxy group and possible mispairing of O4-methylthymidine with guanine

O4-Methylthymidine (O4medT) is a promutagen. To correlate its biological properties to changes in the electronic, geometric, and conformational properties of the pyrimidine base resulting from the keto to enol shift arising from methylation, an X-ray study of O4medT was undertaken. The crystal data are a = 4.950 (2) A, b = 12.648 (1) A, c = 19.305 (2) A, space group P2(1)2(1)2(1), Z = 4, and R = 0.042. The D-deoxyribofuranosyl ring is puckered in the uncommon 1T2 twist conformation with the phase angle of pseudorotation P = 133.8 (5)degrees. The amplitude of puckering tau m = 31.4 (3)degrees shows that the ring is considerably flattened. The base is in the anti conformation [chi CN = 40.6 (4)degrees], and the exocyclic C(4')-C(5') bond (psi) is gauche+ [46.2 (5)degrees]. Methylation produces cytosine-like conjugation for the thymine base. The methoxy group takes the syn-periplanar conformation. Two types of mispairings with guanine are possible, and both require the anti conformation for the O(4) methoxy group. Semiempirical energy calculations have been carried out and reveal that the anti conformation can be energetically assumed in the double helix by widening the exocyclic angles C(5)-C(4)-O(4) and C(4)-C(5)-C(7) and the angle C(4)-O(4)-C(8) at the methoxy group. Such coordinated expansion relieves unfavorable interactions between the C(7) and C(8) methyl groups.     

REVIEWS

Books reviewed in this article:
Physiology of plants and their cells. By J. A. G ross .
Plant Physiology. By M eiron T homas , S. L. R anson and J. A. R ichardson .
Rate Control of Biological Processes. Symposium No. 27 of the Society for Experimental Biology. Ed. by D. D. D avies
Towards an Understanding of the Mechanism of Heredity. By H. L. K. W hitehouse .
Chromosome Botany and the Origins of Cultivated Plants. By C. D. D arlington .
Handbook of Phycological Methods: Culture Methods and Growth Measurements. By J anet R. S tein (Ed.)
Drawings of British Plants. Part XXXI. Lemnaceae, Alismataceae, Butomaceae, Junca-ginaceae, Scheuchzeriaceae, Potamagetonaceae, Ruppiaceae, Zannichelliaceae, Zosteraceae, Najadaceae, Eriocaulaceae. By S tella R oss -C raig .
Census Catalogue of the Flora of Ireland. By M. J. P. S cannell and D. M. S ynnot .
The Genus Lesquerella (Cruciferae) in North America. By R. C. R ollins and E. A. S haw .
Allgemeine Geobotanik. By H. W alter .
Einführung In Die Botanik. By R. B ornkamm .
Ökologie der Pflanzen. By W. L archer .     

Recent advances in the molecular analysis of inherited disease

Many important human genes have been cloned during the last ten years. In some cases, using reverse genetic techniques [Orkin, S. H. (1986) Cell 47, 845-850], disease-causing genes have been isolated whose product was previously unknown. Important examples include the dystrophin protein which, when mutated, gives rise to either Duchenne or Becker muscular dystrophy [Koenig, M., Hoffman, E. P., Bertelson, C. J., Monaco, A. P., Feener, C. and Kunkel, L. M. (1987) Cell 50, 509-517; Monaco, A. P., Bertelson, C. J., Liechti-Gallati, S. & Kunkel, L. M. (1988) Genomics 2, 90-95; Koenig, M., Monaco, A. P. & Kunkel, L. M. (1988) Cell 53, 219-228] and the cystic fibrosis transmembrane conductance regulator (CFTR) [Riordan, J. R., Rommens, J. M., Kerem, B.-S., Alon, N., Rozmahel, R., Grzelczak, Z., Zielenski, J., Lok, S., Plavsic, N., Chou, J.-L., Drumm, M. L., Ianuzzi, M. C., Collins, F. S. & Tsui, L.-C. (1989) Science 245, 1066-1073]. Recently the technology for systematically detecting single base-pair changes by chemical methods, enzymatic methods or direct DNA sequencing has greatly expanded and simplified. In addition to providing structural information about these clinically important genes and information on disease-causing mutations, these studies have led to an increased understanding of mechanisms of mutation, to the discovery of novel genetic mechanisms and to important clinical applications of carrier detection and pre-natal diagnosis. The recent rapid progress has been made possible by the development of DNA amplification using the polymerase chain reaction (pcr) invented by Saiki and colleagues [Saiki, R. K., Chang, C-A., Levenson, C. H., Warren, T. C., Boehm, C. D., Kazazian, H. H. & Ehrlich, H. A. (1988) N. Engl. J. Med. 319, 537-541].