Collagen stability: insights from NMR spectroscopic and hybrid density functional computational investigations of the effect of electronegative substituents on prolyl ring conformations

Collagen-like peptides of the type (Pro-Pro-Gly)(10) fold into stable triple helices. An electron-withdrawing substituent at the H(gamma)(3) ring position of the second proline residue stabilizes these triple helices. The aim of this study was to reveal the structural and energetic origins of this effect. The approach was to obtain experimental NMR data on model systems and to use these results to validate computational chemical analyses of these systems. The most striking effects of an electron-withdrawing substituent are on the ring pucker of the substituted proline (Pro(i)) and on the trans/cis ratio of the Xaa(i-1)-Pro(i) peptide bond. NMR experiments demonstrated that N-acetylproline methyl ester (AcProOMe) exists in both the C(gamma)-endo and C(gamma)-exo conformations (with the endo conformation slightly preferred), N-acetyl-4(R)-fluoroproline methyl ester (Ac-4R-FlpOMe) exists almost exclusively in the C(gamma)-exo conformation, and N-acetyl-4(S)-fluoroproline methyl ester (Ac-4S-FlpOMe) exists almost exclusively in the C(gamma)-endo conformation. In dioxane, the K(trans/cis) values for AcProOMe, Ac-4R-FlpOMe, and Ac-4S-FlpOMe are 3.0, 4.0, and 1.2, respectively. Density functional theory (DFT) calculations with the (hybrid) B3LYP method were in good agreement with the experimental data. Computational analysis with the natural bond orbital (NBO) paradigm shows that the pucker preference of the substituted prolyl ring is due to the gauche effect. The backbone torsional angles, phi and psi, were shown to correlate with ring pucker, which in turn correlates with the known phi and psi angles in collagen-like peptides. The difference in K(trans/cis) between AcProOMe and Ac-4R-FlpOMe is due to an n-->pi interaction associated with the Bürg-Dunitz trajectory. The decrease in K(trans/cis) for Ac-4S-FlpOMe can be explained by destabilization of the trans isomer because of unfavorable electronic and steric interactions. Analysis of the results herein along with the structures of collagen-like peptides has led to a theory that links collagen stability to the interplay between the pyrrolidine ring pucker, phi and psi torsional angles, and peptide bond trans/cis ratio of substituted proline residues.     

Photoisomerization and photochemistry of matrix-isolated 3-furaldehyde

3-Furaldehyde (3FA) was isolated in an argon matrix at 12 K and studied using FTIR spectroscopy and quantum chemistry. The molecule has two conformers, with trans and cis orientation of the O=C-C=C dihedral angle. At the B3LYP/6-311++G(d,p) level of theory, the trans form was computed to be ca. 4 kJ mol(-1) more stable than the cis form. The relative stability of the two conformers was explained using the natural bond orbital (NBO) method. In fair agreement with their calculated relative energies and the high barrier of rotamerization (ca. 34 kJ mol(-1) from trans to cis), the trans and cis conformers were trapped in an argon matrix from the compound room temperature gas phase in proportion ~7:1. The experimentally observed vibrational signatures of the two forms are in a good agreement with the theoretically calculated spectra. Broad-band UV-irradiation (λ > 234 nm) of the matrix-isolated compound resulted in partial trans → cis isomerization, which ended at a photostationary state with the trans/cis ratio being ca. 1.85:1. This result was interpreted based on results of time-dependent DFT calculations. Irradiation at higher energies (λ > 200 nm) led to decarbonylation of the compound, yielding furan, cyclopropene-3-carbaldehyde, and two C(3)H(4) isomers: cyclopropene and propadiene.     

Synthesis and reactivity of silyl ruthenium complexes: the importance of trans effects in C-H activation,Si-C bond formation,and dehydrogenative coupling of silanes

A series of octahedral ruthenium silyl hydride complexes, cis-(PMe(3))(4)Ru(SiR(3))H (SiR(3) = SiMe(3), 1a; SiMe(2)CH(2)SiMe(3), 1b; SiEt(3), 1c; SiMe(2)H, 1d), has been synthesized by the reaction of hydrosilanes with (PMe(3))(3)Ru(eta(2)-CH(2)PMe(2))H (5), cis-(PMe(3))(4)RuMe(2) (6), or (PMe(3))(4)RuH(2) (9). Reaction with 6 proceeds via an intermediate product, cis-(PMe(3))(4)Ru(SiR(3))Me (SiR(3) = SiMe(3), 7a; SiMe(2)CH(2)SiMe(3), 7b). Alternatively, 1 and 7 have been synthesized via a fast hydrosilane exchange with another cis-(PMe(3))(4)Ru(SiR(3))H or cis-(PMe(3))(4)Ru(SiR(3))Me, which occurs at a rate approaching the NMR time scale. Compounds 1a, 1b, 1d, and 7a adopt octahedral geometries in solution and the solid state with mutually cis silyl and hydride (or silyl and methyl) ligands. The longest Ru-P distance within a complex is always trans to Si, reflecting the strong trans influence of silicon. The aptitude of phosphine dissociation in these complexes has been probed in reactions of 1a, 1c, and 7a with PMe(3)-d(9) and CO. The dissociation is regioselective in the position trans to a silyl ligand (trans effect of Si), and the rate approaches the NMR time scale. A slower secondary process introduces PMe(3)-d(9) and CO in the other octahedral positions, most likely via nondissociative isomerization. The trans effect and trans influence in 7a are so strong that an equilibrium concentration of dissociated phosphine is detectable (approximately 5%) in solution of pure 7a. Compounds 1a-c also react with dihydrogen via regioselective dissociation of phosphine from the site trans to Si, but the final product, fac-(PMe(3))(3)Ru(SiR(3))H(3) (SiR(3) = SiMe(3), 4a; SiMe(2)CH(2)SiMe(3), 4b; SiEt(3), 4c), features hydrides cis to Si. Alternatively, 4a-c have been synthesized by photolysis of (PMe(3))(4)RuH(2) in the presence of a hydrosilane or by exchange of fac-(PMe(3))(3)Ru(SiR(3))H(3) with another HSiR(3). The reverse manifold - HH elimination from 4a and trapping with PMe(3) or PMe(3)-d(9) - is also regioselective (1a-d(9)() is predominantly produced with PMe(3)-d(9) trans to Si), but is very unfavorable. At 70 degrees C, a slower but irreversible SiH elimination also occurs and furnishes (PMe(3))(4)RuH(2). The structure of 4a exhibits a tetrahedral P(3)Si environment around the metal with the three hydrides adjacent to silicon and capping the P(2)Si faces. Although strong Si...HRu interactions are not indicated in the structure or by IR, the HSi distances (2.13-2.23(5) A) suggest some degree of nonclassical SiH bonding in the H(3)SiR(3) fragment. Thermolysis of 1a in C(6)D(6) at 45-55 degrees C leads to an intermolecular CD activation of C(6)D(6). Extensive H/D exchange into the hydride, SiMe(3), and PMe(3) ligands is observed, followed by much slower formation of cis-(PMe(3))(4)Ru(D)(Ph-d(5)). In an even slower intramolecular CH activation process, (PMe(3))(3)Ru(eta(2)-CH(2)PMe(2))H (5) is also produced. The structure of intermediates, mechanisms, and aptitudes for PMe(3) dissociation and addition/elimination of H-H, Si-H, C-Si, and C-H bonds in these systems are discussed with a special emphasis on the trans effect and trans influence of silicon and ramifications for SiC coupling catalysis.     

Synthesis of Complexes with Abnormal “Protic” N‐Heterocyclic Carbenes

Neutral 4‐iodo‐N‐ethylimidazole 3 oxidatively adds to [Pt(PPh₃)₄] to give, in the presence of different tetraalkylammonium salts, complexes trans‐[ 4 ], trans‐[ 5 ], and trans‐[ 6 ] containing an anionic C4‐bound heterocycle with an unsubstituted ring‐nitrogen atom. Complex trans‐[ 4 ] reacts with the proton source NH₄I under protonation of the ring‐nitrogen atom to produce complex trans‐[ 7 ]I which bears an NH,NR‐substituted aNHC ligand. The reaction of trans‐[ 4 ] with CH₃I yields the complex trans‐[ 8 ]I which has a classical aNHC ligand with two alkylated ring‐nitrogen atoms.     

Enantioselective trans dihydroxylation of nonactivated C-C double bonds of aliphatic heterocycles with Sphingomonas sp. HXN-200

The bacterial strain Sphingomonas sp. HXN-200 was used to catalyze the trans dihydroxylation ofN-substituted 1,2,5,6-tetrahydropyridines 1 and 3-pyrrolines 4 giving the corresponding 3,4-dihydroxypiperidines 3 and 3,4-dihydroxypyrrolidines 6, respectively, with high enantioselectivity and high activity. The trans dihydroxylation was sequentially catalyzed by a monooxygenase and an epoxide hydrolase in the strain with epoxide as intermediate. While both epoxidation and hydrolysis steps contributed to the overall enantioselectivity in trans dihydroxylation of 1, the enantioselectivity in trans dihydroxylation of the symmetric substrate 4 was generated only in the hydrolysis of meso-epoxide 5. The absolute configuration for the bioproducts (+)-3 and (+)-6 was established as (3R,4R) by chemical correlations. Preparative trans dihydroxylation of 1a and 4b with frozen/thawed cells of Sphingomonas sp. HXN-200 afforded the corresponding (+)-(3R,4R)-3,4-dihydroxypiperidine 3a and (+)-(3R,4R)-3,4-dihydroxy pyrrolidine 6b in 96% ee both and in 60% and 80% yield, respectively. These results represent first examples of enantioselective trans dihydroxylation with nonterpene substrates and with bacterial catalyst, thus significantly extending this methodology in practical synthesis of valuable and useful trans diols. Enantioselective hydrolysis of racemic epoxide 2a with Sphingomonas sp. HXN-200 gave 34% of (-)-2a in >99% ee, which is a versatile chiral building block. Further hydrolysis of (-)-2a with the same strain afforded (-)-(3S,4S)-3a in 96% ee and 92% yield. Thus, both enantiomers of 3a can be prepared by biotransformation with Sphingomonas sp. HXN-200.     

Selection of the cis and trans phosph(III)azane macrocycles [{P(mu-NtBu)}2(1-Y-2-NH-C6H4)]2(Y=O, S)

The 1 : 1 reactions of [ClP(mu-NtBu)]2 with the difunctional aromatic amines 1,2-1-YH-2-NH2-C6H4 in the presence of Et3N give the dimeric phosph(III)azane macrocycles [{P(mu-NtBu)2(1-Y-2-HN-C6H4)]2, predominantly as the cis isomer in the case of Y=O (1.cis) and as the trans isomer for Y=S (2.trans). Model M.O. calculations suggest that the selection of the cis and trans isomers is not thermodynamically controlled. The alternative isomers 1.trans and 2.cis are generated exclusively by the deprotonation of the model intermediates [(1-Y-2-NH2-C6H4)P(mu-NtBu)]2[Y=O (3), S (4)] with nBuLi followed by cyclisation with [ClP(mu-NtBu)]2. The solid-state structures of 1.cis/trans(50 : 50), 2.cis, 3 and 4 are reported.     

Photophysical studies of the trans to cis isomerization of the push-pull molecule: 1-(pyridin-4-yl)-2-(N-methylpyrrol-2-yl)ethene (mepepy)

Organic molecules possessing intramolecular charge-transfer properties (D-pi-A type molecules) are of key interest particularly in the development of new optoelectronic materials as well as photoinduced magnetism. One such class of D-pi-A molecules that is of particular interest contains photoswitchable intramolecular charge-transfer states via a photoisomerizable pi-system linking the donor and acceptor groups. Here we report the photophysical and electronic properties of the trans to cis isomerization of 1-(pyridin-4-yl)-2-(N-methylpyrrol-2-yl)ethene ligand (mepepy) in aqueous solution using photoacoustic calorimetry (PAC) and theoretical methods. Density functional theory (DFT) calculations demonstrate a global energy difference between cis and trans isomers of mepepy to be 8 kcal mol(-1), while a slightly lower energy is observed between the local minima for the trans and cis isomers (7 kcal mol(-1)). Interestingly, the trans isomer appears to exhibit two ground-state minima separated by an energy barrier of approximately 9 kcal mol(-1). Results from the PAC studies indicate that the trans to cis isomerization results in a negligible volume change (0.9 +/- 0.4 mL mol(-1)) and an enthalpy change of 18 +/- 3 kcal mol(-1). The fact that the acoustic waves associated with the trans to cis transition of mepepy overlap in frequency with those of a calorimetric reference implies that the conformational transition occurs faster than the approximately 50 ns response time of the acoustic detector. Comparison of the experimental results with theoretical studies provide evidence for a mechanism in which the trans to cis isomerization of mepepy results in the loss of a hydrogen bond between a water molecule and the pyridine ring of mepepy.     

Role of axial donors in the ligand isomerization processes of quadruply bonded dimolybdenum(II) compounds

Quadruply bonded dimolybdenum(II) complexes with NP-R (2-(2-R)-1,8-naphthyridine; R = thiazolyl (NP-tz), furyl (NP-fu), thienyl (NP-th)) and 2,3-dimethyl-1,8-naphthyridine (NP-Me 2) have been synthesized by reactions of cis-[Mo2(OAc)2(CH3CN)6][BF4]2 with the corresponding ligands. The products cis-[Mo2(NP-tz)2(OAc)2][BF4]2 (1), trans-[Mo2(NP-fu)2(OAc)2][BF4]2 (2), trans-[Mo2(NP-th)2(OAc)2][BF4]2 (3), and trans-[Mo2(NP-Me2)2(OAc)2][BF4]2 (4) were isolated and characterized. The NP-R ligands with stronger R = pyridyl and thiazolyl donors result in cis isomers whereas the weaker furyl and thienyl appendages lead to compounds having a trans orientation of the ligands. The use of NP-Me2 leads to a trans structure with a tetrafluoroborate anion occupying one of the axial sites. Complete replacement of two acetate groups by acetonitrile in 1 and 2 resulted in the cis isomers [Mo2(NP-tz)2(CH3CN)4][OTf]4 (5) and [Mo2(NP-fu)2(CH3CN)4][OTf]4 (6) respectively. The combination of one acetate and two acetonitriles as ancillary ligands, however, yields trans-[Mo2(NP-tz)2(OAc)(CH3CN)2][BF4]3 (7) in the solid state as determined by X-ray crystallography. (1)H NMR spectra of the products are diagnostic of the cis and trans dispositions of the ligands. Solution studies reveal that the ligand arrangements observed in the solid state are mostly retained in the acetonitrile medium. The only exception is 7, for which a mixture of cis and trans isomers are detected on the NMR time scale. The isolation of trans compounds 2- 4 from the cis precursor [Mo2(OAc)2(CH3CN)6][BF4]2 indicates that an isomerization process occurs during the reactions. The mechanism involving acetate migration through axial coordination has been invoked to rationalize the product formation. Compounds 1- 7 were structurally characterized by single-crystal X-ray methods.     

Metallacycles of porphyrins as building blocks in the construction of higher order assemblies through axial coordination of bridging ligands: solution- and solid-state characterization of molecular sandwiches and molecular wires

Treatment of the octahedral Ru(II)-dimethyl sulfoxide complexes trans-RuCl(2)(dmso-S)(4) (1), trans-RuCl(2)(dmso-O)(2)(CO)(2) (2), and trans-RuCl(2)(dmso)(3)(CO) (3) with a stoichiometric amount of 5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin (4'-cis-DPyP) yields, after chromatographic purification, the novel 2+2 molecular squares of formula [trans,cis,cis-RuCl(2)(dmso-S)(2)(4'-cis-DPyP)](2) (4), [trans,cis,cis-RuCl(2)(CO)(2)(4'-cis-DPyP)](2) (5), and [trans,cis,cis-RuCl(2)(dmso-S)(CO)(4'-cis-DPyP)](2) (6), respectively. Compound 6 exists as an equimolar mixture of the isomeric metallacycles 6a and 6b, depending on whether the 4'-N(py) rings of 4'-cis-DPyP's are trans to CO or to dmso-S. Compounds 4-6 were fully characterized by NMR and IR spectroscopy and by FAB mass spectrometry. Treatment of 5 with excess zinc acetate in chloroform/methanol mixtures led to the isolation of the corresponding zinc adduct [trans,cis,cis-RuCl(2)(CO)(2)(Zn x 4'-cis-DPyP)](2) (5Zn). Treatment of 5Zn with 1 equiv of a trans ditopic N-donor ligand L (L = 4,4'-bipy, 5,15-bis(4'-pyridyl)-2,8,12,18-tetra-n-propyl-3,7,13,17-tetramethylporphyrin (4'-trans-DPyP-npm), or 5,15-bis(4'-pyridyl)-10,20-diphenylporphyrin (4'-trans-DPyP)) leads readily and selectively, according to (1)H NMR spectroscopy, to the quantitative assembling of 2:2 supramolecular adducts of stacked metallacycles of formula [(5Zn)(2)(mu-L)(2)] (7-9), which were thoroughly characterized in solution by NMR spectroscopy. NMR features indicate that, at ambient temperature, the equilibrium between 5Zn and L to yield [(5Zn)(2)(mu-L)(2)] has an intermediate to slow rate on the NMR time scale (relatively broad signals for L) and is totally shifted toward the 2:2 product (all or nothing process). Single-crystal X-ray investigations showed that, depending on the nature of the bridging ligand, in the solid state these sandwich structures can either be maintained or originate polymeric chains formulated as [(5Zn)(mu-L)](infinity). When L = 4'-trans-DPyP, both solution- and solid-state data indicate that [(5Zn)(2)(mu-4'-trans-DPyP)(2)] (9) is a discrete supramolecular assembly of two molecular squares of metalloporphyrins axially connected through other porphyrins. In this molecular box, the two bridging porphyrins are coplanar at a distance of about 11.4 A. When L = 4,4'-bipy, the corresponding adduct 7 has the anticipated sandwich-like discrete architecture [(5Zn)(2)(mu-4,4'-bipy)(2)] in solution, but it assumes a stair-like polymeric wire structure in the solid state. The polymer [(5Zn)(mu-4,4'-bipy)](infinity) is made by 5Zn squares bridged by 4,4'-bipy ligands which are axially coordinated alternatively on the two opposite sides of each square. Our work clearly established that relatively simple supramolecular adducts of porphyrins, such as molecular squares, are suitable building blocks for the construction of more elaborate assemblies of higher order by axial coordination of bridging ligands.     

聚丁二烯~(13)C-NMR谱图中一个新的共振吸收峰(T_4)

<正> 最近解析锂系和铁系含乙烯基聚丁二烯的~(13)C-NMR谱图,发现并确认了一个新的共振吸收峰。即聚丁二烯反式1,4-序列的第四峰,简称T_4峰。 Furukawa等对钼系和章哲彦等对铁系等二元聚顺-1,4-1,2-丁二烯的研究都指出在聚丁二烯的~(13)C-NMR谱图上,有四个谱峰属于顺1,4-序列。本文给出其化学位     

A photoactivated trans-diammine platinum complex as cytotoxic as cisplatin

The synthesis and X-ray structure (as the tetrahydrate) of the platinum(IV) complex trans,trans,trans-[Pt(N(3))(2)(OH)(2)(NH(3))(2)] 3 are described and its photochemistry and photobiology are compared with those of the cis isomer cis,trans,cis-[Pt(N(3))(2)(OH)(2)(NH(3))(2)] 4. Complexes 4 and 3 are potential precursors of the anticancer drug cisplatin and its inactive trans isomer transplatin, respectively. The trans complex 3 is octahedral, contains almost linear azide ligands, and adopts a layer structure with extensive intermolecular hydrogen bonding. The intense azide-to-platinum(IV) charge-transfer band of complex 3 (285 nm; epsilon=19 500 M(-1) cm(-1)) is more intense and bathochromically shifted relative to that of the cis isomer 4. In contrast to transplatin, complex 3 rapidly formed a platinum(II) bis(5'-guanosine monophosphate) (5'-GMP) adduct when irradiated with UVA light, and did not react in the dark. Complexes 3 and 4 were non-toxic to human skin cells (keratinocytes) in the dark, but were as cytotoxic as cisplatin on irradiation for a short time (50 min). Damage to the DNA of these cells was detected by using the "comet" assay. Both trans- and cis-diammine platinum(IV) diazide complexes therefore have potential as photochemotherapeutic agents.     

Switching of molecular insertion in a cyclic molecule via photo- and thermal isomerization

Two new cyclic ligands were synthesized: a ligand with two trans-azobenzene moieties and one bipyridine moiety, trans(2)-oAB-O13, and a ligand with two trans-azobenzene moieties and two bipyridine moieties, trans(2)-oAB-bpy. Both ligands underwent reversible trans-cis isomerization at the azobenzene moieties. The mole ratios of the trans(2) form:trans-cis form:cis(2) form, evaluated by (1)H NMR spectroscopy of the photostationary states prepared by 1 h illumination, were 0.13:0.27:0.60 (365 nm irradiation) and 0.41:0.47:0.12 (436 nm irradiation) for oAB-O13, and 0.18:0.12:0.70 (365 nm irradiation) and 0.36:0.43:0.21 (436 nm irradiation) for oAB-bpy. When trans(2)-oAB-O13 was mixed with Cu(I), both the bipyridine units and the polyether chains coordinated to the copper center. Addition of a noncyclic bipyridine ligand, trans(2)-oAB-2OH, afforded a bis(bipyridine)copper(I) complex, [Cu(trans(2)-oAB-O13)(trans(2)-oAB-2OH)]BF(4). The bis(bipyridine) ligand, trans(2)-oAB-bpy, formed a 1:1 complex with Cu(I), [Cu(trans(2)-oAB-bpy)]BF(4). [Cu(cis(2)-oAB-bpy)]BF(4) did not undergo the ligand substitution reaction with a noncyclic ligand with two azobenzene moieties and one bipyridine moiety, oAB, whereas its thermal isomerization in the presence of oAB caused the formation of [Cu(trans(2)-oAB-bpy)(trans(2)-oAB)]BF(4), indicating that the isomerization and ligand exchange reactions synchronized via a conformational change of the cyclic ligand.     

Oxidative Addition of the Dithiobis(formamidinium) Cation to Platinum(II) Chloro Am(m)ine Compounds: Studies on Structure, Spectroscopic Properties, Reactivity, and Cytotoxicity of a New Class of Platinum(IV) Complexes Exhibiting S-Thiourea Coordination

The oxidative addition of the salt [{SC(NMe(2))(2)}(2)]Cl(2).2H(2)O (1), the disulfide-like dimerized form of 1,1,3,3-tetramethylthiourea (tmtu), to Pt(II) chloro am(m)ine compounds is described. Oxidation of the [PtCl(3)(NH(3))](-) anion with 1 in methanol yields cis-[PtCl(4)(NH(3))L] (2; L = tmtu) as the result of the trans addition of one tmtu and one chloro ligand. The same mode of oxidation is found in reactions of 1 with [PtCl(dien)](+) (dien = diethylenetriamine) and trans-[PtCl(2)(NH(3))(2)]. In these cases, however, the oxidation is followed by (light-independent) cis,trans isomerizations, giving trans,mer-[PtCl(2)(dien)L]Cl(2) (4) and fac-[PtCl(3)(NH(3))(2)L]Cl.0.5MeOH (6), respectively. The single-crystal X-ray structures of 2 and trans,mer-[PtCl(2)(dien)L](BF(4))(2) (4a) have been determined. 2: monoclinic, space group P2(1)/n, a = 6.280(1) ?, b = 13.221(3) ?, c = 16.575(2) ?, beta = 96.45(1) degrees, Z = 4. 4a: monoclinic, space group C2/m, a = 21.093(5) ?, b = 8.9411(9) ?, c = 14.208(2) ?, beta = 124.65(2) degrees, Z = 4. The tmtu ligands are S-bound. In 2 a pronounced trans influence of the S-donor ligand on the Pt-Cl bond (2.370(1) ?) trans to sulfur is observed. The unusual acidity of the Pt(IV) complexes exhibiting tmtu coordination trans to chloride is attributed to hydrolysis of the labilized Pt-Cl(trans) bond, which is supported by ion sensitive electrode measurements. An upfield shift of the (195)Pt resonances is found on changing the ligand combination from NCl(4)S (2) to N(3)Cl(2)S (4). This order correlates with the trans influences of the ligands: tmtu > am(m)ine > chloride. The cytotoxicity of 2 and 6 in L1210 cell lines is reported and discussed in terms of a possible mechanism of action of the compounds invivo. It is suggested that tmtu may act as a lipophilic carrier ligand and therefore enhance the cellular uptake of the new potential Pt(IV) drugs.     

Synthesis and Surface-Active Properties of New Photosensitive Surfactants Containing the Azobenzene Group

Several water-soluble cationic surfactants, 4-alkylazobenzene-4'-(oxy-2-hydroxypropyl)trimethylammonium methylsulfate (AZMS) (AZMS-0, AZMS-1, AZMS-2, AZMS-4, and AZMS-8), containing alkylglycidylether and azoarene have been synthesized with high yields of 63-78% and their surface-active properties have been investigated upon irradiation with UV/vis light. All of the trans-AZMS surfactants are isomerized to cis-trans mixtures containing 92.5% cis isomer by UV light irradiation at 350 nm. The cis isomers in the mixtures are reverted to trans isomers by visible light irradiation (lambda>445 nm). Such photoisomerization induces changes in the surface activity of each surfactant. The critical micelle concentration (cmc) of the trans form of AZMS-8 surfactant is about 1.28x10(-4) mol/l. At the photostationary state, 92.5% of the trans form is changed to the cis form which exhibits a slightly higher cmc (3.41x10(-4) mol/l). The new cmc of AZMS surfactants upon photoisomerization is similar to that of the ideal mixed micellar system. In particular, the ratio of cmc(cis) to cmc(trans) of AZMS derivatives is about 1.87-2.85 which increases proportionally with the chain length of alkyl group. The minimum average area per molecule (A(min)(a/w)) for the trans and cis isomers of AZMS-8 is 0.60 and 0.74 nm(2), respectively. The difference in the A(min)(a/w) may originate from the structural differences in the two isomers. These values are quite different as compared to those of the conventional azobenzene surfactants. Copyright 2000 Academic Press.     

1H NMR study of protected and unprotected kainoid amino acids: facile assignment of C-4 stereochemistry

The kainoid amino acids exhibit potent neuroexcitatory activity in the mammalian central nervous system. Around their pyrrolidine ring, a trans disposition between the C-2 and C-3 substituents and a cis relationship between the C-3 and C-4 substituents are crucial for their potent biological activity. During synthetic studies into the kainoids, we have established a straightforward, empirical rule, which allows the facile assignment of C-4 stereochemistry to both protected and unprotected kainoids. When pairs of C-4 epimers are available, the rule indicates that, when their (1)H NMR spectra are compared, one of the methylene protons on the C-3 side chain appears at significantly lower chemical shift in the C-3, C-4 cis isomer than the corresponding signal for the proton in the spectrum for the C-3, C-4 trans isomer. In addition, the rule states that the difference in chemical shift between the two individual protons on the C-3 side chain of the C-3, C-4 cis isomer is significantly greater than the corresponding difference for the C-3, C-4 trans isomer. The rule is demonstrated for kainoids possessing an unsaturated substituent at C-4 and when comparing spectra in D(2)O for pairs of unprotected C-4 epimers, the spectra were recorded at approximately the same pD.     

Synthesis and structural characterization of trivalent amino acid derived chiral phosphorus compounds

Reaction of the N-toluenesulfonyl derivatives of (S)-alanine, phenylalanine, and valine (4-6) with PhPCl(2) gave in high yield the 4-methyl, benzyl, and isopropyl derivatives (7-9) of 2-phenyl-1-p-toluenesulfonyl-1,3,2-oxazaphospholidin-5-one. The ratios of the (2S,4S)/(2R,4S) diastereomers (cis/trans isomers) were 1:1, 2:1, and 10:1 for the methyl, benzyl, and isopropyl derivatives 7a,b, 8a,b, and 9a,b, respectively. For 7a,b, both isomers could be crystallized, but for the others only the major isomers were isolable. The X-ray crystal structure of 9a shows that the isopropyl and phenyl groups are mutually cis and that the tolyl moiety is oriented s-trans to both the isopropyl and phenyl groups. Reaction of 6 with Cl(2)PCH(2)CH(2)PCl(2) (10) gave a 56:38:7 mixture of the cis/cis, cis/trans, and trans/trans diphosphorus heterocycles 11a-c. The major isomer could be crystallized and isolated free of the other diastereomers. Reaction of 6 with EtPCl(2) gave a 6:1 mixture of cis/trans isomers of the ethyl-substituted heterocycles 12a,b as an inseparable oil but allowed confirmation of the structure of 11a. Slow epimerization at phosphorus may occur by inversion but more likely by ring opening/closure, since 7b, 9a, and 11a give rise upon standing in solution to mixtures containing starting material and 7a, 9b, and 11b, respectively, along with the free amino acid derivatives 4 and 6. The NMR spectra, and in particular the coupling constants between the alpha-hydrogen atom of the amino acid moiety and phosphorus, were used to establish the identities of the cis and trans isomers. Reaction of 9a with (THF)W(CO)(5) gave the phosphorus-ligated adduct (9a)W(CO)(5) (13), and the IR spectrum of this complex shows that 9a is a strongly electron-withdrawing ligand. The geometry of the sulfonamide moiety is discussed in detail, as are the (1)H NMR coupling constants. The data are consistent with the presence of little steric interaction between the cis isopropyl and phosphorus substituent in 9a, 11a, and 12a and orientation of the tolyl moiety s-cis to the isopropyl group in 9b, 12b, and 13.     

Reactions of the dirhenium(II) complex Re2Cl4(mu-dppm)2 with pyridinecarboxylic acids. Examples of bidentate O,O versus N,O coordination and tridentate O,N,O coordination and structural isomers that contain the pyridine-2,6-dicarboxylate ligand

Pyridine-2-carboxylic acid, pyridine-2,3-dicarboxylic acid, and pyridine-2,4-dicarboxylic acid or their [(Ph(3)P)(2)N](+) salts react with the triply bonded dirhenium(II) complex Re(2)Cl(4)(mu-dppm)(2) (dppm = Ph(2)PCH(2)PPh(2)) in refluxing ethanol to afford unsymmetrical substitution products of the type Re(2)(eta(2)-N,O)Cl(3)(mu-dppm)(2), where N,O represents a chelating pyridine-2-carboxylate ligand (N,O = O(2)C-2-C(5)H(4)N (1), O(2)C-2-C(5)H(3)N(-3-CO(2)Et) (3), or O(2)C-2-C(5)H(3)N(-4-CO(2)H) (4)). The carboxylate groups in the 3- and 4- positions are not bound to the metal centers; in the case of 3 this group undergoes esterification in the refluxing ethanol solvent. Structure determinations have shown that 1, 3, and 4 possess similar structures in which there is an axial Re-O (carboxylate) bond (collinear with the Re(triple bond)Re bond) and the mu-dppm ligands are bound in a trans,cis fashion to the two Re atoms which have the ligand atom arrangement [P(2)NOClReReCl(2)P(2)]. The tridentate dianionic pyridine-2,6-dicarboxylate ligand (dipic) reacts with Re(2)Cl(4)(mu-dppm)(2) in ethanol at room temperature to give a compound Re(2)(dipic)Cl(2)(mu-dppm)(2) (6) in which the dipic ligand is bound in a symmetrical eta(3)-(O,N,O) fashion to one Re atom, with the N atom in an axial position (collinear with the Re(triple bond)Re bond) and with preservation of the same trans,trans coordination of the mu-dppm ligands that is present in Re(2)Cl(4)(mu-dppm)(2). Under reflux conditions, this kinetic product isomerizes to the thermodynamically favored isomer 5 with an unsymmetrical structure in which the dipic ligand chelates to one Re atom (as in 1, 3, and 4) and uses its other carboxylate group to bridge to the second Re atom. The isomerization of 6 to 5, which also results in a change in the coordination of the pair of mu-dppm ligand to trans,cis, is believed to occur by a partial "merry-go-round" process, a mechanism that probably explains the structures of the thermodynamic products 1, 3, and 4. The reaction of Re(2)Cl(4)(mu-dppm)(2) with pyridine-3-carboxylate gives the trans isomer of Re(2)(mu:eta(2)-O(2)C-3-C(5)H(4)N)(2)Cl(2)(mu-dppm)(2) (2) in which a pair of carboxylate bridges are present and the pyridine N atom is not coordinated. Single-crystal X-ray structural details are reported for 1-6.     

NMR and theoretical calculations: a unified view of the cis/trans isomerization of 2-substituted thiazolidines containing peptides

The cis/trans isomerization of peptides containing the pseudoproline (4R)-thiazolidine-4-carboxylic acid Cys(Psi (R1,R2) pro) is investigated from both an experimental and a theoretical point of view by NMR and DFT calculations. A series of Ac-Cys(Psi(R1,R2) pro)-OCH3 and Ac-Cys(Psi(R1,R2) pro)-NHCH3 peptides were prepared to assess the influence of the substitution at the C2 position as well as of the amide following the thiazolidine residue. For each compound, the cis/trans ratio along with free energy, the puckering of the thiazolidine ring and the free rotational energy barrier are reported and discussed. We observe there is a pronounced effect of the C2 substituents and of the chirality upon the cis/trans ratio with the population of the cis content in the order (2R)-Cys(Psi(CH3,H) pro)<(2S)-Cys(Psi(H,CH3) pro)相似文献   

A kinetically controlled trans bifunctionalized organoimido derivative of the Lindqvist-type hexamolybdate: synthesis, spectroscopic characterization, and crystal structure of (n-Bu4N)2{trans-[Mo6O17(NAr)2]} (Ar = 2,6-dimethylphenyl)

A kinetically controlled trans bifunctionalized organoimido derivative of hexamolybdate, (n-Bu(4)N)(2){trans-[Mo(6)O(17)(NAr)(2)]} (Ar = 2,6-dimethylphenyl) 1, in which the two 2,6-dimethylphenylimido groups are bonded to hexamolybdate at the trans positions, has been successfully synthesized in ca. 60% yield under mild reaction conditions. Its trans structure has been confirmed by a single-crystal X-ray diffraction study. In the crystals, cluster anions of 1 self-assemble into a 3D netlike structure via two different kinds of C-H...O hydrogen bondings, in which 1D supramolecular rectangular channels containing tetrabutylammonium cations form along the a axis. Compound 1 has also been characterized by (1)H NMR, IR, and UV-vis spectroscopic studies. UV-vis-near-IR reflectance spectroscopy measurements reveal the compound's nature of semiconductivity with an optical energy gap of 2.55 eV.