Chirality inversion in the bilirubin molecular exciton

The bichromophoric pigment bilirubin acts as a molecular exciton in its UV-visible and circular dichroism (CD) spectroscopy. In both polar and nonpolar solvents, an optically active analog, (beta R,beta 'R)-dimethylmesobilirubin-XIII alpha (1), exhibits intense bisignate CD Cotton effects in the region of its long wavelength UV-vis absorption near 400 nm: Delta epsilon(434)(max) + 337, Delta epsilon(389)(max) - 186 (CHCl(3)), and Delta epsilon(431)(max) + 285, Delta epsilon(386)(max) - 177 (CH(3)OH). However, introduction of an amine into a CHCl(3) solution of 1 causes the Cotton effect signs to become inverted, e.g., after addition of NH(3), Delta epsilon(433)(max) - 345, Delta epsilon(389)(max) + 243, and after addition of ethylene diamine, Delta epsilon(435)(max) - 420, Delta epsilon(390)(max) + 299. The sign inversions imply inversion of molecular chirality of the bilirubin and the phenomenon appears to be general for amines, including alpha,omega-diamines. 1,8-Diaminooctane was found to be more effective than longer or shorter chain analogs in producing CD sign inversion.     

Addition reactions at the 16(17) double bond of 3-methoxy-13alpha-estra-1,3,5(10),16-tetraene

The epoxidation, the addition of hypobromous acid, and the hydroboration of 3-methoxy-13alpha-estra-1,3,5(10),16-tetraene 1 with diborane, catecholborane, and 9-BBN were investigated in order to determine the stereochemical outcome and to synthesize new 13alpha-estra-1,3,5(10)-trienes for biological and conformational investigations. It was shown that the sterically demanding reagent 9-BBN participated in a preferred beta attack (53% 16betaOH 10, 34% 17betaOH 8, 13% 16alphaOH 11). This stereochemical result is in agreement with that from another cis addition reaction, the recently described OsO4 dihydroxylation of 1 [Steroids 68 (2003) 113]. With smaller reagents such as B2H6, catecholborane, or magnesium monoperoxyphthalate, a diminished stereoselectivity was observed with only a slight excess of beta attack. The ionic trans addition of hypobromous acid gave two 17-bromo-16-alcohols with 16beta,17alpha (4, 76%) and 16alpha,17beta configuration (5, 24%) formed by trans cleavage of the 16,17alpha- and beta-bromonium ion at position 16. The same regioselective and stereoselective course was found for the cleavage of the 16alpha,17alpha- and 16beta,17beta-epoxides (3 and 2) with hydrazoic acid (3-->16betaN3,17alphaOH 7, 2-->16alphaN3,17betaOH 6). The stereochemistry of the addition reactions to 1 can be explained in terms of a twist-boat conformation involving the C ring of compound 1. From a synthetic viewpoint the synthesis of the beta-epoxide 2 from the bromohydrin 4, the cleavage of this epoxide to 16alpha-substituted-17beta-hydroxy compounds, such as 6, and hydroboration/oxidation with 9-BBN to the hitherto unknown 16beta-hydroxy compound 10 are useful procedures. The bromohydrin 5 is the first 13alpha-steroid with a 17beta-bromo substituent. X-ray analysis revealed twist-boat and 16beta-envelope conformations for rings C and D, respectively.     

Synthesis of oligosaccharides with group specificity for blood types A, B and H (Type 3)

Chemical synthesis of A, B, and H (type 3) human blood group determinant oligosaccharides (as R-glycosides, R = OCH2CH2CH2NHCOCF3) and their polymeric derivatives are reported. 4,6; 4',6'-Di-O-benzylidene derivative of Gal beta 1----3GalNAc alpha 1----R was chloroacetylated selectively at 3'-OH, the chloroacetate was alpha-fucosylated and dechloroacetylated to give protected H (type 3) trisaccharide bearing free 3'-OH. alpha-Glycosylation of the trisaccharide with 2-azido-3,4,6-tri-O-benzyl-beta-D-galactopyranosyl chloride and 2,3,4,6-tetra-O-benzyl-alpha-D-galactopyranosyl bromide gave rise to protected A and B tetrasaccharides, respectively. Deprotected R-glycosides were converted to OCH2CH2CH2NH2 derivatives. Their reaction with poly(4-nitrophenylacrylate) affords polyacrylamide-coupled conjugates with A, B, and H (type 3) specificity.     

Structural transitions involved in a novel amyloid-like beta-sheet assemblage of tripeptide derivatives

Self-assembly of two tripeptide derivatives containing three nonpolar isoleucine moieties and polar oxyethylene groups are studied in methanol. Peptide A [CH3(OCH2CH2)3OCH2CO(Ile)3OCH3] and peptide B [CH3(OCH2CH2)3OCH2CO(Ile)3NH (CH2CH2O)3CH3] take a mixture of unordered and helical conformation at low concentration (8.5 x 10(-4) M). However, at high concentration (2 x 10(-3) M), both the peptide showed significant increase in the helical conformation. An interesting conformational transition of peptides A and B at various methanol contents was observed in the solvated films of these compounds by spectroscopic methods like the far-uv circular dichroism and Fourier transform infrared (FT-IR) techniques. Peptide B, which contains more polar oxyethylene groups than A, showed a highly cooperative conformational transition when the methanol content was decreased. This transition was characterized by a large increase of beta-sheet, retaining a alpha-helical contribution. Peptide A showed a conformational transition resulting in a beta-sheet in the aggregated state. From the CD spectra, the ratio in the ellipticity indicates that peptide B forms twisted antiparallel beta-sheet conformation, whereas peptide A takes a parallel beta-sheet conformation. The results obtained in this work indicates the role of polar derivatization on the conformational preference of peptides having similar sequence.     

Misdirected π-donor ligands: (η-C5H5)2Zr(Cl) (SR) with sterically more demanding R groups have lower rotational barriers

Rotational barriers about the M-S bonds of 16-electron bent metallocene monothiolates (η⁵-C₅H₅)₂Zr(Cl) (SR) (R = −CH₃, −CH₂CH₃, −CH(CH₃)₂, −C(CH₃)₃) (1a–d) have been measured by dynamic ¹H NMR methods: 32, 33, 35 and 26 kJ mol⁻¹, respectively. The ground-state orientation about the Zr-S bonds of 1 that maximizes Spπ → Mdπ bonding (Cl-Zr-S-R ≈ 90°) also maximizes CpR steric interaction, whereas the rotational transition-state orientation (Cl-Zr-S-R ≈ 0°) is one that minimizes Spπ → Mdπ bonding and maximizes ClR steric interaction. Deviation from a ground-state orientation that is ideal for Spπ → Mdπ bonding might be expected as the size of the R group and CpR steric interaction increases. Thus, the aberrant trend for the R = −C(CH₃)₃ derivative could be attributed to a ground-state steric effect where the sterically demanding −C(CH₃)₃ group forces an unfavorable (misdirected) orientation for Mdπ-Spπ bonding, but a favorable orientation with respect to CpR and ClR steric interactions. However, the solid-state structures of (η⁵-C₅H₅)₂Zr(SR)₂ (R = −CH₃, −CH₂CH₃, −CH(CH₃)₂, −C(CH₃)₃) (2a–d) exhibit regular variation of their metric parameters as evidenced by their Zr-S-C bond angles of 108, 109, 113, and 124° and S-Zr-S′ bond angles of 97, 99, 100 and 106°, respectively. Neither the S′-Zr-S-R torsion angles nor the dihedral angles that describe the relationship between the S/Zr/S′ and Cp(centroid)/Zr/Cp′ (centroid) planes (both indicators of the relative orientation of the Zr dπ acceptor orbital and the thiolate S pπ donor orbital) reflect the steric demand of the R group. Thus, the size of the R group imposes a measured effect on the geometry of 2 and the tert-butyl group is not extraordinary. Although the enthalpic and entropic effects could not be deconvoluted for rotation about the Zr-S bond of 1 in the present study, literature precedents suggest that both enthalpic and entropic effects may play a role in determining the irregular trend that is observed.     

Crystal packing and hydrogen bonding in platinum(II) nucleotide complexes: X-ray crystal structure of [Pt(MeSCH(2)CH(2)SMe)(5'-GMP-N7)(2)].6H(2)O

We have synthesised the complex [Pt(CH(3)SCH(2)CH(2)SCH(3))(5'-GMP-N7)(2)].6H(2)O (1), where 5'-GMP is 5'-guanosine monophosphate, and determined its X-ray crystal structure. Pt(II) adopts a square-planar geometry in which the bases are coordinated head-to-tail (HT) in the Delta configuration. The nucleotide conformation in this complex is almost identical to that in the previously reported complex [Pt(en)(5'-GMP-N7)(2)].9H(2)O (2), in which there is outer sphere macrochelation via intramolecular H-bonding between the monoanionic phosphate groups and the coordinated ethylenediamine (en) NH. It is therefore apparent that intermolecular interactions rather than intramolecular H-bonding determines the orientation of the sugar-phosphate side-chain in these Pt(II) bisnucleotide complexes in the solid state.     

Trans-diaryl epoxides: asymmetric synthesis, ring-opening, and absolute configuration

Anthryl-phenyl, phenanthryl-phenyl, and naphthyl-phenyl trans-epoxides (1, 2, and 3, respectively) having enantiomeric purities of 95%, 99%, and 96% were synthesized from a diastereo and enantiopure sulfonium salt derived from Eliel's oxathiane. The determination of their (1R,2R) absolute configurations was achieved by application of the CD exciton chirality method using a Zn-porphyrin tweezer on the corresponding alcohols obtained after opening of these epoxides with LiAlH(4). The R-configuration at C2 of these epoxides, (-)-1, (+)-2, and (-)-3, is consistent with our previous results concerning asymmetric synthesis of monoaryl epoxides, cyclopropanes, and aziridines. The (1S,2R)-configuration of the cis isomer (when present) was also confirmed. Moreover, the agreement between the negative exciton chirality for conjugates of (S)-configuration predicted by molecular modeling and the observed CD spectra helps to clarify the relative steric size of phenyl and CH(2)-aryl (phenanthryl or anthryl), which is critical when the tweezer method is applied for absolute configurational assignment (phenyl = medium group; anthacenyl CH(2) and phenanthryl CH(2) = large group).     

A minimized human integrin alpha(5)beta(1) that retains ligand recognition

Two isolated recombinant fragments from human integrin alpha(5)beta(1) encompassing the FG-GAP repeats III to VII of alpha(5) and the insertion-type domain from beta(1), respectively, are structurally well defined in solution, based on CD evidence. Divalent cation binding induces a conformational adaptation that is achieved by Ca(2+) or Mg(2+) (or Mn(2+)) with alpha(5) and only by Mg(2+) (or Mn(2+)) with beta(1). Mn(2+) bound to beta(1) is highly hydrated ( approximately 3 water molecules), based on water NMR relaxation, in agreement with a metal ion-dependent adhesion site-type metal coordination. Each fragment saturated with Mg(2+) (or Mn(2+)) binds a recombinant fibronectin ligand in an RGD-dependent manner. A conformational rearrangement is induced on the fibronectin ligand upon binding to the alpha(5), but not to the beta(1) fragment, based on CD. Ligand binding results in metal ion displacement from beta(1). Both alpha(5) and beta(1) fragments form a stable heterodimer (alpha(5)beta(1) mini-integrin) that retains ligand recognition to form a 1:1:1 ternary complex, in the presence of Mg(2+), and induces a specific conformational adaptation of the fibronectin ligand. A two-site model for RGD binding to both alpha and beta integrin components is inferred from our data using low molecular weight RGD mimetics.     

Structural and conformational properties of (Z)-beta-(1-naphthyl)- dehydroalanine residue

To understand how chemical structure of beta-substituted alpha, beta-dehydroalanine (particularly size and pi conjugation of beta substituent) affects conformational property, x-ray crystallographic analysis was performed on Boc-Ala-Delta(Z) Nap-Val-OMe [Boc: t-butoxycarbonyl; Delta(Z) Nap: (Z)-beta-(1-naphthyl)dehydroalanine; OMe: methoxy] having the naphthyl group as a bulky beta substituent. Single crystals were grown by slow evaporation from an ethanol solution in the triclinic space group P1 with a = 9.528 (3) A, b = 12.410(4) A, c = 5.975(2) A, alpha = 96.77(3) degrees, beta = 102. 81(2) degrees, gamma = 88.74(3) degrees, V = 684.1(4) A3, and Z = 1. Phase determination was carried out by a direct method (SHELEXS), and the final structure was refined to R = 8.1% and R(w) = 9.0% for 1964 observed reflections. The bond lengths and bond angles of the Delta(Z)Nap residue, characterized by a sp(2) hybridized C(alpha) atom, did not differ from those of other dehydroresidues such as Delta(Z) Phe, Delta(Z) Leu, and DeltaVal essentially. The peptide backbone took a type II beta-turn conformation involving an intramolecular hydrogen bond between CO(Boc) and NH(Val), similar to di- or tripeptides containing a Delta(Z) Phe or Delta(Z) Leu residue in the second positions. Here the naphthyl group was found to be nonplanar [chi(2) = 55(1) degrees ] relative to the C(alpha)==C(beta)==C(gamma) plane. The nonplanarity was supported by conformational energy calculation. The molecular packing was stabilized by two kinds of intermolecular hydrogen bonds and van der Waals interactions. Naphthyl groups were arranged in a partially overlapped face-to-face orientation with a center-to-center distance of 5.97 A. For additional information, peptide Boc-(Ala-Delta(Z) Nap-Leu)(2)-OMe was synthesized and its solution conformation was investigated by (1)H-NMR spectroscopy. The hexapeptide showed the tendency to form a 3(10)-helical conformation in solution essentially. Conformational properties of Delta(Z) Nap residue, characterized by a type II beta-turn and 3(10)-helix, were supported by a conformational energy contour map of the Delta(Z)Nap residue.     

Conformation of the N(CH3)2 group in cytosine and in simple model pyrimidines and pyridines. Steric effects of ortho-methyl substitution on infrared spectra and molecular dipole moments

Infrared spectra of amino and dimethylamino derivatives with and without an ortho-methyl group of 4- and 5-substituted pyrimidines, 4-substituted pyridine, benzene and of the respective cytosines were recorded in the region of skeletal ring vibrations. Integrated intensities of ring vibration(s) v8 at about 1600 cm-1 sensitive to the presence of electron-donating substituents were used for elucidation of the steric effects of ortho-methyl on the mesomeric interaction between the -N(CH3)2 group and the ring. Molecular dipole moments were also determined experimentally in benzene for simple pyrimidine and pyridine derivatives and analysed vectorially with the use of component group moments in terms of the N(CH3)2 group conformation. The data point to a progressive twist of the dimethylamino group in hindered derivatives in the order: pyrimidine-5 greater than pyridine-4 greater than pyrimidine-4. They are also in agreement with the essential planarity of sterically crowded m41,4,4,5cytosine.     

Tetraspanin CD151 regulates glycosylation of (alpha)3(beta)1 integrin

The tetraspanin CD151 forms a stoichiometric complex with integrin alpha3beta1 and regulates its endocytosis. We observed that down-regulation of CD151 in various epithelial cell lines changed glycosylation of alpha3beta1. In contrast, glycosylation of other transmembrane proteins, including those associated with CD151 (e.g. alpha6beta1, CD82, CD63, and emmprin/CD147) was not affected. The detailed analysis has shown that depletion of CD151 resulted in the reduction of Fucalpha1-2Gal and bisecting GlcNAc-beta(1-->4) linkage on N-glycans of the alpha3 integrin subunit. The modulatory activity of CD151 toward alpha3beta1 was specific, because stable knockdown of three other tetraspanins (i.e. CD9, CD63, and CD81) did not affect glycosylation of the integrin. Analysis of alpha3 glycosylation in CD151-depleted breast cancer cells with reconstituted expression of various CD151 mutants has shown that a direct contact with integrin is required but not sufficient for the modulatory activity of the tetraspanin toward alpha3beta1. We also found that glycosylation of CD151 is also critical; Asn(159) --> Gln mutation in the large extracellular loop did not affect interactions of CD151 with other tetraspanins or alpha3beta1 but negated its modulatory function. Changes in the glycosylation pattern of alpha3beta1 observed in CD151-depleted cells correlated with a dramatic decrease in cell migration toward laminin-332. Migration toward fibronectin or static adhesion of cells to extracellular matrix ligands was not affected. Importantly, reconstituted expression of the wild-type CD151 but not glycosylation-deficient mutant restored the migratory potential of the cells. These results demonstrate that CD151 plays an important role in post-translation modification of alpha3beta1 integrin and strongly suggest that changes in integrin glycosylation are critical for the promigratory activity of this tetraspanin.     

Quantitative structure-activity relationships of estrogenic steroids substituted at C14, C15

The uterotropic activity of thirty 3-methoxyestradiol derivatives is measured and discussed on the basis of X-ray crystallographic results and quantitative structure-activity relationship analyses involving hydrophobic substituent constants pi and f as well as steric parameters Pr and L. In addition, estrogenicity is compared to data of interceptive activity and receptor binding affinity. All the biological data exhibit a high degree of intercorrelation. 17 beta-Hydroxysteroids having 14 alpha configuration reveal a generally better capability of high-affinity binding than those being 14 beta configurated. Between the uterotropic activity and the hydrophobicity of C14, C15 substituents, statistically significant correlations are found which suggest a close contact between the steroidal D-ring subsite and the receptor protein (e.g. for 14 alpha steroids: log UDD = -0.996 pi -0.392; n = 9, r = -0.943, s = 0.235, t = -7.5, alpha less than 0.001). The hydrophobic nature of both 14 alpha and 14 beta medium-sized substituents employed is shown by QSAR regressions to exert a stronger influence than steric effects. Furthermore, there are indications to additional hydrogen bonding and steric repulsion phenomena. As to the receptor-binding models discussed in the literature, it is concluded that the receptor protein has a high conformational flexibility to accommodate very different drug structures all having the common phenolic ring A. But, if an appropriate spacing of steroidal key atoms is recognized by the receptor and, consequently, the steroid-receptor complex is formed, the binding is complemented by hydrophobic interactions also in the D-ring region.     

Inhibition of the 3 beta-hydroxysteroid oxidoreductase of Pseudomonas testosteroni by steroids.

55 Steroids of the estratriene and androstane type with substituents in pos. 16 alpha, 17 alpha or 17 beta were tested for inhibition of the 3beta-hydroxysteroid oxidoreductase of Pseudomonas testosteroni. Estratrien-3-ols were strong and competitive inhibitors (Ki less than 1 micron). Substituents in pos. 16 alpha of estradiol influenced the inhibitory activity distinctly. Substituents in 17 alpha- or 17 beta-position were of slight influence. 3-Methoxy estratrienes gave no inhibition of the enzymic 3 beta-OH-dehydrogenation. The 4-unsaturated 3-oxo-steroids tested were moderate inhibitors (Ki 2.4-70 micron). The activity was slightly influenced by 17 alpha-substituents. It was increased by 10 beta-substituents in the order H less than CH3 less than N3. The inhibition test can be used to select and eliminate very strong synthetic inhibitors, which are known to disturb the metabolism of steroid hormones.     

Enzymatic transfer of a preassembled trisaccharide antigen to cell surfaces using a fucosyltransferase.

The Lewis alpha (1-->3/4)-fucosyltransferase (Le-FucT) is known to fucosylate both Type I (beta Gal(1-->3) beta GlcNAc) and Type II (beta Gal(1-->4) beta GlcNAc) sequences even when these are sialylated at OH-3 or fucosylated at OH-2 of the terminal Gal residues. These acceptor sequences are ubiquitous on mammalian cell-surface glycoproteins and glycolipids. The Le-FucT enzyme is therefore a potential candidate as a universal reagent for the modification of cell surfaces. We have found that a readily accessible, partially purified Le-FucT from human milk, which normally uses GDP-fucose (a 6-deoxy sugar) as the donor for the transfer of a single fucose residue, will also transfer a fucose residue substituted on C-6 by a very large sterically demanding structure, in this instance, a synthetic blood group antigen. As a demonstration of the ability of the Le-FucT to modify glycoconjugates in a mild and specific manner, we chemically synthesized the complex sugar-nucleotide alpha Gal(1-->3) [alpha Fuc(1-->2)]-beta Gal-O-(CH2)8COHN(6)-beta-L-fucose-GDP (13) which is a GDP-fucose analog where the human blood group B trisaccharide antigen is covalently linked to C-6 of fucose through an amino group. It is shown that, in enzyme-linked immunosorbent assays, the Le-FucT uses both immobilized beta Gal(1-->3) beta GlcNAc-bovine serum albumin conjugates and fetuin as acceptor substrates and renders them blood group B-active as detected by a monoclonal anti-B blood-grouping antibody. The fucose residue to which the B-trisaccharide is linked therefore becomes covalently attached to the acceptor oligosaccharide chains of those glycoproteins. Incubation of type "O" erythrocytes with the Le-FucT and complex donor 13 results in the covalent transfer of alpha Gal(1-->3) [alpha Fuc(1-->2)] beta Gal-O-(CH2)8COHN(6)-beta-L-Fuc to cell-surface acceptors since the cells become phenotypically "B" and are agglutinated by the same antibody. It is proposed that the Le-FucT represents a powerful new tool with the ability to label animal cell surfaces with preassembled oligosaccharide and possibly also other complex recognition markers.     

Comparative sensitivity of 2 carboxylesterases from the reindeer liver to various inhibitors

The sensitivity to the inhibitor of two forms of reindeer liver carboxylesterases differing in electrophoretic mobility and conventionally termed as "slow" and "fast" forms were investigated. The rate constants for the interaction of organophosphorous irreversible inhibitors--diisopropylfluorophosphate (DPP) and two methylthiophosphonic acid thioesters--C5H11O(CH3)P(O)S(CH2)SCH2C(O)OCH3 (Sh-205) and, C8H17O(CH3)P(O)S(CH2)SCH2C(O)OCH2 (Sh-207)--with the "fast" form are hundreds of times as high as those with the "slow" one. The rate constants for irreversible carbamate inhibitor interaction byehone with both carboxylesterase forms were approximately equal to 1.2 X 10(3) M-1 X min-1 and 2.0 X 10(3) M-1 X min-1, respectively. The reversible inhibitors potassium benzylate and kathapin also inhibited the "fast" carboxylesterase form in the indophenylacetate (IPA) hydrolysis reaction (770 and 1700-fold, respectively). On the contrary, N-methylpiperidinyl ester of benzyl acid inhibited the "slow" form three times stronger. Carbophos reversibly inhibited IPA hydrolysis in the presence of both enzyme forms, but the carboxyester carbophos group was hydrolyzed at a measurable speed only by the "slow" form.     

A study on the quaternary structure change of hemoglobin in the ligation process

In order to inquire into the molecular mechanism underlying the cooperative ligand binding to hemoglobin (Hb), conformational interaction at the interfaces between subunits are investigated on the basis of the atomic coordinates of human deoxy and human carbonmonoxy Hbs. Hypothetical intermediate structures are used, each of which is obtained from the procedure where one or more subunits in deoxy Hb are replaced by the corresponding CO-liganded subunits in carbonmonoxy Hb using the method of superimposition of two sets of atomic coordinates. When either alpha or beta subunit is substituted with the corresponding subunit in carbonmonoxy Hb, serious steric hindrances are produced between alpha 1FG4(92)Arg and beta 2C3(37)Trp or between alpha 1C6(41)Thr and beta 2FG4(97)His, all of which belong to the allosteric core affected directly by ligand binding. These steric hindrances become more serious when both alpha 1(alpha 2) and beta 2(beta 1) subunits are substituted. Therefore the change in the relative distance between iron atom and porphyrin by ligation results in strain in the C-terminal residues as an effect of the steric hindrance between the FG and C segments. However, no steric hindrance can be seen between subunits when the subunits in carbonmonoxy Hb are substituted with the corresponding subunits in deoxy Hb. The nature of the quaternary structural change from liganded to deoxy Hb seems to be different from that from deoxy to liganded Hb.     

The conformational behaviour of complexes of alpha-cyclodextrin with p-chlorophenol and p-hydroxybenzoic acid in water as studied by molecular dynamics simulations.

Molecular dynamics simulations were performed to obtain information about the conformational behaviour and stabilization of alpha-cyclodextrin (alpha CD) complexes in water. Simulations of p-chlorophenol and p-hydroxybenzoic acid in alpha CD showed that the complex is a very flexible system. The guest compound rotates inside the cavity and partly moves in and out. alpha CD continuously adapts its conformation to the orientation of the guest compound (or vice versa): the hexagon of the glycosidic oxygen atoms is stretched parallel to the plane of the aromatic ring of the guest compound during 80% of the simulation. This suggests that Van der Waals interactions play an important role in the stabilization of the complex. Each intramolecular hydrogen bond between neighbouring glucose units is formed during 30-80% of the simulation. Hydrogen bonds between alpha CD and the guest compound, on the other hand, are rarely formed. Thus, intermolecular hydrogen bonding seems to play a minor role in the stabilization of alpha CD complexes.     

A helical arrangement of beta-substituents of dehydropeptides: synthesis and conformational study of sequential nona- and dodecapeptides possessing (Z)-beta-(1-naphthyl)dehydroalanine residues

Sequential nona- and dodecapeptides possessing three and four (Z)-beta -(1-naphthyl)dehydroalanine (Delta(Z)Nap) residues, Boc-(L-Ala-Delta(Z)Nap-L-Leu)(n)-OCH(3) (n = 3 and 4; Boc = t-butoxycarbonyl), were synthesized to design a rigid 3(10)-helical backbone for a regular arrangement of functional groups using dehydropeptides. Their solution conformations were investigated by NMR and CD analyses, and theoretical energy calculations. Both peptides were found to adopt a 3(10)-helical conformation in CDCl(3) from their nuclear Overhauser effect spectroscopy (NOESY) spectra, which showed intense cross peaks for N(i)H-N(i+1)H proton pairs, but no cross peaks for C(alpha)(i)H-N(i+4)H pairs. The predominance of a 3(10)-helix was also supported by solvent accessibility of NH resonances. CD spectra of both peptides in tetrahydrofuran showed strong exciton couplets at around 228 nm assignable to naphthyl side chains, which are regularly arranged along a right-handed helical backbone. Chain-length effects on conformational preference in sequential peptide -(Ala-Delta(Z)Nap-Leu)(n)- were discussed based on spectroscopic analysis, energy minimization, and molecular dynamics simulations. Consequently, the repeating number n > or = 3 forms predominantly a right-handed 3(10)-helical conformation. The energy calculation also revealed that the midpoint naphthyl groups of peptide n = 4 are highly restricted to one stable orientation. In conclusion, beta-substituted alpha,beta-dehydroalanine is expected to be a unique tool for designing a rigid molecular frame of 3(10)-helix along which beta-functional groups are regularly arranged in a specific manner.     

Two-dimensional proton NMR studies on poly(VPGVG) and its cyclic conformational correlate, cyclo(VPGVG)3

Two-dimensional nuclear Overhauser enhancement (2D NOESY) data are reported for the polypentapeptide of elastin, poly(VPGVG), and the cyclopentadecapeptide, cyclo(VPGVG)3. In both, the repeating type II Pro2-Gly3 beta-turn can be derived from the NOE data, providing confirmation of many previous studies. In addition, other through-space connectivities are detailed that also compare favorably with previously determined crystal and solution structures for cyclo(VPGVG)3. Also, near identical data for the cyclopentadecapeptide and the polypentapeptide demonstrate the cyclic conformation-linear (helical) conformational correlate relationship between the two molecules. The 2D NOESY experiment is seen to be an effective means of establishing the presence or absence of a conformational relationship between a cyclic repeating sequence and its higher molecular weight linear counterpart. This is an approach of substantial practical value when developing the conformation of sequential polypeptides and when attempting to identify the presence of the conformation of a repeating peptide sequence within a more complex primary structure. Having established the basic conformational relationship between a cyclic conformation and its linear helical counterpart, cross peaks present in the linear helical structure that are not present in the cyclic conformational correlate can provide information on the interactions between adjacent turns of the helix. In this connection, a Val gamma CH3 in equilibrium Pro beta CH2 interaction is reported that can be the basis for determining the number of pentamers per turn of helix once it is determined whether it is dominantly the Val1 or Val4 gamma CH3 that is interacting with the Pro2 beta CH2.     

Understanding bilirubin conformation and binding. Circular dichroism of human serum albumin complexes with bilirubin and its esters

Human serum albumin binds tightly and noncovalently to a wide variety of hydrophobic bilirubins, including (4Z,15Z)-bilirubin-IX alpha, its dimethyl ester and mono methyl esters, its mono 2-butyl esters and amides, the dimethyl ester of (4Z,15Z)-mesobilirubin-IV alpha, and even (4Z,15Z)-etiobilirubin-IV gamma. The heteroassociation complexes formed from these highly water-insoluble pigments and the protein can be prepared in pH 7.4 aqueous by using a small quantity of dimethyl sulfoxide as amphiphilic carrier. In those solutions the protein acts as a water-soluble chiral complexation agent to induce an asymmetric transformation of the bound pigment. This is recognized by positive chirality, bisignate induced circular dichroism (CD) Cotton effects that fall in the region of the bichromophoric pigment's long wavelength UV-visible absorption band and are characteristic of intramolecular exciton coupling of the bilirubin component pyrromethenone chromophores. The same-signed CD spectra shared by all the pigments of this work indicate selection at the protein binding site for a positive chirality conformer and suggest a common binding site. The CD intensities, which are greatest ([delta epsilon[ congruent to 50) for pigments with one or two free carboxyl groups, are consistent with a binding model where one salt linkage plays a major role in the enantioselectivity of the right-handed folded conformation stabilized by inter- and intramolecular hydrogen bonds.