The X-ray structure analysis of a crystalline sample of 2-azabicyclo-[2,2,2]-octanone-3 or 3-isoquinuclidone shows that the molecules of this compound are associated in centrosymmetrical dimers stabilized by two N? H? O?C hydrogen bonds in which the N,H,O atoms are nearly collinear. As a consequence of this interaction, the H atom is shifted from its usual position and the Cα? N? H angle is increased to 125°. Using infrared spectroscopy (νN–H frequency range), it is possible to demonstrate that 3-isoquinuclidone is mainly in a dimeric form when dissolved in an inert solvent such as CCl4 and to observe the dimer-monomer equilibrium on dilution from saturation to a low concentration (0.005 mole/l.). On the contrary, dimers are broken off when operating in a polar medium (acetonitrile, deuterochloroform). In the same experimental conditions, measurements of the J vicinal coupling constant, by nuclear magnetic resonance spectroscopy, afford a concentration-dependent result in the case of CCl4 solutions (increasing from 5.4 to 5.7 Hz when diluting from 0.5 to 0.005 mole/l.) and a constant one (5.8 Hz) in the case of CH3CN or CDCl3 solutions. Then the 0.4-Hz difference can be attributed to geometrical changes in the Hα? Cα? N? H system when dimers are broken off and the valence angle Cα? N? H consequently decreases from 125° to its standard value (about 115°). This experimental observation is consistent with the result of a theoretical analysis performed by the INDO method. Then it seems that the use of the formulas proposed by Karplus to account for the valence angle distorsions in ethane-like systems, in the case of the Hα? Cα? N? H sequence, could yield overstimated corrections. 相似文献
The 1H-nmr spectra of co-oligopeptides of tryptophan and glycine with structure H-Gly-Trp-(Gly)n-Trp-Gly-OH (n = 0–2) and those of several di- and tripeptides have been recorded at 360 MHz with CD3OD solutions containing 0.1N NaOD. The assignment of resonance signals was generally possible by comparing the spectra of structurally related peptides with each other. In order to solve the remaining ambiguities in the assignment, H-(αL,βS)(α,β-d2)Trp-OH, H-Trp-(αL,βS)(α,β-d2)Trp-OH, and H-Trp-(δ1,ε2,ζ2,ζ3,η2-d5)Trp-OH have been prepared and their spectra compared with those of the undeuterated compounds. The distribution of rotamers around the χ1 and (in two cases) χ2 torsion angles of the side chains has been obtained from the vicinal coupling constants 3J and from the long-range coupling constants 4J. These data and an analysis of the chemical shifts of the Gly-Cα protons suggest that the orientation of the aromatic side chain is influenced by the following order of decreasing interaction with the functional groups at N- and C-side: -NH2 > –NHCO– > –CONH–> –COO?. This rule does not hold for the second Trp residue of di- and tripeptides containing the -Trp-Trp- sequence, which has tentatively been attributed to steric effects. 相似文献
A 20-membered cyclic peptide disulfide has been synthesized as a conformational model for disulfide loops of limited ring size. 1H-nmr studies at 270 MHz establish the presence of three intramolecular hydrogen bonds involving the Leu, Val, and methylamide NH groups in CDCl3. Evidence for peptide aggregation in CDCl3 is also presented. A structural transition involving loosening of the hydrogen bond formed by the Val NH group is observed upon the measured addition of (CD3)2SO to CDCl3. Hydrogen-bonding studies, together with unusually low field positions of the Cys(1) and Cys(6) CαH resonances and high J values provide support for an intramolecular antiparallel β-sheet conformation, facilitated by a chain reversal at the Aib-Ala segment. Extensive nuclear Overhauser effect studies provide compelling evidence for the proposed conformation and also establish a type I′ β-turn at the Aib-Ala residues, the site of the chain reversal. 相似文献
Solution conformations of the cyclic pentapeptide plant-hormone malformin A, whose conformational freedom is constrained by an intramolecular disulfide bridge, are derived and presented here. The nmr and CD data of Ptak are used to place restrictions on the search for possible malformin A solution conformers of low energy. Only two distinct conformers were found to be consistent with Ptak's data. Both structures are characterized by an internally buried (solvent-shielded) D -Cys2 amide proton, a seven-membered (1–3)hydrogen bond between (N–H) and (O?C), and a disulfide bridge conformation with a P chirality as manifested in the nmr study by the temperature independence of the amide proton chemical shifts for the D -Cys2 and D -Leu4 residues and the negative sign of the long wavelength maximum in the CD spectrum, respectively. Inspection of space-filling molecular models of both structures indicates severe steric barriers to their rapid interconversion. Thus, it appears that only one of the two conformers may be present in solution. The difference in their calculated dipole moments (4.6 and 6.9D) suggests an experimental method for distinguishing between the two proposed solution structures. 相似文献
Tetrapeptides with proline in position 2, asparagine or leucine in position 3, and glycine in positions 1 and 4, with end groups free or blocked on the N-terminal side, were studied in their various ionic states in 2H2O and in Me2SO-d6 by 1H- and 13C-nmr. In order to clarify or refine some details, successive substitutions of the residues in these peptides with amino acids enriched to 85% in 13C, or to 85% 13C plus 97% 2H were carried out. The 1H and 13C chemical shifts as well as the 1H-1H, 13C-13C, and 13C-1H coupling constants and the signal intensities show strong similarity of behavior between the tetrapeptides of asparagine and leucine. The main conformational characteristics are (1) the almost total stabilization of the trans conformer in the type I β-turn structure when the peptide is in the zwitterion state dissolved in Me2SO. This is deduced from the 3J and the 3J coupling constants, which both furnish a dihedral angle of ?3 = ?90°, and from the positive value of the temperature coefficient of the glycine-4 amide protons, which suggests a type 4 → 1 hydrogen bond; (2) the evolution of cis and trans isomer fractions which change with the ionic state of the peptides in Me2SO, whereas they remain constant in aqueous solution; and (3) the conformation of the pyrrolidine ring as it follows the variations in cis:trans isomer populations together with the side-chain rotamer fractions of the residue in position 3. In the β-turn conformation the isomer cis is less abundant and the pyrrolidine ring is more flexible; this explains the perfect accommodation of the proline residue in position 2 of a bend. The interdependence of these phenomena where interactive forces play a predominant role underlines the importance of cooperative effects in the molecule. The results also suggest that the cis isomer of proline can adapt itself just as well as the trans isomer to position 2 of a type I β-turn. 相似文献
We report the determination of two- and three-bond 1H-15N spin–spin couplings in the nmr spectra of a polypeptide. The 1H- and 15N-nmr spectra of 99.2% 15N-enriched alumichrome have been studied at 360 MHz and 10.1 MHz, repectively. While some 2J and 3J coupling are of the order of 5 Hz, most splitting resulting from the heteronuclear interaction are ?2 Hz, which introduces strigent requirements of spectral resolution. In the 1H spectra these requirements were met by digital deconvolution with a sine bell routine combined with positive exponential filtering. Although the 15N spectra clearly exhibit features of fine structure, mainly because of the intrinsic higher nmir sensitivity of protons, observation of 1H-15N spin–spin couplings was found to be more practical in the 1H than in the 15N spectra. We find that the alumichrome data do not satisfy a simple cyclic relationship linking the heteronuclear couplings to the crystallographic ψ dihedral angles. It is suggested that a formal treatment of the ψ-related interresidue 1H-15N coupling might have to take into account a more complex dependence of the intervening 3J on the overall local electronic structure, which is dependent on ?,ψ, and ω simultaneoulsy. In contrast, our analysis indicates that χ1 can be readily determined from the measurement of the corresponding heteronuclear 3J coupling in the 1Hβ or in the amide 15N resonances. Karplus relationships are proposed that relate this heteronuclear 3J to the corresponding dihedral angle θ and which, on average, yield 相似文献
Three freshwater and one marine algal species were grown under inorganic carbon limitation in laboratory continuous cultures. Comparisons were made between HCO3? alkalinity and bubbled CO2 as carbon sources. HCO3? alkalinity was an excellent source of inorganic carbon below specific pH levels, but chemical precipitation at high pH placed an upper limit on productivity that was far lower than potential light-limiting levels. With bubbled CO2 it was possible to achieve light limitation. The main factor controlling productivity was the mass flux of inorganic carbon added to the culture, which is the product of gas flow rate and influent P level. Small bubbles were more efficient than large bubbles at low gas flow rates and P levels, but led to froth flotation of algal cells and concomitant reductions in productivity at high bubble rates. At 1% CO2 productivity was still dependent on mass fluxes of added carbon, but was independent of bubble size. At high bubble rates with 1% CO2 narcosis was evident. Maximum yields occurred at intermediate dilution rates when inorganic carbon was supplied via bubbled gas. 相似文献
A survey has been made, by using computer methods, of the types of helices which polypeptide chains can form, taking into account steric requirements and intramolecular hydrogen-bonding interactions. The influence on these two requirements, of small variations in the bond angles of the peptide residues, or of small changes in the overall dimensions of the helix (pitch and residues per turn), have been assessed for the special case of the α-helix. Criteria for the formation of acceptable hydrogen bonds have also been applied to helices of other types, viz., the 3, γ?, ω?, and π-helices. It was shown that the N? H … O and H … O? C angles in hydrogen bonds are sensitive to changes in either the NCαC′ bond angle or in the rotational angles about the N? Cα and Cα? C′ bonds. However, the variants of the α-helix observed experimentally in myoglobin can all be constructed without distortion of the hydrogen bonds. For α-helices, the steric and hydrogen bonding requirements are more easily fulfilled with an NCαC′ bond angle of 111°, rather than 109.5°. The decreased stability observed for the left-handed α-helix relative to the right-handed one for L -amino acids is due essentially only to interactions of the Cβ atom of the side chains with atoms in adjacent peptide units in the backbone, and interactions with atoms in adjacent turns of the helical backbone are not significantly different in the two helices. Restrictions in the freedom of rotation of bulky side chains may have significant kinetic effects during the formation of the α-helix from the “random coil” state. 相似文献
Modes of aggregation fo alanine-, norvaline- and valine-contaiing dpepetides with the general formula R1? C1O1? N2H2? CHR2? C2O2? N3H3? R3 have been studied in CCl4 solution by using infrared and nuclear magnetic resonance spectroscopies. Solutions of the pure L isomer and of the racemic mixture do not give identical data. At a given concentration, the racemic mixtrue is more aggregated than the pure enantiomer, and the difference, negligible in the case of alanine derivative, increases wiht the bulkiness of the side cahin R2. The results show that a selective interaction takes place between enantiomeric molecules, resulting ina dimer associating tow inverse configurated C5 conformers. The stabilizaion of this dimer proceeds from two symmetrical and intermolecular H3 … O1 hydrogen bondings. 相似文献
Trichoderma QM 9414 was aerobically grown on glucose as the sole carbon and energy sources in a chemostat culture. The specific rates of glucose consumption (QG), oxygen consumption (Q), and carbon dioxide production (Q) at the steady state were measured to estimate the growth and maintenance requirements. From the results it was estimated that 2 mol adenosine triphosphate (ATP) were produced when1 mol NADH was oxidized through the respiratory chain of this microorganism. The true growth yield for ATP (YATP) and specific ATP consumption rate for maintenance (Q) calculated with this value were 0.0106 g dry cell/mmol ATP and 5.2 mmol ATP/g dry cell/hr, respectively. Using the relationships between specific growth rate (μ) and (Q) and between μ and QG obtained from chemostat-culture data, cell and glucose concentration histories were estimated from the carbon dioxide production rate during the batch culture. The estimated cell concentrations agreed with the experimentally measured values. Glucose concentration were slightly overestimated. 相似文献
Empirical force-field calculations and ir and 1H-nmr spectra indicate that five-membered (C5) and seven-membered (C) hydrogen-bonded rings are the preferred conformations of acetyl-L -Phe p-acetyl and p-valeryl anilides in nonpolar media. The C5/C ratio was found to be dependent on the dryness of the solute and the solvent. This fact and the results from conformational-energy calculations suggest that a molecule of water participates in the stabilization of the C conformation. 相似文献
Proton magnetic resonance spectra of model dipeptide molecules R1–C′1O1–N2H2–CHR2–C′2O2–N3H3–R3 in CCl4 solutions exhibit splited signals when investigating on mixtures of L and D enantiomers differing from the racemic composition. The major effect is observed on amide proton signals which are the ones most sensitive to the ratio of aggregation. The stereoselective dimerization of enantiomeric molecules in the so-called C5 conformational state is shown to be responsible for such a phenomenon, the intensity of which depends on the bulkiness of the side chain R2. A theoretical approach is proposed which gives predictions in close agreement with our own experimental findings. 相似文献
The molecular theory of the previous paper in this series is extended to determine the effect of divalent metal ions on helix stability relative to coil at fixed ionic strength and nucleotide phosphate concentration. Specification of the state of condensed counterions, as well as their concentration, is essential for the solution of this problem, and it is assumed that they translate freely within a thin cylindrical shell close to the polynucleotide. As a function of divalent counterion concentration m the relative stability of the helix is highly nonlinear. Although the overall trend is that the helix stability increases with addition of divalent metal ion, there is a narrow concentration range for which it decreases slightly. The behavior of the relative stability as a function of m is determined by the translational degrees of freedom of the counterions, both univalent and divalent, both condensed and uncondensed. Detailed comparison of the theory with data is not given here, but it is pointed out that the calculated values of the relative stability are consistent with the order of magnitude of the observed effect Mg2+ on the melting temperature. 相似文献
We use the nmr data concerning the CαH? CβH fragment in eight peptides with rigid side chains to parametrize a Karplus correlation between the vicinal proton Jαβ coupling constant and the dihedral angle θ. When considering molecules containing the fragment CαHα? CβHβHβ′, the three-dimensional structure of the model peptides does not need to be known with accurate precision, since each set of Jαβ and Jαβ′ coupling constants is then related to the coefficients of the Karplus equation. A good correlation is observed with the Karplus equation, which is in substantial agreement with the Jαβ coupling constants reported for rigid as well as rotating Cα? Cβ bonds in peptides. 相似文献