Kalorimetrische und Infrarotspektroskopische Untersuchung von mechanisch hervorgerufenen Konformationsumwandlungen bei Poly-[L-alanin]

Zusammenfassung Es wurden die Wärmestrom- und Kraft-Dehnungskurven von Poly[L-alanin]-Fasern und -Folien bei uniaxialer Verstreckung in einem Verstreckungskalorimeter gemessen. Aus der zum Verstrecken benötigten Arbeit A und der zugehörigen Wärmetönung, ergab sich die Änderung der inneren Energie U der Proben. Es zeigte sich, daß im allgemeinen bis etwa 1 % Dehnung, wobei weitgehend reversibles energieelastisches Verhalten vorherrscht, U um 0,25-0,3 J/g (60–70 mcal/g) zunimmt. Bei weiterer Dehnung steigt U erheblich stärker an (bis 1,5 J/g bei 2% Dehnung), jedoch ist U stark abhängig von der Vorbehandlung z. B. durch Vorverstreckung oder vom Gehalt an niedermolekularen, als Weichmacher wirkenden Substanzen.Da die irreversiblen Änderungen in der Probe hier besonders interessieren, vor allem im Hinblick auf Konformationsumwandlungen wie z. B. , wurde stufenweise verstreckt und nach jedem Dehnzyklus (Dehnen + Entlastung) die Summe der Energieänderung gegen den maximalen Dehnungsgrad aufgetragen. Oberhalb 1 % Dehnung ist diese Energieänderung > 0 und geht mit einer bleibenden Dehnung einher, abhängig von Probenart und Vorgeschichte. Es wurde daher die Änderung von U nach einem Dehnzyklus ( U _D + U _E ) auf diese bleibende Dehnung bezogen.Für die Ermittlung der mit einer Konformationsänderung verbundenen Änderung der inneren Energie ist es erforderlich,die Anteile der verschiedenen Konformationen zu ermitteln. Hierzu wurden die IR-Spektren verwendet. Für die Bestimmung des -Helix-Anteils dienten die Absorptionsbanden bei 1270 bzw. 370 cm^–1, für die der -Faltblattstruktur die Doppel-Banden bei 1220 () und 1240 () cm^–1 und die bei 440 cm^–1, die aus den Banden bei 445 cm^–1 () und 432 cm^–1 () besteht. Dabei wurden als Normierungsbanden die weitgehend konformationsunabhängigen Banden bei 1380 cm^–1 sowie die bei 1050 cm^–1 verwendet.Es zeigte sich, daß nach dem Verstrecken -helicaler Poly-[L-alanin]-Proben um 5% eine bleibende Dehnung von um 2% auftritt, wobei der -Helix-Gehalt um 4% abnimmt. Bei Orientierung in Verstreckrichtung bedeutet dies auf Grund des Längenverhältnisses eines Aminosäurerestes in der -Helix zu der in einer gestreckten Anordnung von 1,5:3,5 = 1:2,3, daß der Anteil an ß-Struktur um 1,5 % zugenommen hat. Man kann demnach davon ausgehen, daß die restlichen 2,5 % -Helices in eine nichtperiodische Konformation übergegangen sind. Aus den dehnungskalorimetrischen Messungen ergab sich aufgrund der Zunahme von U um 2,1 J/g bei 1% irreversibler Dehnung eine Energieänderung von 19 kJ/mol (4,6 kcal/mol). Geht man davon aus, daß -Helix und ß-Faltblattstruktur dieselbe innere Energie haben, so entspricht das einer Energiedifferenz von 16 kJ/mol (3,8 kcal/mol) zwischen den o. a. periodischen Konformationen und der nichtperiodischen, was mit dem vonBirshtein undSkvortsov theoretisch ermittelten Wert von 3,5 kcal/mol gut übereinstimmt (30).

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Summary The heat-flux and stress-strain-curves of poly-[L-alanine]-fibers and films were measured using a stretching-calorimeter. The change in the internal energy U was determined from the mechanical workA necessary for stretching and the heat Q. Up to 1% stretching reversible energyelastic behaviour is predominant and U increases at about 0,25-0,3 J/g (60–70 mcal/g). At higher degrees of stretching U increases considerably more (e. g. up to 1,5 J/g at 2% stretch), however, U depends strongly on the pretreatment of the sample like prestretching or the content of low molecular weight substances acting as a plastifier.In respect to conformation changes of the poly-[L-alanine] e. g. , irreversible changes of the sample during stretching were of special interest. Therefore the samples were stretched stepwise and the sum of the energy changes after each stretching cycle (i. e. after loading and deloading) was plotted against the maximum degree of stretching. Above 1 % this energy change is > 0 and is accompagnied by an irreversible change in length, depending on pretreatment and the kind of sample. According to this, the change of U after one stretching cycle ( U _D + U _E ) was divided by this irreversible change in length.For determining the change in internal energy U due to a conformation change it is essential to determine the content of the different conformation types like , and random. IR-spectroscopy seems to be the most suited method at present for this purpose. For the determination of the -helix content we used the absorption bands at 1270 cm^–1 resp. 370 cm^–1 and for the ß-pleated sheet-structure the double-bands at 1220 cm^–1 () and 1240 cm^–1 () and that at 440 cm^–1, consisting of the bands at 445 cm^–1 () and 432 cm^–1 (). As reference bands which are independent on conformation to a high degree we used those at 1380 cm^–l and 1050 cm^–1.After stretching of almost pure a-helical poly-[L-alanine] at about 5% an irreversible stretch of 2% occurs and the -helix content decreases at about 4%. This means that, if molecules are oriented in stretching direction, the content of ß-structure has increased by 1,5% because the relation in length of one amino-acid residue in the a-helix and in the ß-structure is 1,5 : 3,6 = 1 : 2,4. Therefore it seems that 2,5% -helix content have changed into a non-periodic (random)-conformation. From the increase in U of 2,1 J/g at 1% irreversible stretch a change of 19 kJ/mole (4,6 kcal/mol) occurs. Assuming that the internal energy of -helix and ß-pleated-sheet-structure is the same, this is equivalent to a difference in H between these periodic structures and the non-periodic one of 16 kJ/mole (3,8 kcal/mole) which is in a reasonable agreement with that of 3,5 kcal/mole calculated byBirshtein andSkvortsov from theoretical considerations (30).

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Herrn Prof. Dr.G. Rehage zum 60. Geburtstag gewidmet.

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Mit 9 Abbildungen und 2 Tabellen     

First and second thermodynamic mixing properties of ethylbenzene +n-alkanes: Experimental and theory

Isobaric expansibilities _P and isothermal compressibilities _T have been determined at 25 and 45°C for binary mixtures of ethylbenzene + n-tetradecane and + n-hexadecane and the corresponding excess functions (V ^E /T)_P and (V ^E /P)_T have also been obtained. With these data and supplementary literature values, the following second order mixing properties are also reported at 25°C: _S ^E , (V ^E /P)_T, C_V and (VT). All mixing quantities have been compared with the results obtained at 25°C by using the Prigogine-Flory-Patterson theory of liquid mixtures. The predicted values suggest that the ability of ethylbenzene as a breaker of the pure n-C_n orientations is similar to what we found for toluene and higher than for p-xylene.     

Marked differences between acetonitrile/water and methanol/water mobile phase systems on the thermodynamic behavior of benzodiazepines in reversed phase liquid chromatography

Summary Two different methods were used to determine the separation factor at different temperatures and the Gibbs-Helmholtz parameters ((H), (S)) of two adjacent benzodiazepines on a chromatogram were obtained from plots of ln versus 1/T. We first studied each factor (fraction of water in the ACN/water mixture and column temperatureT), which controls the retention mechanism, and then we examined the simultaneous variation of all these factors. The changes in (H) and (S) in relation to a volume fraction of water in an ACN/water mixture were examined. In the ACN/water system, (H) was fairly constant in the acetonitrile region of 0.52 and appears to be a roughly linear function of for 0.52. In this system (S) is approximately a parabolic function of with an optimum at 0.52. The retention mechanism of ten benzodiazepines was found to be significantly different in the methanol/water and ACN/water mixtures. The separation optimization of these ten benzodiazepines was then considered. A fraction of water of 0.43 in the ACN/water mixture and a column temperature of 44°C gave the most efficient separation conditions in the ACN/water mixture.     

Thermodynamics of protonation of amino acid carboxylate groups from 50 to 125°C

Flow calorimetry has been used to study the interaction of glycine, DL--alanine, DL-2-aminobutyric acid, -alanine, 4-aminobutyric acid, and 6-aminocaproic acid with protons in aqueous solutions from 323.15 K to 398.15 K and at 1.52 MPa. LogK, H°, S°, and C _p ^° for the protonation of the carboxylate groups of these amino acids have been obtained at each temperature studied. Equations are given expressing these values as functions of temperature. The protonation reactions are exothermic at lower temperatures and become endothermic as temperature increases. The logK, H°, and S° values are close together over the temperature range studied for the protonation of -amino acids, i.e., glycine, DL--alanine, and 2-aminobutyric acid. At each temperature, the magnitudes of these thermodynamic quantities increase as the number of methylene groups between the amino group and the carboxylate group increases. The C _p ^° value for the protonation of the carboxyl group is found to lie between those of an isocoulombic reaction and a charge reduction reaction. At 323.15 K, the protonation reactions of the carboxylate groups have larger C _p ^° values which approach those associated with charge reduction reactions. As the temperature increases, C _p ^° decreases and approaches those found for isocoulombic reactions. This result is explained by considering long-range and short-range solvent effects. The trend in H° and S° with temperature and with charge separation in the zwitterions is interpreted in terms of solvent-solute interactions and the electrostatic interaction of the two oppositely charged groups within the molecule.     

Sorption of C6 Alkanes in Aluminophosphate Molecular Sieve, AlPO4-5

Sorption of C₆ alkanes viz.,n-hexane, 2-methylpentane, 2,2-dimethyl butane, 2,3-dimethyl butane, cyclohexane and methyl cyclopentane in AlPO₄-5 is studied and sorption capacity and thermodynamic parameters (H⁰, H, G, S and S_a) at various sorption coverage for these sorbates have been estimated. The initial heat of sorption (H⁰) is found to increase with the degree of branching in the alkane chain. The sorbate-sorbate interactions are found to be more prominent during the sorption of doubly branched alkanes and cyclic alkanes and a higher sorption capacity has been observed for cyclic alkanes. Based on the results obtained, a packing model has been proposed for various C₆ alkanes inside the AlPO₄-5 channel.     

The position of plutonium/III/ in the lanthanide series in respect to thermodynamic functions of complex formation with nitrates and thiocyanates

The position of Pu/III/ within lanthanides in respect to G⁰, H⁰ and S⁰ of complex formation with nitrate and thiocyanate ligands was determined by the extraction method. It was found that in respect to G⁰, Pu/III/ is a light pseudolanthanide for nitrate ligands and a heavy pseudolanthanide for thiocyanate ligands. A comparison of the positions of Pu/III/ and Am/III/ in respect to G⁰, H⁰ and S⁰ shows that the radius of plutonium is greater than that of americium in the An/NO₃/ ₅ ^2– complex and smaller in the An/NCS/₃/TBP/_n complex. The increase in the radii between plutonium and americium in the thiocyanate complex points out to a contribution from 5f orbitals to bonding.     

Circulardichroismus. XXX

Zusammenfassung Es wird eine einfache Anordnung beschrieben, die es gestattet, auf einem Zweitschreiber zum Roussel-Jouan-Dichrographen wahlweise direkt oder / aufzuzeichnen. Integration der letzteren Kurve mit einem Planimeter gibt (unter Berücksichtigung der entsprechenden Faktoren) dann die Rotationsstärke der betrachteten CD-Bande, auch wenn diese starke Peinstruktur zeigt.

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Circular dichroism XXX. On the direct determination of the rotatory strength by the roussel-jouan dichrograph

A simple device is described which can be used with the Roussel-Jouan Dichrograph to give direct readings of s and /, resp. Integration of the latter curve with a planimeter yields the rotatory strength of any partial CD-band.

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Der Deutschen Forschungsgemeinschaft danken wir sehr für die großzügige Unterstützung unserer Arbeiten.

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XXIX. Mitteilung: Snatzke, G., u. K. Schaffner [10].

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Herrn Prof. Dr. E. Asmus zum 60. Geburtstag gewidmet.     

13C NMR Spectroscopy on the Complexation of α-Cyclodextrin with 1-Alkanols and 1-Alkanoate Ions

Binding constants for -cyclodextrin ( -CD) complexes with 1- alkanols and 1- alkanoate ions were determined by analyzing changes ( ) in chemical shifts of guest ¹³C NMR signals with -CD concentration. The guests gave well-separated ¹³C NMR signals. The curve-fitting analyses of for individual carbons upon assumption of a simple 1:1 complexation gave virtually the same binding constants (K₁) in the cases of guests with relatively short alkyl chains, such as 1-propanol, 1-butanol, 1- pentanol , 1- butanoate , 1- pentanoate , and 1-hexanoate. However, thus obtained individual K₁ values were significantly different from one another, when the guests had relatively long alkyl chains such as 1- hexanol , 1- heptanol , and 1-octanoate. In these guests, satisfactorily consistent K₁ values were obtained by the curve-fitting analyses of for individual carbons upon an assumption that not only 1:1 but also 2:1 (host:guest) complexation occurs.     

Thermodynamic behavior of mixed benzodiazepines by a new liquid chromatographic method

Summary Using a rapid chemometric methodology to determine the separation factor, , at different temperatures, Gibbs Helmholtz parameters ( (H), (S), (G)) of two adjacent benodiazepines on a chromatogram were obtained from ln versus T^–1 plots. A temperature dependent reversal of the elution order was studied and the mobile phase composition and column temperature were optimized to obtain the best separation. A flow rate of 0.80 ml min^–1 with 52.6% methanol in the methanol-water mixture and a column temperature of 48°C gave the most efficient separation of ten benzodiazepines.     

Features of the Tautomerism of Some α-Substituted Pyridines

The AM1 quantum-chemical semi-empirical method was used to calculate the values of the enthalpy of activation (H^#), the heats of reaction (H), and the tautomeric equilibrium constants (K_T) for several -substituted pyridines. It was found that the keto-enol tautomeric conversion in -pyridone occurs more readily than the amino-imino conversion in 2-aminopyridine but in 2-methylpyridine the tautomeric equilibrium does not exist at all.     

Structural relationships in silatrane molecules

An updated compendium of silatrane structural data is presented. The relationships between nonplanarity at silicon (Si) andd(N-Si), the length of the dative bond, and nonplanarity at nitrogen N andd(N-Si) are examined. Excluding a platinum-substituted derivative: the Si relationship is strong and predicts the limiting length of a strong N Si bond, while the N relationship is weak. A good relationship between _I for the four (non-N) exo- and endocyclic substituents attached to Si holds for silatranes, methyl, keto, and diketo derivatives as well as for 2-carbasilatranes. The relationship fails for strong -donor exocyclic substituents, implicating resonance, and for benzo- and tribenzosilatranes, where steric effects dominate.On Leave of Absence from Academia Sinica, Beijing, People's Republic of China.     

Thermodynamics of Proton Association of Polyacrylates and Polymethacrylates in NaCl(aq)

Enthalpies of protonation of polyacrylates and polymethacrylates with different molecular weights in aqueous NaCl solutions, 0 I 2 mol-L^–1 were determined by titration calorimetry at 25°C. H values are dependent on both the neutralization degree, , and the molecular weight of polyacids. T S of protonation was obtained using pK values already reported and the present H results. Empirical equations for the dependence on I, , and molecular weight are reported for both H and T S.     

Strain Energies of α-Alkylsubstituted Styrenes, 1,1-Di-phenyl-ethene,Tri-, and Tetra-phenyl-ethene

The standard (p° = 0.1 MPa) molar enthalpies of formation _fH°_m (1 or cr) at the temperature T = 298.15 K were determined by using combustion calorimetry for -ethyl-styrene (A), -iso-propyl-styrene (B), -tert-butyl-styrene (C), 1,1-di-phenyl-ethene (D), tri-phenyl-ethene (E), and tetra-phenyl-ethene (F). The standard molar enthalpies of vaporization _l ^g H°_m or sublimation _cr ^g H°_m of compounds A to F were obtained from the temperature variation of the vapor pressure measured in a flow system. Molar enthalpies of fusion _cr ^l H°_m of solid compounds were measured by d.s.c. Resulting values of _fH°_m (g) were obtained at the temperature T = 298.15 K and used to derive strain enthalpies of phenylalkenes. The interactions of the substituents are discussed in terms of deviations of _fH°_m (g)from the group additivity rules. These values provide a further improvement on the group-contribution methodology for estimation of the thermodynamic properties of organic compounds.     

Complex formation of hexakis(2,6-di-O-methyl)-α-cyclodextrin with substituted benzenes in aqueous solution

The host-guest interaction of hexakis(2,6-di-O-methyl)--cyclodextrin (DM--CDx) with substituted benzenes in aqueous solution has been investigated by circular dichroism spectra. From the resemblance of the spectra, it is concluded that the guest molecules are included in the DM--CDx ring in a manner similar to that of corresponding -cyclodextrin complexes. Thermodynamic parameters of the complex formation were determined on the basis of the temperature dependent intensity change of the spectra. In spite of the considerable variation in H and AS values, the free energy does not change much among complexes because of the strong compensation effect. The compensation temperature was 309 ± 11K. Negative values of H and S, suggesting that the complex formation is not governed by the hydrophobic interaction, can be interpreted in terms of the tight binding of the guest molecule within the host cavity.     

Binding of Acridinedione Dyes with α, β and γ-Cyclodextrins: Fluorescence Quenching and Estimation of Thermodynamic Parameters

Using absorption and fluorescence spectroscopy, acridinedione (ACD) was found to be included into the -cyclodextrin (-CD) cavity to form a 1 : 1 inclusion complex. As a consequence of inclusion, the fluorescence of ACD is strongly quenched. Lifetime data confirm that the quenching is purely static. The association constant for the formation of the complex was calculated using linear and non-linear equations. H and S values obtained from the temperature dependent association constants of the -CD/ACD(B) system are reported. It was also observed that these dyes complexed very weakly with and -CD.     

Substituent effect and enthalpy-entropy compensation on the inclusion ofβ-cyclodextrin with 1-substituted naphthalenes

The inclusion complexation of-CD with 1-substituted naphthalenes has been investigated by fluorescence spectroscopy. It was observed that the association constants were influenced by the molar refraction (R _m), hydrophobic constant ( _x ), and Hammett constant ( _x ) of substituents in the guest compounds. The thermodynamic parameters G⁰, H ⁰, and S ⁰ determined by measuring the temperature-dependentK _a values shows that inclusion complex formation is enthalpy driven. The results are discussed in terms of enthalpy-entropy compensation.     

Synthesis and Dehydrohalogenation of 3 β-Chloro-5 α,7 α-dibromo-6-ketosteroids of the Stigmastane and Cholestane Series

3-Chloro-5,7-dibromo-6-ketosteroids 5a and 5b are synthesized from -sitosterol (1a) and cholesterol (1b). Dehydrohalogenation of these forms 7-bromo-2,4-dien-6-ones (6a-b), 2,4-dien-6-ones (7a-b), and 14-hydroperoxy-2,4,7-trien-6-ones (8a-b). Woodward hydroxylation of dienone 6a produces 2-iodo-7-bromo-3-acetoxy-⁴-6-ketone 9 and 7-bromo-2,3-diacetoxy-⁴-6-ketone 10. 2-Iodo-3-acetoxy-^4,7,14-trien-6-one 11 is prepared analogously from trienone 8a.     

Synthesis and rotatory dispersion of cisoid bicyclic enamino ketones

Optically active cisoid, bicyclic, enamino ketones — N-(-phenylethyl)-4-keto-^9,10-octa-hydroquinoline and N-(-phenylethyl)-4-keto-^8,9-hexahydropyridine — were synthesized by the condensation of ethyl -[N-(-phenylethyl)amino]propionate with cyclohexanone and cyclopentanone, while ethyl -(N-benzylamino)propionate and cyclohexanone gave N-benzyl-4-keto-^9,10-octahydroquinoline. A study of the rotatory dispersion of the compounds obtained demonstrated that a strong positive Cotton effect at 330–350 nm, which is associated with the presence of a cis-enamino ketone chromophore, is characteristic for them.Communication XXVIII of the series Stereochemical Investigations. See [1] for communication XXVII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 541–545, April, 1972.     

The relation between the nonlinear dielectric effect and solvent polarity

The relation between solvent polarity expressed through the Dimroth-Reichardt spectroscopic parameter E _T (30) and the nonlinear dielectric effect (NDE) expressed through the parameter /E² is demonstrated where is a change in the electric permittivity of a solvent in an external strong electric field E. Both E _T (20) and /E², determined in quite different ways, are extremely sensitive to the dielectric properties of a solvent which depend on molecular interactions. Linear correlations between /E² and E _T (30) have been found for n-alkanols representing hydrogenbond donor solvents, and for halogenobenzenes which are dipolar, aprotic, weakly-associated solvents.Part of this work was presented at The 22^nd International Conference on Solution Chemistry in Linz, Austria, July 1991.