Showing papers on "Intramolecular force published in 1972"

Secondary Bonding to Nonmetallic Elements

Abstract: Publisher Summary A number of recent crystal structure determinations on compounds of the nonmetals have discovered intramolecular distances that are much longer than normal bonds, and intermolecular distances that are much shorter than van der Waals distances. In this chapter, these interactions are examined and a qualitative explanation is attempted. It will become clear that in most of them an approximately linear arrangement is found, Y-A—X where Y-A is a normal bond and A—X is a short intermolecular distance. It is with these approximately linear interactions that we are particularly concerned, and it will be our contention that they are the result of directed forces and that their behavior is sufficiently regular and understandable for the name secondary bond to be appropriate. The only conclusive method of establishing the presence of secondary interactions is by crystal structure determinations. An intermolecular interaction can be recognized as being significant by being shorter than the expected intermolecular (van der Waals) distance, but if it is the result of directed forces— that is, bonds rather than electrostatic or nondirectional van der Waals forces.

Molecular Conformation and Packing of Poly(vinylidene fluoride). Stability of Three Crystalline Forms and the Effect of High Pressure

Abstract: The formation of three crystalline forms of poly(vinylidene fluoride) was studied in detail by using an apparatus for heat treatments under high pressure up to 5000 atm. Form II consisting of TGTG-type molecular chains was the most stable under atmospheric pressure, and form I, composed of planar zigzag-type chains, was formed under special conditions, such as tension, high pressure, etc. Form III may be an intermediate modification between I and II. Based upon these experimental facts, as well as the potential energy calculations of the intra- and intermolecular interactions in the crystal lattices due to the van der Waals and electrostatic forces, the relation between the conditions of formation of the three forms and their structures was examined. The planar zigzag-type conformation (in forms I and III) is considered to be less stable than the TGTG type (form II) because of the steric hindrances and electrostatic dipole interactions. In spite of the difference in the intramolecular potential energy between these two molecular conformations, the stabilities of the three crystalline forms are not so very different because of the more favorable intermolecular interaction in form I.

Structure and efficiency in intramolecular and enzymic catalysis. Catalysis of amide hydrolysis by the carboxy-group of substituted maleamic acids

Abstract: The efficiency of intramolecular catalysis of amide hydrolysis by the carboxy-group of a series of substituted N-methylmaleamic acids is remarkably sensitive to the pattern of substitution on the carbon–carbon double bond. A single alkyl group increases the rate of hydrolysis by a factor which increases with its size. A second alkyl substituent has a disproportionately larger effect, which is sharply reduced when the two groups are joined together in a ring. The rates of hydrolysis of a series of dialkyl-N-methylmaleamic acids range over more than ten powers of ten, and the ‘effective concentration’ of the carboxy-group of the most reactive compound studied is greater than 1010M. This amide, dimethyl-N-n-propylmaleamic acid, is converted into the more stable dimethylmaleic anhydride with a half-life of less than 1s at 39 °C below pH 3. The mechanism of catalysis, and the factors responsible for this extremely high reactivity, are discussed.

The crystal and molecular structure of trehalose dihydrate

Abstract: The crystal structure of trehalose dihydrate has been solved by direct methods and refined to an R index of 0-057 using anisotropic refinement. The space group is P212121 and four formula units of C12HzzOz~.2HzO are contained in the unit cell of dimensions a= 17.90, b= 12.21 and c= 7.586 A. The two glucopyranose residues in the trehalose molecule have the chair 4C1 form, and are bonded by the a 1 --+ 1 glycosidic link in approximate twofold symmetry. Departures from symmetry are found in torsion angles about the e 1 -+ 1 link and in conformations of the primary alcohol groups O(6)H and O(6')H. The C-O bond lengths show systematic trends similar to those in other e-pyranose sugars and at the same time show some characteristic features related to the e 1 -~1 link of the two glucose residues. There are two indirect intramolecular hydrogen bonds incorporating water molecules. Neither the ring oxygens nor the glycosidic linkage oxygen accept hydrogen bonds. The molecular packing in the crystal is mainly determined by the hydrogen bonds.

Classical Dynamics of the Reaction of Fluorine Atoms with Hydrogen Molecules. III. The Hot‐Atom Reactions of 18F with HD

Abstract: Excitation functions for the various product channels in reactions of hot 18F atoms with ground‐state HD are calculated using the Monte Carlo quasiclassical trajectory technique and an optimized semi‐empirical potential energy surface. The production of H18F and D18F and dissociation into 18F+H+D are reported as a function of the center‐of‐mass collision energy over the range 0.1–65.0 eV. The calculated excitation functions for H18F and D18F cross at ∼ 7 eV indicating an inversion in the intramolecular isotope effect with increasing collision energy. Features of these excitation functions and the calculated product energy distributions are discussed in terms of simple mechanistic models. The trajectory results, on the average, correlate well with the predictions of the spectator stripping model from epithermal collision energies up to the limiting energy where this model would lead only to dissociation. The high‐energy tail of the abstraction excitation functions, however, is shown to be attributable to a...

Precision neutron diffraction structure determination of protein and nucleic acid components. VIII: the crystal and molecular structure of the β-form of the amino acidl-glutamic acid

Abstract: A neutron diffraction study of the β-form of L-glutamic acid, C5H9NO4, has been carried out. The structure is orthorhombic: space groupP212121,a= 5·159(5),b= 17·30(2),c= 6·948(7) A andz = 4. Least-squares refinements based on 803 reflexions led to a final conventionalR value of 0·026, and bond lengths involving hydrogen atoms have been determined with an average precision of 0·004 A. The molecule is in the zwitterion form, and no intramolecular hydrogen bonds have been found. The hydrogen atom involved in a strong hydrogen bond between two carboxyl groups in adjacent molecules (0 ... 0 distance 2·519(2) A) is covalently bonded to the carboxyl group belonging to the side chain of the amino acid. This side chain is buckled with Cδ gauche to Cα with respect to the C β —C γ bond. The bond angles involving carbon atoms in the side chain are accordingly strained.

Experimental study of the influence of specific intramolecular interactions on the conformation of model molecules. (Peptides and oligopeptides).

Abstract: Several experimental methods have been used for studying the conformations taken by dipeptidic molecules when dissolved, at a very low concentration. in an inactive solvent such as carbon tetrachloride. Through systematic investigations performed by infrared and nmr spectroscopy. it has been possible to obtain accurate data about these conformations and to compare with the results of various theoretical treatments. Several typical examples are reported which concern aminoacid derivatives containing a paraffinic. an aromatic or a polar side substituent. It appears that the conformations taken by such molecules are mainly determined by specific intramolecular forces. These results agree only with the conclusions of the theoretical calculations in which such interactions have been considered.

Electronic Structure of the Silver (1+)—Ethylene Complex

Abstract: The electronic structure of the silver (1+)—ethylene complex has been calculated using the nonempirical self‐consistent field theory in an extended Gaussian orbital basis set and in a top‐complex geometry corresponding to Dewar's original two‐way donor‐acceptor model. A purely electronic binding energy of just under 1.0 eV is predicted for this complex in the gas phase. For comparison purposes an identical calculation was carried out on ethylene in the presence of a unit point charge replacing the Ag+ ion. Based on an examination of core electron binding energies and basis orbital populations it is concluded that (a) the point charge model is capable of describing most of the ethylene intramolecular electron rearrangement in the complex, and (b) the metal olefin σ bond is more important than the π bond. The energy ordering of the excited states of the complex is found to be dominated by the upper orbital energy. A number of considerations and results mitigate against the alternative side complex as the most stable geometric configuration for AgC2H4+.

Evidence for the Presence of Hydrogen-Bonded Secondary Structure in Angiotensin II in Aqueous Solution

Abstract: Automated tritium-hydrogen exchange measurements have been made on the linear octapeptide Val5-angiotensin II amide. All six amide hydrogens of the peptide backbone are observable, and are resolved into three classes according to their exchange rates. The rate of exchange of the slowest class, t1/2 of 300 min at 0°C (pH 2.5), is compared with that of hydrogens that exchange abnormally slowly in other peptides. It is concluded that these slow hydrogens in angiotensin II are involved in secondary structure with either one or both forming stable, intramolecular hydrogen bonds. This finding demonstrates that linear peptides may have hydrogen-bonded conformations in aqueous solutions. Analysis of the pH dependence of the rate of exchange indicates that one peptide amide hydrogen, namely that of the Asn1-Arg2 peptide bond, is not involved in hydrogen bonding and is freely accessible to the solvent. Thus, the finding of internal hydrogen bonding, together with the assignment of the environment of one peptide bond, places major constraints on the number of allowable conformations of this linear polypeptide hormone.

Mechanism of the 2,3-diphosphoglycerate-dependent phosphoglycerate mutase from rabbit muscle.

Abstract: 1. The properties and kinetics of the 2,3-diphosphoglycerate-dependent phosphoglycerate mutases are discussed. There are at least three possible mechanisms for the reaction: (i) a phosphoenzyme (Ping Pong) mechanism; (ii) an intermolecular transfer of phosphate from 2,3-diphosphoglycerate to the substrates (sequential mechanism); (iii) an intramolecular transfer of phosphate. It is concluded that these mechanisms cannot be distinguished by conventional kinetic measurements. 2. The fluxes for the different mechanisms are calculated and it is shown that it should be possible to distinguish between the mechanisms by appropriate induced-transport tests and by comparing the fluxes of (32)P- and (14)C-labelled substrates at chemical equilibrium. 3. With (14)C-labelled substrates no induced transport was found over a wide concentration range, and with (32)P-labelled substrates co-transport occurred that was independent of concentration over a twofold range. (14)C-labelled substrates exchange at twice the rate of (32)P-labelled substrates at chemical equilibrium. The results were completely in accord with a phosphoenzyme mechanism and indicated a rate constant for the isomerization of the phosphoenzyme of not less than 4x10(6)s(-1). The intramolecular transfer of phosphate (and intermolecular transfer between two or more molecules of substrate) were completely excluded. The intermolecular transfer of phosphate from 2,3-diphosphoglycerate would have been compatible with the results only if the K(m) for 2-phosphoglycerate had been over 7.5-fold smaller than the observed value and if an isomerization of the enzyme-2,3-diphosphoglycerate complex had been the major rate-limiting step in the reaction. 4. The very rapid isomerization of the phosphoenzyme that the experiments demonstrate suggests a mechanism that does not involve a formal isomerization. According to this new scheme the enzyme is closely related mechanistically and perhaps evolutionarily to a 2,3-diphosphoglycerate diphosphatase.

Intramolecular hydrogen bonding in hydroxy-keto-steroids

Abstract: The i.r. spectra of hydroxy-keto-steroids with an equatorial-hydroxy-group α to the ketone show a band due to hydrogen bonding (OH ⋯ O type); those with an axial hydroxy-group α or β to the ketone exhibit two absorption bands, due to a free and a bonded hydroxy-group (OH ⋯π type). The mode of multiple hydrogen bonding in epimeric pairs of monohydroxy-diketo-steroids and dihydroxy-monoketo-steroids is discussed. Hydrogen bonding in dihydroxy-keto-steroids is more extensive than in monohydroxy-keto-steroids.