Single-bond Character Research Articles

Kinetic study on the acylation step of alpha-chymotrypsin-catalyzed hydrolysis of acylimidazole. A model reaction of specific peptide substrate activated by binding to the enzyme.

As a model reaction of the enzyme-bound activated peptide substrates in proteinase-catalyzed hydrolysis, the alpha-chymotrypsin-catalyzed hydrolysis of three acylimidazoles (trans-cinnamoylimidazole, indoleacryloylimidazole, and furylacryloylimidazole) was studied, especially as regards the acylation process. The complicated pH-dependences of the reactions, due mainly to the existence of the protonated forms of these acylimidazoles, were analyzed based on a reaction scheme considering the so-called inactive monomeric enzyme and the dimeric enzyme. The intrinsic reaction parameters for individual species were evaluated. The acylation through the mono-protonated enzyme-substrate complex (ESH) reflects the rate of attach by Ser-195 O gamma on the carbonyl carbon of acylimidazolium, where the formation of the possible tetrahedral intermediate is rate-determining. The acylation via the non-protonated enzyme-substrate complex (ES) is intermediate between those of esters (as regards the single bond character of the cleaved linkage) and of amides (with respect to the proton transfer as a key process of the acylation). This was confirmed by the observation of acceleration of the acylation rate upon addition of external proton donors. Based on these results, the mechanism of acylation in peptide substrate hydrolysis is discussed.

Molecular structure and intermolecular interactions of N1'-methoxycarbonylbiotin methyl ester: a model for carboxybiotin.

The crystal structure of N1'-methoxycarbonylbiotin methyl ester, a model for N1'-carboxybiotin, has been determined. The ureido carbonyl bond has more double bond (keto) character than does the corresponding bound in free biotin, which has single bond (enolate) character. In addition, there is an interesting intermolecular interaction between the ureido carbonyl oxygen and a methyl group. Comparison of the molecular structure and crystal packing with those of free biotin suggests that the coenzyme may have evolved with the incorporation of the ureido moiety because the electronic configuration of this region of the molecule is sensitive to N1' carboxylation. On decarboxylation, the ureido carbonyl bond becomes more polarized (C-O-), thereby facilitating the deprotonation of N1' and increasing its nucleophilicity. As a result, carboxylation can occur readily. On carboxylation, the carbonyl bond is depolarized (C = O), allowing the carboxylated coenzyme to interact with nonpolar groups and carboxylate them. Thus, the carboxylation and decarboxylation of biotin appear to act as a mechanistic switch, turning off and on the polarization of the ureido carbonyl bond as well as modulating the nucleophilicity of N1'.

Spectroscopic investigation on the molecular and electronic structure of [Mo 3S 13] 2−, a discrete binary transition metal sulfur cluster

The only known discrete transition metal sulfur cluster [Mo 3S 13] 2− has been investigated spectroscopically (XPS, UV/VIS, IR and Raman) and by a cyclovoltammetric study. It turns out that the cluster consists of three Mo atoms with the oxidation state +IV and six S 2− 2 ligands. A simplified MO treatment of the metal system in [Mo 3S 13] 2− explains the diamagnetism of the compound, as well as the single bond character of the MoMo bonds. The two absorption bands in the visible region are due to a transition within the Mo 3 metal system and the charge transfer transitions π * v(S 2− 2) → d(Mo).

X-Ray crystal structure of [AsPh4]+[NSF2NSO2F]–: three different types of S–N bonds in the anion

X-Ray crystal structure determination of [AsPh4]+[NSF2NSO2F]– has revealed that the anion [NSF2NSO2F]– has three different types of S–N bonds with triple, double, and single bond character.

Crystal structure and bond character study of 2,4-Dichloroacetanilide by NQR

Two NQR lines were observed in 2,4-dichloroacetanilide at liquid-nitrogen and room temperatures. The Zeeman spectrum of both lines was investigated using a single crystal of the compound. Analysis of the zero-splitting loci show two non-equivalent directions for the electric field gradient principal axes for each of the lines. The observations are in agreement with the C 2 h symmetry for the crystal as reported by Groth. The c axis is inclined at an angle of 13° to the growth axis. The two CCl bonds in the molecule make an angle of 123° with one another. The crystal contains two sets of benzene rings with their planes inclined at angle of 24° to one another. The unit cell contains a minimum of 4 molecules. The ionic, single-bond, and double-bond character for both chlorines in all of the molecules are in the ratio 25:72:3.

Bindungssystem der N-acyl-sulfilimine—II: Ir spektroskopische untersuchung der N-Dihalogenacetyl-sulfilimine

N-Dihaloacetyl-sulphilimines (RR′SNCOQ; R,R′ = aliphatic or aromatic groups, Q = CHCl 2, CHBr 2) can be prepared from dihalogeno acetamide and thioether by a slightly modified Mann-Pope condensation. In the IR spectra of these compounds the ν SN and ν CO band may be found at 820-780 cm −1 and at 1640-1600 cm −1 respectively. The ν SN and ν CO values depend on the kind of halogen atoms of the Q group, but they are independent of the character of the R and R′ groups. On the basis of these facts, the bonding system of N-dihaloacetyl-sulphilimines shows a strong conjugative effect, but this is limited to the SNCO bonding system. As a result of the conjugation the SN and CO bonds are strongly polar, the first one having nearly single bond character, and consequently the SN dπ bond is very weak. The relative lability of sulphilimines containing the N-carboacyl group is due to their highly polar structure. In the case of N-dihaloacetyl-sulphilimines the —I effect of the Q group represents a stability-enhancing factor.

The crystal and molecular structure of the disodium salt of 2,4,6,8-tetrahydroxypyrimido[5,4-d]pyrimidine (2,6,8,10-tetrahydroxyhomopurine)

The crystal structure of the title compound, C6H2N4O4Na2,4H2O, at room temperature, has been determined by three-dimensional methods. There are two formula units in the monoclinic unit cell, a= 3·57, b= 15·36, c= 10·54 Å, β= 98° 30′, with space group P21/c. The disodium salt is formed by the neutralisation of the two β-hydroxy-groups. The molecule is planar but only partly aromatic: the α-oxygen atoms are in the carbonyl form, the carbon–carbon central bond has a high double-bond character, and the other carbon–carbon a high single-bond character; the carbon–nitrogen distances vary from 1·37 to 1·40 Å. Each sodium atom is surrounded by an octahedron of six oxygen atoms, three from water and three from homopurine molecules. The water molecules are arranged in such a manner as to form donor–acceptor hydrogen bonds with the α-oxygen and nitrogen atoms of the molecule.

Some Bromine, Iodine, and Indium Nuclear Quadrupole Interaction Frequencies

Quadrupole interaction frequencies due to Br79 and I127 are reported in a number of crystalline solids. Br79 interaction frequencies in bromobenzene derivatives and I127 frequencies in iodobenzene derivatives are correlated with Hammett's σ. The splitting between interaction frequencies found in s-tribromophenol and related di- and tribromobenzene derivatives is explained tentatively on the basis of hindered rotation. Coupling constants and asymmetry parameters for I127 in iodobenzene derivatives are reported and discussed in terms of the single bond, double bond, and ionic character of the carbon-iodine bond. Owing to the large asymmetry (∼45%) of the I127 quadrupole coupling in HIO3, |Δm| = 2 transition could be observed as well as the two |Δm| = 1 transitions. Eight interaction frequencies were found in InI3, two of which (at 26.93 Mc/sec and 36.33 Mc/sec) are attributed to (±7/2 ↔±5/2) and (±9/2 ↔±7/2) transitions of In115. From these two frequencies a coupling constant and asymmetry parameter of 219.8±0.3 Mc/sec and 32.0±4.0%, respectively, are evaluated for In115 in this compound.

Resonance Energy of the Tetrachlorethylene Molecule

RECENTLY1, it has been shown that the interpretation of a general potential function determining the modes of vibration for the C2Cl4 molecule supports the hypothesis of resonance among several electronic structures, leading for the C—C bond to a single-bond character of about 15 per cent. This result has been deduced partly from the fact that the C—C force constant lies between 8·0 and 8·5 × 105 dynes/cm., as compared with 9·0 × 105 dynes/cm, for ethylene, and partly from general considerations on the signs of the cross-terms. (From a private communication from Prof. C. Manneback, we adopt 9·0 instead of 9·3 as in our previous publications; in this case, the conclusions are even reinforced.) If so, following Pauling's arguments2, the actual energy of formation of the molecule must be greater than the calculated value by a certain amount, called resonance energy of the system. The definite bond diagram used for our calculation corresponds to the classical distribution of valencies and gives It is to be noted that the value Qcalc.= E (C = C) + 4 E (C—Cl) — 2 Lc is not influenced by the not yet definitely established value of Lc. According to Pauling2, the C=C bond energy is 100 kcal./mole. the the C—Cl bond energy is 66·5 kcal./mole and Lc is 124·3 kcal./mole. Hence the heat of formation Qcalc. is 117·4 kcal./mole.

The vibrations and the molecular structure of urea and guanidonium

The vibrations of urea and guanidonium have been calculated for a field containing valence and angle forces. The assumption is made that urea has the symmtery C 2 v and guanidonium C 3 h . It is shown that it is possible to assign every observed frequency of these two substances to definite modes of vibration under these assumptions. The force constants have been evaluated and have been found to be f C-N = 7.1 x 10 5 dynes/cm. for guanidonium, and f C-N = 6.6 x 10 5 dynes/cm. and f C=0 = 9.7 x 10 5 dynes/cm. for urea. These values are compatible with the ’hypothesis that quantum mechanical resonance occurs in both molecules, with the result that the C—N bond in urea has approximately 28 % double-bond character and the C=O linkage a corresponding single-bond character. The guanidonium ion shows complete resonance; each C—N bond has 1/3 double-bond character. Curves have been drawn to illustrate the relation between the valence force constants and the bond character.