Investigation of the thermal isomerization of 1α,25-dihydroxycholecalciferol by nuclear magnetic resonance spectroscopy

This investigation of the kinetics and thermodynamics of the thermal isomerization reaction of 1α,25-dihydroxycholecalciferol, the physiologically active metabolite of vitamin D₃, is based on the simultaneous determinations of 1α,25-(OH)₂D₃ and its previtamin analog by nuclear magnetic resonance spectroscopy which distinguishes these compounds from possible impurities. The kinetics at different temperatures are used to obtain the activation parameters for the sigmatropic [1,7] thermal interconversion process which is shown to be compatible with a reaction that is unimolecular and concerted. The nature of the transition state of the activated complex, the reaction energetics, and the relative stabilities of 1α,25-(OH)₂D₃ and vitamin D₃ are discussed.     

Carbon-13 nuclear magnetic resonance spectra. VIII—18α- and 18β-glycyrrhetic acid derivatives

The ¹³C NMR data of six pairs of 18α/18β-glycyrrhetic acid derivatives are presented. It is shown that the configuration at C-18 can easily be recognized by inspecting the chemical shifts of several characteristic carbons, e.g. C-12, C-13, C-18 and C-28. The shifts of these carbons originated by the change of the D/E ring junction proved to be largely independent of the substitution at C-3 and C-20.     

NMR Studies of ortho and meta- fluorocinnamate-α-chymotrypsin complexes

Fluorine magnetic resonance spectroscopy has been used to examine the complexes formed between o- and m-fluorocinnamate ions and the enzyme α-chymotrypsin. Study of the frequency dependence of fluorine spin-lattice and spin-spin relaxation rates suggests that, like the p-isomer, both the o- and m-fluorocinnamates bind equally well to monomeric, dimeric and trimeric forms of the protein. Dipole-dipole interactions with protons of the enzyme play an important role in relaxing the protein-bound fluorine nucleus. Various deuterated forms of the ortho isomer were used to obtain evidence suggesting that the conformation of this molecule bound to the enzyme is similar if not identical to the conformation found in free solution. NMR methods were used to estimate the rates of dissociation of the complexes and the chemical shifts of the bound fluorine atoms were determined to be substantially downfield of their positions in the absence of enzyme.     

13C nuclear magnetic resonance spectra. XV—the γ-anti substituent effect and its dependence on substitution at the α- and γ-carbon atom

It is shown that the γ_α effects of hetero substituents on the ¹³C chemical shift (γ_α SCS) are enlarged by +2 to +5 ppm by substituting the α-hydrogen atom by any group or atom (e.g. CH₃, OR, F, Cl, Br). The same is encountered when the axial γ-hydrogen is replaced by CH₃, OH or F. If, however, the substituting atom at the γ_α-carbon atom is a higher-row halogen (Cl or Br), diamagnetic γ_α SCS for this signal are observed which may even exceed those for unsubstituted γ_α-carbon atoms. The removal of a 1,3-diaxial hydrogen-hydrogen interaction and the existence of a still unspecified ‘heavy halogen effect’–both diamagnetic contributions to the γ_γ SCS of hetero substituents–are responsible for these findings. Methyl groups do not behave like hetero substituents with respect to the γ_α SCS.     

Anionic-nonionic surfactant interaction by nuclear magnetic resonance

Summary The interaction between anionic (sodium dodecyl benzene sulfonate) surfactant and nonionic (Tri and Tetra propylene glycol monomethyl ether) surfactant was studied using nuclear magnetic resonance measurement. It was observed that the addition of sodium dodecyl benzene sulfonate to the solution of nonionic surfactant (Tri and Tetra propylene glycol mono methyl ether) caused an upfield shift of the central protons of the nonionic surfactants. The aromatic protons of sodium-dodecyl benzene sulfonate undergo a very small, almost negligible, downfield shift. The changes in the chemical shift values and the integration values of the polypropylene protons and benzene protons was interpreted in terms of mixed micelle formation with the simultaneous presence of highly fluid mixed micelles of varying compositions.With 2 figures and 2 tables     

Study of molecular motion in polysilastyrene by pulsed 1H nuclear magnetic resonance

Pulsed NMR spectra of protons in polysilastyrene, $ \rlap{--} [{\rm Si(CH}_{\rm 3} {\rm )}_{\rm 2} {\rm  Si(CH}_{\rm 3} )({\rm C}_6 {\rm H}_5 )\rlap{--} ]_n $, with n ≈ 60, have been measured in the temperature range 80–450 K. The linewidth is constant at 7.4 G up to 200 K and narrows considerably above 250 K to a constant value of 0.3 G above 360 K. The motion responsible for this effect has an activation energy of 43.7 kJ/mol and is identified with the large-scale motion occurring in the vicinity of the glass transition temperature. The spin-lattice relaxation time T₁ was measured by the π-t-½π pulse sequence as a function of temperature. Two motional minima in T₁ were observed. The low-temperature motion has an activation energy of 3.7 kJ/mol and is identified with methyl group reorientation. The high-temperature motion has an activation energy of 29.1 kJ/mol and might be due to segmental motion.     

Studies of the γ ← α transition in syndiotactic polystyrene

In this work we compare calorimetric and X-ray diffraction experiments realized on annealed sPS in helical γ forms resulting by different treatements: From clathrate δ form and from interaction of amorphous sample with acetone. The experimental results show that the γ form obtained by acetone converts into the more ordered final α” form modification; while the γ form, obtained by thermal treatments of δ form, transforms into the poorly ordered final α' form.     

A molecular mechanics analysis of molecular recognition by cyclodextrin mimics of α-chymotrypsin

The method of molecular mechanics is used to investigate the structural and electrostatic features of molecular recognition by β-cyclodextrin and capped β-cyclodextrin models of α-chymotrypsin. Since capped β-cyclodextrin has been shown to be the more effective biomimetic catalyst, these features of molecular recognition can be interpreted in terms of the relationship between molecular structure and catalytic function. Calculations in vacuo show that the addition of an N-methylformamide “cap” substituent to each glucose unit appears to change the relative orientation of some glucose fragments from that found in the X-ray structure of the β-cyclodextrin macrocycle. These results indicate that certain structural components of molecular recognition, such as the orientation of the secondary hydroxyls and the related orientation of the caps, may be implicated in the catalysis. In addition, the electrostatic component of molecular recognition was investigated by the analysis of molecular electrostatic potential maps calculated in planes parallel to the average plane of the glycosidic oxygen atoms. The results indicate that the addition of the caps to the β-cyclodextrin macrocycle subtly alters the pattern of the maps in each plane. However, the general qualitative features of electrostatic recognition by β-cyclodextrin and capped β-cyclodextrin are similar.     

Relaxation-allowed nuclear magnetic resonance transitions by interference between the quadrupolar coupling and the paramagnetic interaction

Of the various ways in which nuclear spin systems can relax to their ground states, the processes involving an interference between different relaxation mechanisms, such as dipole-dipole coupling and chemical shift anisotropy, have become of great interest lately. The authors show here that the interference between the quadrupolar coupling and the paramagnetic interaction (cross-correlated relaxation) gives rise to nuclear spin transitions that would remain forbidden otherwise. In addition, frequency shifts arise. These would be reminiscent of residual anisotropic interactions when there are none. While interesting from a fundamental point of view, these processes may become relevant in magnetic resonance imaging experiments which involve quadrupolar spins, such as (23)Na, in the presence of contrast agents. Geometrical constraints in paramagnetic molecule structures may likewise be derived from these interference effects.     

Carbon-13 magnetic resonance: γ-anti substituent effects

γ-anti Substituent effects of the first row elements (CH₃, NH₂, OH, F) and the higher-row halogens on ¹³C chemical shifts are shown to have separate linear relationships with element electronegativity. The halogens generally show an increasing upfield shift with increasing electronegativity, but the response of the effect of the first-row elements to changing electronegativity is dependent upon the substitution pattern of the atoms involved. In molecules with the substituent at a bridgehead, increasing electronegativity causes increasing downfield shift of the γ-anti carbon, whereas in other systems which have the substituent at a secondary carbon, increasing electronegativity causes an increasing upfield shift. Examples are cited to show that the γ-anti effect of a hydroxyl group is more shielding (by about 2 ppm) when a 1,3-diaxial interaction between an α and a γ hydrogen is present, than when this interaction is removed by replacement of either of the hydrogens. This correlation is taken as evidence of this hydrogen-hydrogen interaction as one pathway for the transmission of the γ-anti substituent effects of first-row elements.     

Synthesis and nuclear magnetic resonance spectroscopic studies of 1‐arylpyrroles

A series of m‐ and p‐substituted 1‐phenyl, 1‐benzyl, 1‐benzoyl, and 1‐(2‐phenylethyl)pyrroles was prepared and their ¹H and ¹³C nmr spectroscopic characteristics were examined. In general, good correlations were observed between the chemical shift values of the β? H and the β? C of pyrroles [except 1‐(2‐phenylethyl)pyrroles] and the Hammettt σ. The observation may be explained in terms of the electronic effects of the substituents which are transmitted through bonds and through space by interaction of the p orbitals between β? Cs of the pyrrole ring and m‐ and p? Cs of the phenyl ring. Substituent constants of 1‐pyrrolyl, 1‐pyrrolylmethyl, and 1‐pyrroloyl groups for the ¹H and ¹³C chemical shifts of phenyl ring are also presented.     

Studies on kinetics of complexation of lanthanide α-hydroxycarboxylates with 1,10-phenanthroline

Kinetics of the coordination reaction of lanthanide (La^III, Eu^III) α-hydroxycarboxylates [LnL₃(H₂O)₂] with 1,10-phenanthroline (phen) in methanol-water (v/v, 3:2) were studied at 25°C by calorimetric titration. A one-step reaction process in accordance with the rate law has been suggested. The reaction is found to be first order for both lanthanide α-hydroxycarboxylates and phen. We have evaluated rate constants of the reactions. It is found that a linear free energy relationship exists between the stability constants of the lanthanide-α-hydroxycarboxylate-phen ternary complex and the rate constants. It is also found that a linear free energy relationship exists between the rate constants of La-hydroxycarboxylate with phen and the acid strength of α-hydroxy-acid as primary ligand, but the linear free energy relationship does not exist in the Eu-α-hydroxycarboxylate-phen ternary complex. The influence of other factors upon the reaction rate constants was also discussed.