Huilan Chen

Synthesis and crystal structure of pyridine[N,N′-ethylenebis(salicylideneiminato)](isobutyl)cobalt(III)

Abstract The title complex pyridine[N,N′-ethylenebis(salicylideneiminato)](iso-butyl)-cobalt(III) [C i 4 H 9 Co(salen)(py)] has been synthesized and characterized and its crystal structure has been determined by X-ray diffraction methods. The crystal consists of discrete molecules with distorted octahedral stereochemistry around the cobalt(III) ion. The tetradentate “salen” ligand is in the equatorial plane and C i 4 H 9 and pyridine occupy the two axial positions. The lengths of axial CoC and CoN bonds are 2.003(4) and 2.184(3) A, respectively.

Thermolysis of β-cyclodextrin/alkylcobaloxime inclusion complexes in the solid state

Abstract The solid state stability of a series of β-cyclodextrin/alkylcobaloxime inclusion complexes has been investigated by differential scanning calorimetery (DSC) and thermogravimetric analysis (TGA). Four distinct transitions in the TGA scans, corresponding to dehydration (Tc1), dealkylation (Tc2), breakdown of equatorial oxime ligands (Tc3), and decomposition of β-CD (Tc4) were generally seen. Three distinct transitions during DSC, corresponding to Tc1 (endotherm), Tc2 (exotherm) and Tc4 (endotherm), were observed in most cases. In addition, the temperatures of dealkylation were higher in β-cyclodextrin/alkylcobaloxime complexes than those of the corresponding alkylcobaloximes. The Tc2 changes were related to the substitution groups on the alkylcobaloximes and paralleled the stability constants of these inclusion complexes.

Study of the thermal decomposition, axial-base equilibrium, and CoC bond dissociation energy of 2′, 5′-Dideoxyadenosylcobalamin in ethylene glycol

Abstract The anaerobic thermal decomposition for 2′, 5′-dideoxyadenosylcobalamin (2′-dAdoCbl) with an added 2,2,6,6-tetramethylpipyridinyl-1-oxy(TEMPO) radical trap in ethylene glycol has been studied. The temperature dependence of the rate for 2′-dAdoCbl CoC homolysis was obtained from 85–115°C, ΔH on ≠ = 36.8 ± 0.9 kcal·d mol −1 , and ΔS on ≠ = 22.1+- 1.5 cal · mol −1 · K −1 . The temperature dependence of the axial-base equilibrium of 2′-dAdoCbl in ethylene glycol was independently determined from 10–75°C, ΔH = −6.9 ± 0.2 kcal·mol −1 , and ΔS = −18.2 ± 0.6 cal·mol −1 ·K −1 . The CoC bond dissociation energy was estimated at 32.4 ± 1.5 kcal·mol −1 for 2′-dAdoCbl in ethylene glycol if one assumes a cage efficiency factor Fc = 1. Comparing the CoC BDE of 2′-dAdoCbl with that for 5′-deoxyadenosylcobalamin (30.0 ± 2 kcal·mol −1 _, the cage chemistry effect probably can account for this ∼2 kcal·mol −1 .

The first study on 2′,5′-dideoxynucleosidyl radicals formed from the anaerobic photolysis of 2′,5′-dideoxynucleosidylcobalamins by the ESR spin-trapping technique

Abstract Anaerobic photolysis of coenzyme B 12 (Ia) and its four analogues of 2′,5′-dideoxynucleosidylcobalamins (Ib → Ie) containing 2′,5′-dideoxycytidyl, 2′,5′-dideoxyuridinyl, 2′,5′-dideoxyadenosyl, and 5′-deoxythymidyl groups (R) as axial ligands in the presence of t-nitrosobutane (tNB) in H 2 O, DMSO, and DMF affords the spin-trapped nitroride radicals Bu t N(Ȯ)R (IIa → IIe) and B 12r . This result reveals anaerobic Co III C homolysis for Ib → Ie, as for coenzyme B 12 . The spectra of all Bu t N(Ȯ)R radicals show 1:1:1:1 quartet and line width alternations were observed due to two unsymmetrical conformers caused by a chiral β-carbon atoms. Magnetic nuclei on the α and β-carbon atoms produce informative secondary and third structures of nitroxide radicals. The effects of the solvent and temperature on ESR spectra and parameters were rationalized.

Studies on the Formation of β-Cyclodextrin/Alkylcobaloxime Inclusion Complexes by Using NMR Methods

Abstract It has been established that alkylcobaloxime(R=i-C4H9, n-C4H9, n-CsH11, c-C6H11, PhCH2) and β-cyclodextrin form a kind of 1: 1 inclusion complexes in aqueous solution by 1H NMR spectroscopy. Ka and −ΔGθ were calculated according to chemical shift changes of different probe protons. The stability constant(Ka) has been found increasing in the order: PhCH2 > n-C5H11 > c-C6H11 > i-C4H9 > n-C4H9.

THE INTERACTION OF POLYSACCHARIDES WITH A SPECTROSCOPIC PROBE: THE ANION EFFECT ON THE BINDING SITE OF HEPARIN

Heparin (Hep) has a variety of biological activities, most of them due to Heps high anionic groups. The average binding number (n) is proposed to express the density of anions group (binding site) on Hep. To gain insight into the binding site of Hep under the interference of anions, we have developed a method that makes use of isosbestic points in the reaction system in which two reagents compete for the same dye ion, and one complex is colorless. It is also necessary to determine the molar absorptivities of the various species concerned.

The assignment of Co-C bond stretching vibrational frequency of CH3Co(DH)2H2O in IR and Raman spectra

Abstract In order to aid assignment of Co-C bond stretching vibrational frequency of CH3Co(DH)2H2O (DH=dimethylgIyoximato monanion) in IR and Raman spectra, its isotopic substitution CD3Co(DH)2H2O has been synthesized and normal coordinate analyses on the two complex have been made. The bands were assigned in terms of potential energy distribution. The results provide definitive band assignment of the Co-C bond and Co-N bond stretching modes which are coupling at 511 cm−1.

The Coenzyme B12 analogs — ribosylcobalamins: Synthesis, characterization, and photolysis studies☆

Two analogs of Coenzyme B12, i.e., 1-methyl-5-deoxy-β-D(−)ribofuranos-5-ylcobalamin (RibCbl) and 1-methyl-5-deoxy-2,3-isopropylidene-β-D(−)-ribofuranos-5-yl-cobalamin(RibCbl∗), were synthesized. They were characterized by UV-vis and 2D1 HNMR spectroscopies. Furthermore, aerobic and anaerobic photolysis of the two compounds have been studied. Kinetic data of anaerobic photoinduced homolysis for two ribosylcobalamins(RibCbl∗, RibCbl) and 5′-deoxyadenosylcobalamin(AdoCbl) have been determined. The half-times under the same experimental conditions are: RibCbl∗, t12 = 488 min; RibCbl, t12 = 122 min; and AdoCbl, t12 = 3 min, respectively. Therefore, both of ribosylcobalamins are less photosensitive than Coenzyme B12. These results are discussed in terms of the in-cage chemical process as well as steric and electronic effects of the axial ligand on the CoC bond homolysis.

UV-visible spectra and thermodynamic studies of 2′,5′-dideoxynucleosidylcobalamins

The electronic spectra and pKa values for the protonation and displacement of 5,6-dimethylbenzimidazole base have been measured for four coenzyme B12 analogues with axial 2′,5′-dideoxynucleosidyl ligands. The equilibria between the 6-coordinate base-on species and 5-coordinate unprotonated base-off species have been studied, and values of ΔH and ΔS determined as ∼ 7 kcal mol−1 and ∼ 18 cal K−1 mol−1 in neutral and ∼ 4 kcal mol−1 and ∼ 10 cal K −1 mol−1 at pH 4.3, respectively. In equilibrium mixture the main species of the four analogues were found to be 6-coordinate base-on form at pH 6.9 ( ∼ 95%). With pH decreased, the equilibria were remarkedly shifted in favor of the 5-coordinate base-off form. Comparisons of the four analogues with coenzyme B12 and other organocobalamins were made.

Two Dimensional 1HNMR Studies on 2′5′-Dideoxyadenosylcobalamin

Abstract The 1HNMR spectrum of 2′,5′-dideoxyadenosylcobalamin, a Coenzyme B12(5′-deoxyadenosylcobalamin) analogue, has been assigned by 2D COSY. Its proton coupling constants have also been measured by J-resolved experiment. The comparison between the analogue and Coenzyme B12 was made.