title: Negative kinetic temperature effect on the hydride transfer from NADH analogue BNAH to the radical cation of N-benzylphenothiazine in acetonitrile.
abstract: The reaction rates of 1-(p-substituted benzyl)-1,4-dihydronicotinamide (G-BNAH) with N-benzylphenothiazine radical cation (PTZ(*+)) in acetonitrile were determined. The results show that the reaction rates (k(obs)) decreased from 2.80 x 10(7) to 2.16 x 10(7) M(-1) s(-1) for G = H as the reaction temperature increased from 298 to 318 K. The activation enthalpies of the reactions were estimated according to Eyring equation to give negative values (-3.4 to -2.9 kcal/mol). Investigation of the reaction intermediate shows that the charge-transfer complex (CT-complex) between G-BNAH and PTZ(*+) was formed in front of the hydride transfer from G-BNAH to PTZ(*+). The formation enthalpy of the CT-complex was estimated by using the Benesi-Hildebrand equation to give the values from -6.4 to -6.0 kcal/mol when the substituent G in G-BNAH changes from CH(3)O to Br. Detailed thermodynamic analyses on each elementary step in the possible reaction pathways suggest that the hydride transfer from G-BNAH to PTZ(*+) occurs by a concerted hydride transfer via a CT-complex. The effective charge distribution on the pyridine ring in G-BNAH at the various stages-the reactant G-BNAH, the charge-transfer complex, the transition-state, and the product G-BNA(+)-was estimated by using the method of Hammett-type linear free energy analysis, and the results show that the pyridine ring carries relative effective positive charges of 0.35 in the CT-complex and 0.45 in the transition state, respectively, which indicates that the concerted hydride transfer from G-BNAH to PTZ(*+) was practically performed by the initial charge (-0.35) transfer from G-BNAH to PTZ(*+) and then followed by the transfer of hydrogen atom with partial negative charge (-0.65). It is evident that the present work would be helpful in understanding the nature of the negative temperature effect, especially on the reaction of NADH coenzyme with the drug phenothiazine in vivo.
authors.affiliation: Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China. xqzhu@nankai.edu.cn
authors.forename: Xiao-Qing
authors.initials: XQ
authors.lastname: Zhu
authors.forename: Jian-Yu
authors.initials: JY
authors.lastname: Zhang
authors.forename: Jin-Pei
authors.initials: JP
authors.lastname: Cheng
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D002412:Cations; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D009243:NAD / Q000031:analogs & derivatives* / Q000737:chemistry; D010640:Phenothiazines / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry; D013696:Temperature; D013816:Thermodynamics
chemical_list: C445701:1-benzyl-1,4-dihydronicotinamide; D000097:Acetonitriles; D002412:Cations; D005609:Free Radicals; D010640:Phenothiazines; D011725:Pyridines; D009243:NAD; C032159:acetonitrile