Determination of a low isomerisation barrier in a push-pull alkene: Conversion of the (Z)-isomer to the (E) 1-methyl-4-(2′-methylthiovinyl)-pyridinium iodide
The (Z)-1-methyl-4-(2'-methylthiovinyl)-pyridinium iodide 4 was prepared from the corresponding pyridine under conditions where the salt 4 was precipitated before undergoing the irreversible isomerisation to (E) isomer 3. The kinetics of the isomerisation of (Z) isomer 4 to (E) 3 was studied by H-1-NMR. The activation parameters were calculated from the Eyring equation and found to be Delta H-not equal +14 kcal mol(-1) (59 kJ mol(-1)) and Delta S-not equal -26 eu (-108 Jmol(-1) K-1. This is certainly one of the lowest barriers to an irreversible isomerisation which has been determined. (C) 1997 Elsevier Science Ltd.
Determination of the transglycosidation activity of NAD+ glycohydrolases with 4-(2′-alkyl-sulfanyl-vinyl)-pyridine derivatives generating chromophoric NAD+ analogs
The base exchange of nicotinamide with pyridine derivatives 1a-5a, catalyzed by pig brain NAD(+) glycohydrolase and ADP-ribosyl cyclase from Aplysia californica, generated the corresponding NAD(+) analogs 1b-5b. These analogs exhibited a high absorbance band in the visible region. The transglycosidation rate was determined by monitoring the absorbance increase. Among the tested derivatives, (E)-4-[2-(methylsulfanyl)-vinyl]-pyridine 1a was the most suitable substrate for pig brain NAD(+) glycohydrolase while 4-[1,3]-dithiolan-2-ylidenemethyl-pyridine 3a was the most efficient for ADP-ribosyl cyclase from A. californica. (C) 2003 Elsevier Science (USA). All rights reserved.