Calorimetric Measurements of the Complexation of Cyclosporin A, Ascomycin, Fujimycin, and Rapamycin with Lithium Chloride and with an Immunophilin
✍ Scribed by Dieter Seebach; Hans G. Bossler; Robert Flowers; Edward M. Arnett
- Publisher
- John Wiley and Sons
- Year
- 1994
- Tongue
- German
- Weight
- 894 KB
- Volume
- 77
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
Solutions (2 ml) of small linear and cyclic peptides (411), of a peptolide containing nine amino acids and a lactate moiety (12), of the cyclic undecapeptide cyclosporin A (CS, l), and of the macrolides ascomycin, fujimycin, and rapamycin (1S-15) in THF were added to excess LiCI, LiBr, or LiCIO, (up to 3000 equiv. in 40 ml THF) in a calorimeter (calorimetric titration). The enthalpies of interaction measured are in the range of AH = -8 to -37 kcal/mol. A similar experiment was carried out with one of the binding proteins of cyclosporin, the human cyclophilin A, to give the thermodynamic parameters for the complexation AHo = -16, AG" = -10 kcal/mol, and AS' = -20 cal/mol .deg. at 25' which corresponds to an equilibrium constant K = 2. lo7 l/mol, in good agreement with the result of independent measurements using different methods. NMR Measurements of the macrolides in (D8)THF containing LiCl show strong down-field shifts of signals of the H-atoms next to C=O and C-OH groups in these molecules.
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The arrangement with a dihedral angle of 0" (8 in Formula A of Scheme 2 ) would be stereoelectronically favored. The barrier to rotation around the OC-N bond in DMF is raised appreciably in the presence of LiC1, as determined by NMR spectroscopy .