D. Seebach et al., CALORIMETRIC MEASUREMENTS OF THE COMPLEXATION OF CYCLOSPORINE-A, ASCOMYCIN, FUJIMYCIN, AND RAPAMYCIN WITH LITHIUM-CHLORIDE AND WITH AN IMMUNOPHILIN, Helvetica Chimica Acta, 77(1), 1994, pp. 291-305
Solutions (2 ml) of small linear and cyclic peptides (4-11), of a pept
olide containing nine amino acids and a lactate moiety (12), of the cy
clic undecapeptide cyclosporin A (CS, 1), and of the macrolides ascomy
cin, fujimycin, and rapamycin (13-15) in THF were added to excess LiCl
, LiBr, or LiClO4 (up to 3000 equiv. in 40 ml THF) in a calorimeter (c
alorimetric titration). The enthalpies of interaction measured are in
the range of Delta H = -8 to -37 kcal/mol. A similar experiment was ca
rried out with one of the binding proteins of cyclosporin, the human c
yclophilin A, to give the thermodynamic parameters for the complexatio
n Delta H degrees = -16, Delta G degrees = -10 kcal/mol, and Delta S d
egrees = -20 cal/mol.deg. at 25 degrees which corresponds to an equili
brium constant K = 2.10(7) l/mol, in good agreement with the result of
independent measurements using different methods. NMR Measurements of
the macrolides in (D-8)THF containing LiCl show strong down-field shi
fts of signals of the H-atoms next to C=O and C-OH groups in these mol
ecules.