ROTATIONAL BARRIERS OF CIS TRANS ISOMERIZATION OF PROLINE ANALOGS ANDTHEIR CATALYSIS BY CYCLOPHILIN/

The rotational barriers for cis/trans isomerization of different proli ne analogues have been investigated by dynamic H-1 NMR spectroscopy. T o this end the analogues (S)-azetidine-2-carboxylic acid (Aze), (S)-pi peridine 2-carboxylic acid (Pip), (R)-thiazolidine-4-carboxylic acid ( 4-Thz), (4R)-2-methylthiazolidine-4-carboxylic acid (2Me4-Thz), (R)-th iazolidine-2-carboxylic acid (2-Thz), (S)-oxazolidine-4-carboxylic aci d (4-Oxa), (4S,5R)-5-methyloxazolidine-4-carboxylic acid (5Me4-Oxa), a nd (2S,4R)-4-hydroxypyrrolidine-2-carboxylic acid (Hyp) and several N- alkylated amino acids were incorporated into the sequences Ala-Yaa-(4- )nitroanilide and Ala-Gly-Yaa-Phe-(4-)nitroanilide. NMR line-shape ana lyses of various cis and trans proton signals of these peptides were p erformed at different temperatures, and the rate constants of cis/tran s isomerization were fitted to the Eyring equation. The rotational bar riers of all cyclic proline analogues except hydroxyproline were found to be lower than that of proline by about 10 kJ/mol, whereas all nonc yclic analogues and hydroxyproline showed rotational barriers similar to that observed for proline. In addition, the ability of cytosolic po rcine kidney cyclophilin (Cyp18), a member of the peptidyl prolyl cis/ trans isomerase family, to catalyze cis/trans isomerization of the pep tide bond preceding the proline analogues was investigated. By line-sh ape analyses we proved efficient catalysis by Cyp18 for the analogues Ate, 4-Thz, and 2-Thz.