The rate enhancement for prolyl cis-to-trans isomerization of cyclic CPFC peptide is caused by an increase in the vibrational entropy of the transition state.
The conformational preferences and prolyl cis-trans isomerization of oxidized and reduced Ac-Cys-Pro-Phe-Cys-NH2 (CPFC peptides) have been carried out using the ab initio HF/6-31+G(d) and hybrid density functional B3LYP/6-311++G(d,p) levels of theory. The most preferred conformations of oxidized and reduced CPFC peptides with the trans prolyl peptide bond have a type-I beta-turn for the Pro-Phe sequence in common. In particular, the transition states for both forms are stabilized by the intramolecular hydrogen bonds between the prolyl nitrogen and the N-H group of the Phe3 residue. The rotational barrier DeltaGct to the cis-to-trans isomerization for the oxidized CPFC peptide is calculated to be 19.37 kcal/mol at the B3LYP/6-311++G(d,p)//HF/6-31+G(d) level of theory, which is lower by 0.88 kcal/mol than that of the reduced CPFC peptide. This may indicate that the rate constant kc-->t of the prolyl cis-to-trans isomerization for the oxidized form is about 4 times larger than that of the reduced form, which is reasonably consistent with the value deduced from NMR experiments. In particular, the increase in vibrational entropy for the transition state of the oxidized form over that of the reduced form contributes to enhance the rate constant for the prolyl cis-to-trans isomerization of the oxidized form.
Lee
Joo Yun
JY
Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Republic of Korea.
Kang
Young Kee
YK
D000595:Amino Acid Sequence; D003198:Computer Simulation; D007536:Isomerism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000737:chemistry*; D010456:Peptides, Cyclic / Q000737:chemistry*; D011392:Proline / Q000737:chemistry*; D011485:Protein Binding; D011789:Quantum Theory; D013816:Thermodynamics
C529693:Ac-Cys-Pro-Phe-Cys-NH2; D009842:Oligopeptides; D010456:Peptides, Cyclic; D011392:Proline