Impact of azaproline on amide cis-trans isomerism: conformational analyses and NMR studies of model peptides including TRH analogues.
The beta-turn is a well-studied motif in both proteins and peptides. Four residues, making almost a complete 180 degree-turn in the direction of the peptide chain, define the beta-turn. Several types of the beta-turn are defined according to Phi and Psi torsional angles of the backbone for residues i + 1 and i + 2. One special type of beta-turn, the type VI-turn, usually contains a proline with a cis-amide bond at residue i + 2. In an aza-amino acid, the alpha-carbon of the amino acid is changed to nitrogen. Peptides containing azaproline (azPro) have been shown to prefer the type VI beta-turn both in crystals and in organic solvents by NMR studies. MC/MD simulations using the GB/SA solvation model for water explored the conformational preferences of azPro-containing peptides in aqueous systems. An increase in the conformational preference for the cis-amide conformer of azPro was clearly seen, but the increased stability was relatively minor with respect to the trans-conformer as compared to previous suggestions. To test the validity of the calculations in view of the experimental data from crystal structures and NMR in organic solvents, [azPro(3)]-TRH and [Phe(2), azPro(3)]-TRH were synthesized, and their conformational preferences were determined by NMR in polar solvents as well as the impact of the azPro substitution on their biological activities.
Zhang
Wei-Jun
WJ
Department of Biochemistry, Washington University, St. Louis, Missouri 63110, USA.
Berglund
Anders
A
Kao
Jeff L-F
JL
Couty
Jean-Pierre
JP
Gershengorn
Marvin C
MC
Marshall
Garland R
GR
D001372:Aza Compounds / Q000737:chemistry*; D007536:Isomerism; D009010:Monte Carlo Method; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods; D009842:Oligopeptides / Q000138:chemical synthesis / Q000737:chemistry; D011392:Proline / Q000031:analogs & derivatives* / Q000737:chemistry; D011487:Protein Conformation; D018025:Receptors, Thyrotropin-Releasing Hormone / Q000737:chemistry / Q000378:metabolism; D013973:Thyrotropin-Releasing Hormone / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism
D001372:Aza Compounds; D009842:Oligopeptides; D018025:Receptors, Thyrotropin-Releasing Hormone; D013973:Thyrotropin-Releasing Hormone; D011392:Proline