Thermal Stability of Dehydrophenylalanine‐Containing Model Peptides as Probed by Infrared Spectroscopy: a Case Study of an α ‐Helical and a 3 10 ‐Helical Peptide

The temperature‐dependent secondary‐structural changes in the two known helical model peptides Boc‐Val‐ΔPhe‐Ala‐Leu‐Gly‐OMe ( 1 ; α ‐helical) and Boc‐Leu‐Phe‐Ala‐ΔPhe‐Leu‐OMe ( 2 ; 3 10 ‐helical), which both comprise a single dehydrophenylalanine (ΔPhe) residue, were investigated by means of FT‐IR spectroscopy (peptide film on KBr). Both the first‐order and the better‐resolved second‐order derivative IR spectra of 1 and 2 were analyzed. The ν (NH) (3240–3340 cm −1 ), the Amide‐I (1600–1700 cm −1 ), and the Amide‐II (1510–1580 cm −1 ) regions of 1 and 2 showed significant differences in thermal‐denaturation experiments (22°→144°), with the 3 10 ‐helical peptide ( 2 ) being considerably more stable. This observation was rationalized by different patterns and strengths of intramolecular H‐bonds, and was qualitatively related to the different geometries of the peptides. Also, a fair degree of residual secondary‐structural elements were found even in the ‘denatured’ states above 104° ( 1 ) or 134° ( 2 ).