13C Isotope Labeling of Hydrophobic Peptides. Origin of the Anomalous Intensity Distribution in the Infrared Amide I Spectral Region of β-Sheet Structures

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Abstract

A series of isotopically substituted derivatives of the hydrophobic peptide K2(LA)6 including K2LA*(LA)5, K2L*A*(LA)5, and K2 LA*LA*(LA)4 (where the asterisk represents a residue with 13C substitution in the peptide bond CO) has been synthesized. The peptides adopt antiparallel β-sheet conformations as revealed by solution CD and IR measurements. The amide I region of the IR spectrum is substantially altered by the isotopic labeling. Peaks of anomalously large intensity are observed on the low frequency (∼1610 cm-1) side of the major conformation marker at 1625−1630 cm-1 present in the unlabeled isotopomer. The spectral changes cannot be described by the appearance of a pure mode based on the substitution of an oscillator of increased mass within the sequence. A semiempirical model incorporating transition dipole coupling and through-bond interactions within the context of the Wilson GF matrix method produces excellent agreement between calculated and observed amide I spectra with a single set of four parameters (through H-bond interaction force constant, through valence bond interaction force constant, transition dipole magnitude, and physical size) for four amide I β-sheet contours. In addition, the model reproduces the amide I contour for an isotopically labeled derivative of the α-helical peptide K2(LA)10. The excellent agreement between calculated and experimental spectra suggests that the model accounts for the most important interactions between peptide groups with β-sheet or α-helical structures.

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