Computed and Experimental Chemical Shift Parameters for Rigid and Flexible YAF Peptides in the Solid State B

DFT methods were employed to compute the ¹³C NMR chemical shift tensor (CST) parameters for crystals of YAF peptides (Tyr-Ala-Phe) with different stereochemistry for the Ala residue. Tyr-d-Ala-Phe 1 crystallizes in the C2 space group while Tyr- l-Ala-Phe crystallizes in either the P2₁2₁2 space group (2a) or the P6₅ space group (2b). PISEMA MAS measurements for samples with a natural abundance of ¹H and ¹³C nuclei and ²H QUADECHO experiments for samples with deuterium labeled aromatic rings were used to analyze the geometry and time scale of the molecular motion. At ambient temperature, the tyrosine ring of sample 1 is rigid and the phenylalanine ring undergoes a π-jump, both rings in sample 2a are static, and both rings in sample 2b undergo a fast regime exchange. The theoretical values of the CST were obtained for isolated molecules (IM) and clusters employing the ONIOM approach. The experimental ¹³C δᵢᵢ parameters for all of the samples were measured via a 2D PASS sequence. Significant scatter of the computed versus the experimental ¹³C CST parameters was observed for 1 and 2b, while the observed correlation was very good for 2a. In this report, we show that the quality of the ¹³C σᵢᵢ/¹³C δᵢᵢ correlations, when properly interpreted, can be a source of important information about local molecular motions.