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

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Abstract
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.

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