Stereospecific assignments of 1H‐nmr methyl lines and conformation of valyl residues in the lac repressor headpiece

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Abstract
Two‐dimensional 1H‐nmr methods are described to obtain information on the sidechain conformation of valyl residues of the lac repressor headpiece and to assign the resonances of their methyl groups stereospecifically. The spin–spin coupling constants (Jαβ) between Cαand Cβ protons are obtained from two‐dimensional correlated spectroscopy experiments. Large values for Jαβ(10–12 Hz) corresponding to trans orientations for these protons (g+ conformation) are found for all valyl residues in α‐helical segments. For these valyl residues, the distance between one methyl group (γ1)and the valyl amide proton is much shorter than for the other methyl group, so that stereospecific resonance assignments follow from relative intensities of the corresponding cross peaks in a two‐dimensional nuclear Overhauser enhancement spectrum. Thus, streospecific assignments could be made for the methyl groups of Val 9, 20, 23, and 38 (of a total of eight valyl residues).