We have prepared Lactobacillus casei dihydrofolate reductase containing biosynthetically incorporated (2S,4S)-5-fluoroleucine ([5-F]-Leu DHFR) and have obtained its 1H and 19F NMR spectra at 9.4 Tesla. The 19F spectrum of [5-F]-Leu DHFR showed 12 fairly sharp peaks (one containing two overlapped signals) for the 13 leucine residues in DHFR, covering a chemical shift range of 15 ppm. The large range of chemical shifts observed could not be explained solely in terms of the electrostatic field effects due to local charge fields and is thought to have a second contribution from side-chain conformational differences (γ-gauche effects) between different leucine residues, making 19F NMR of aliphatic amino acids in proteins a potentially useful new probe of protein structure.

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A general method IS described for the stereospecific assignment of methyl resonances in protein NMR spectra based on selective deuteration procedures. A selectively deuterated dihydrofolate reductase from L case1 was prepared by incorporating stereoselectively deuterated L-leucine, (2S.4R)[5.5,5-'H,]leucine. By comparing the COSY spectra of the dihydrofolate reductase-methotrexate complexes formed using deuterated and non-deuterated enzyme the stereospecific assignments for resonances of all 13 leucine residues were obtained by noting the absence of cross-peaks in spectra from the deuterated protems.

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