C-13 NMR EVIDENCE OF THE SLOW EXCHANGE OF TRYPTOPHANS IN DIHYDROFOLATE-REDUCTASE BETWEEN STABLE CONFORMATIONS

13C NMR spectra are reported for dihydrofolate reductase of Streptococcus faecium labeled with [γ-13C]tryptophan. Two of the four tryptophans generate unusual resonances indicating slow exchange of the residues between alternative stable conformations. Since 3′, 5′-dichloromethotrexate sharpens two of the resonances, it apparently locks the corresponding residues into one conformation. © 1979.