H-3 NMR-STUDIES OF MULTIPLE CONFORMATIONS AND DYNAMIC PROCESSES IN COMPLEXES OF FOLATE AND METHOTREXATE WITH LACTOBACILLUS-CASEI DIHYDROFOLATE-REDUCTASE

[7,3',5',H-3(3)]- and [7,9-H-3(3)]-folic acid and [7,3',5',H-3(3)]meth otrexate (MTX) have been prepared and H-3-n.m.r. spectra obtained for their complexes with Lactobacillus casei dihydrofolate reductase (DHFR ). The H-3 results confirm the presence of three pH-dependent differen t conformational forms in the complex DHFR.NADP(+).folate. The folate benzoyl ring could be shown to be in essentially the same environment in the different forms, with the major differences being associated wi th the pterin ring. The appearance of a single resonance for the 3',5' -tritons showed that the benzoyl ring is flipping rapidly in all three forms. In contrast, the MTX complex was shown to exist as a single co nformational state with the benzoyl ring flipping rate being too low t o give a single averaged signal for the 3',5'-nuclei over the temperat ure range 283-313 K.