Tetrahydrobiopterin analogs: the solution conformations of 6-methyltetrahydropterin, 7-methyltetrahydropterin, and cis- and trans-6,7-dimethyltetrahydropterins as determined by proton nuclear magnetic resonance

Abstract

The conformation of tetrahydrobiopterin analogues in aqueous solution at 23 degrees C has been determined by analyzing the 200-MHz 1H NMR spectral parameters of the enzymatically active 6-methyltetrahydropterin, 7-methyltetrahydropterin, and cis- and trans-6,7-dimethyl-5,6,7,8-tetrahydropterins. Each of these cofactors, with the exception of the cis-6,7-dimethyl isomer, exhibited an unusually small trans 6H-7H spin-spin coupling (8.5-9.1 Hz). An empirical equation that accounts for the effects of substituent electronegativity and orientation on vicinal couplings [Haasnoot, C. A. G., deLeeuw, F. A. A. M., & Altona, C. (1980) Tetrahedron 36, 2783-2792] predicted this coupling to be 11.3-11.6 Hz. We attribute the discrepancy between the calculated and experimentally observed values of this coupling to hyperconjugation of the axially oriented C7-H bond with the pi orbital of the vinylogous amide protein of the pterin ring (N8-C8a = C4a-C4 = O) rather than conformational averaging. The trans 6H-7H interproton distance in the 6-methyl analogue is calculated to be 3.0 A from the measured decrease in the spin-lattice relaxation rate of the axially oriented C7 proton after specific deuteration at C6. This is consistent with the single-conformer interpretation. Chemical shift comparisons of the methyl resonances of these analogues, NOE measurements from selectively deuterated analogues, and the differential sensitivities of axially vs. equatorially disposed ring protons to protonation at N5 all indicate that (i) the methyl substituents at both the C6 and C7 positions markedly prefer equatorial-like orientations and (ii) the tetrahydropterin ring is, with the exception of a pronounced pucker at C6, nearly planar.(ABSTRACT TRUNCATED AT 250 WORDS)

Hybrid Quantum and Classical Simulations of the Dihydrofolate Reductase Catalyzed Hydride Transfer Reaction on an Accurate Semi-Empirical Potential Energy Surface

“…As for the reduced equilibrium structures (6-Me-H 4 pterin and 6-Me-H 3 pterin – ), the pyrazine ring adopts a half-chair conformation, where C6 and C7 lie below and above the plane, respectively, in a staggered conformation with respect to each other, and the methyl group attached to C6 takes a pseudoequatorial position. These findings are in good agreement with 1 H NMR studies on tetrahydropterin derivatives (spin–spin coupling constants measurements). − A comparison of the crystal structure of 6-Me-H 3 pterin + and the computed geometry at the B3LYP/6-31G(2df,p) level is available in the Supporting Information.…”

A Kinetic and Conformational Study on the Interaction of Tetrahydropteridines with Tyrosine Hydroxylase

“…Thus, our kinetic and docking experiments with PH 4 support the conclusion that the presence of a substituent in the C6position on the pteridine ring represents an important determinant for a correct functional positioning of tetrahydropterins at the active site of TH. A similar conclusion has previously been reached for PAH (56). Uncoupling of rat TH has also been induced by mutating the active site residue Ser395 (52).…”

Pterin-Dependent Amino Acid Hydroxylases

“…From this evidence, Williams and Storm reasoned that (1) there is a single conformer of either monomethylated tetrahydropterin with the CH 3 group preferring an equatorial position and (2) that the tetrahydropterin ring is planar except for C 6 , 547 in agreement with the crystal structures. 548 In general, conformational analysis of tetrahydropterins using NMR tends to suffer from the difficulties posed by rapid exchange at N 5 and N 8 .…”