Isotopic Evidence for Reaction Pathways and Quantum Tunneling in Methanotrophy

Area of Science:

• Biogeochemistry
• Computational Chemistry
• Microbial Metabolism

Background:

• Isotope composition of residual methane offers insights into methanotrophic pathways.
• Accurate kinetic isotope effects (KIEs) from first-principles computations are crucial for mechanistic interpretation.
• Understanding methane isotopologue abundances requires detailed kinetic calculations.

Purpose of the Study:

• To calculate KIEs and predict kinetic isotope fractionation for aerobic and anaerobic methane oxidation pathways.
• To elucidate the roles of specific copper complexes in aerobic methanotrophy.
• To differentiate between proposed methane activation mechanisms in anaerobic methanotrophy.

Main Methods:

• Density functional theory (DFT) methods to compute KIEs, including quantum tunneling.
• Kinetic calculations for various isotopomers and isotopic substitutions.
• Comparison of computed results with experimental data on methane isotopologue abundances.

Main Results:

• Calculated KIEs and fractionation accurately reproduced experimental abundances of methane isotopomers (e.g., 13CH4, 12CH3D).
• Identified an antiferromagnetic bis(μ-oxo)dicopper(II) complex with significant quantum tunneling as the active oxidant in aerobic methanotrophy, explaining observed D and 13C fractionation.
• Supported methane activation by a sulfur-centered radical over a nickel center in anaerobic methanotrophy, with minimal quantum tunneling.

Conclusions:

• A computational workflow using multiple isotopologue compositions can distinguish competing microbial reaction pathways.
• Modern quantum chemical methods can elucidate biochemical mechanisms from isotope effects without extensive labeling.
• The study provides a robust method for analyzing microbial methane oxidation under various conditions.