A Synthetic Isoprenoid Lipoquinone, Menaquinone-2, Adopts a Folded Conformation in Solution and at a Model Membrane Interface

Abstract

Menaquinones (naphthoquinones, MK) are isoprenoids that play key roles in the respiratory electron transport system of some prokaryotes by shuttling electrons between membrane-bound protein complexes acting as electron acceptors and donors. Menaquinone-2 (MK-2), a truncated MK, was synthesized, and the studies presented herein characterize the conformational and chemical properties of the hydrophobic MK-2 molecule. Using 2D NMR spectroscopy, we established for the first time that MK-2 has a folded conformation defined by the isoprenyl side-chain folding back over the napthoquinone in a U-shape, which depends on the specific environmental conditions found in different solvents. We used molecular mechanics to illustrate conformations found by the NMR experiments. The measured redox potentials of MK-2 differed in three organic solvents, where MK-2 was most easily reduced in DMSO, which may suggest a combination of solvent effect (presumably in part because of differences in dielectric constants) and/or conformational differences of MK-2 in different organic solvents. Furthermore, MK-2 was found to associate with the interface of model membranes represented by Langmuir phospholipid monolayers and Aerosol-OT (AOT) reverse micelles. MK-2 adopts a slightly different U-shaped conformation within reverse micelles compared to within solution, which is in sharp contrast to the extended conformations illustrated in literature for MKs.

Structure Dependence of Pyridine and Benzene Derivatives on Interactions with Model Membranes

“…30 · 43–45 Both 1 H- 1 H 2D NOESY and ROESY spectra with a 200 ms mixing time and higher concentration of BA was acquired to explore which experiment would provide the best signal to noise ratio (Figure S13). With the ROESY data producing a better signal to noise ratio than the NOESY for these samples, other ROESY spectra were sought using 100 or 0 ms mixing times.…”

“…By comparing the findings of the RM experiments with phospholipid Langmuir monolayer experiments, the combination can shed light on crucial differences behind the interactions of molecules with the surfactant or lipid interfaces. 30 · 45 …”

Synthesis of Naphthoquinone Derivatives: Menaquinones, Lipoquinones and Other Vitamin K Derivatives

“…The abbreviation used for the menaquinones in this manuscript will indicate the number of isoprene units in the side chain. For example, MK-1 describes a menaquinone with one isoprene unit, and MK-9, the major MK-derivative found in the Mycobacteria , contains nine isoprene units [ 12 ]. If the MK-derivative contains isoprene units that are saturated, it will be indicated by the addition of a Roman numeral to specify the location of the isoprene unit numbered from the naphthoquinone.…”

“…It is highly likely that the redox reaction involving fewer intermediates is important for the biological properties of these compounds. Our group has been investigating the redox properties of some menaquinone systems and found distinctly different properties in organic solvents where the MK-derivatives are soluble [ 12 ], compared to aqueous systems where the menaquinone will be interacting with a membrane or a model membrane system, such as a liposome [ 32 ].…”

“…Despite the polarity of the quinone group, these molecules are very hydrophobic, even though this property does vary with the length and the nature of the isoprene side group. We have recently found by synthesis of truncated MK-derivatives that only MK-1, MK-2, and MK-3, including derivatives with fully or partially saturated counterparts, are soluble in aqueous solution [ 12 , 37 , 38 ]. This means that assays with MK-4, even though it is known as an enzyme substrate in vivo , may not demonstrate enzyme activity even if the aqueous assay includes surfactants [ 39 ].…”