Can NADH diffuse across the cell membrane?

NADH is an important redox cofactor needed by many oxidoreductases such as those employed in biocatalytic reactions. Typically, NADH needs to be regenerated to sustain a biotransformation reaction. But, would the molecule diffuse across the cell membrane? By current understanding, the answer is no. Specifically, studies have shown that NADH could not diffuse across the mitochondrion membrane. By inference, NADH could not pass through the bacterial cell membrane. To transfer electrons into the electron transport chain, shuttle molecules such as FAD or FADH₂ are needed. These molecules help NADH transfer electrons into different proteins of the electron transport chain to the final terminal electron acceptor, oxygen. Overall, current understanding suggests that NADH could not diffuse across the cell membrane.