Conformational preferences and antimicrobial activities of alkanediols

Diols with short aliphatic chains were known to exhibit antimicrobial activities, which were found to depend on the chain length and the position of the hydroxyl groups. We carried out the conformational preferences of 1,n-hexanediols (n = 2–6) and (S)-3-alkoxypropane-1,2-diols in the gas phase and in water using density functional methods and explored the factors important to exhibit antimicrobial activities. In the case of 1,2-, 1,3-, and 1,4-hexanediols, intramolecular H-bonds played a role in stabilizing their preferred conformers in the gas phase, whereas they were remarkably depopulated in water. For 1,5-, and 1,6-hexanediols, no H-bonded conformers were favored both in the gas phase and in water. The C–H⋯O interactions appeared to be of consequence in determining the preferred structures of 1,n-hexanediols (n = 3–6) in water. The stabilization of the preferred conformers of (S)-3-alkoxypropane-1,2-diols could be ascribed to the bifurcated H-bonds both in the gas phase and in water. However, their populations were decreased and the –CH2–O–(CH2)m–CH3 chain became more extended in water. In the optimized structures of dimers and trimers of 1,2-hexanediol and (S)-3-(hexyloxy)propane-1,2-diol in water, the head OH groups are oriented to each other to form intermolecular H-bonds and the aliphatic tails are stretched out away from the head groups, which are likely to form micelle-like structures in water. When two hydroxyl groups become closer and the aliphatic chain becomes longer, the amphipathicity of alkanediol is increased and thus, it is likely to penetrate more easily into membrane bilayers of the microbial cell and may disrupt the membrane structure.