The Bacteriostatic Activity of 2-Phenylethanol Derivatives Correlates with Membrane Binding Affinity

Abstract

The hydrophobic tails of aliphatic primary alcohols do insert into the hydrophobic core of a lipid bilayer. Thereby, they disrupt hydrophobic interactions between the lipid molecules, resulting in a decreased lipid order, i.e., an increased membrane fluidity. While aromatic alcohols, such as 2-phenylethanol, also insert into lipid bilayers and disturb the membrane organization, the impact of aromatic alcohols on the structure of biological membranes, as well as the potential physiological implication of membrane incorporation has only been studied to a limited extent. Although diverse targets are discussed to be causing the bacteriostatic and bactericidal activity of 2-phenylethanol, it is clear that 2-phenylethanol severely affects the structure of biomembranes, which has been linked to its bacteriostatic activity. Yet, in fungi some 2-phenylethanol derivatives are also produced, some of which appear to also have bacteriostatic activities. We showed that the 2-phenylethanol derivatives phenylacetic acid, phenyllactic acid, and methyl phenylacetate, but not Tyrosol, were fully incorporated into model membranes and affected the membrane organization. Furthermore, we observed that the propensity of the herein-analyzed molecules to partition into biomembranes positively correlated with their respective bacteriostatic activity, which clearly linked the bacteriotoxic activity of the substances to biomembranes.

Keywords: 2-phenylethanol; Tyrosol; bacteriotoxic; biomembranes; membrane interaction; methyl phenylacetate; phenylacetic acid; phenyllactic acid.

Figure 1

Chemical structures of 2-phenylethanol (2-PEtOH) and derivatives. 2-phenylethanol (A), phenylacetic acid (B), phenyllactic acid (C), Tyrosol (D), methyl phenylacetate (E), and 1-hexanol (F). The structures were drawn with ChemSketch V5.

Figure 2

Number–density profiles, compound orientation relative to the membrane normal and simulation snapshots. (A–E) The number–density profiles report the molar fraction of different chemical groups in the simulation box, perpendicular to the bilayer. (A) 2-PEtOH, (B) phenylacetic acid, (C) phenyllactic acid, (D) Tyrosol, and (E) methyl phenylacetate. All substances have shifts between the side chain and ring, except Tyrosol. Molar fractions of the solutes have been multiplied by 50 for clarity. All substances spontaneously insert close to the lipid head groups. (F) The angle between membrane normal and compound orientation (defined from side chain to aromatic ring) shows large values for methyl phenylacetate, phenylacetic acid, 2-PEtOH, and phenyllactic acid, indicative of their intercalation in the membrane. Smaller values are reported for Tyrosol, indicating its binding parallel to the membrane surface. (G,H) Representative simulation snapshots of 2-PEtOH and Tyrosol (purple) inserted in the headgroup region of the phospholipid membrane. Water is not shown for clarity.

Figure 3

2-PEtOH and derivatives affect the structure of the model membranes. GP values determined at increasing substance concentrations are shown. The values indicate a fluidizing effect for the more hydrophobic substances 1-hexanol, 2-PEtOH, and methyl phenylacetate. Tyrosol seems to be largely ineffective, while phenyllactic acid and phenylacetic acid seem to have a slight ordering effect.

Figure 4

Determination of MIC50 values. The OD₆₀₀ was plotted against the substance concentration and a dose-response fit was performed for (A) 2-PEtOH, (B) phenylacetic acid, (C) phenyllactic acid, (D) Tyrosol, (E) methyl phenylacetate, and (F) 1-hexanol (n = 3, ±SD). The corresponding MIC50 values are given in Table 1.

Figure 5

The logP values of 2-PEtOH linearly correlate with the log(1/MIC₅₀) values. The logP values of 2-PEtOH and derivatives is plotted against the respective log(1/MIC₅₀) values. The correlation coefficient is r² = 0.987.