Commercial
plant extracts containing
anthraquinones are being increasingly used for
cosmetics, food and pharmaceuticals due to their wide therapeutic and pharmacological properties. In this work, the interaction with model membranes of two representative 1,8-dihydroxyanthraquinones,
barbaloin (Aloe) and
emodin (Rheum, Polygonum), has been studied in order to explain their effects in biological membranes.
Emodin showed a higher affinity for
phospholipid membranes than
barbaloin did, and was more effective in weakening hydrophobic interactions between
hydrocarbon chains in
phospholipid bilayers. Whereas
emodin induced the formation of hexagonal-H(II) phase,
barbaloin stabilized lamellar structures.
Barbaloin promoted the formation of gel-fluid intermediate structures in
phosphatidylglycerol membranes at physiological pH and ionic strength values. It is proposed that
emodin's chromophore group is located at the upper half of the membrane, whereas
barbaloin's one is in a deeper position but having its glucopyranosyl moiety near the
phospholipid/water interface. Moreover, membrane disruption by
emodin or
barbaloin showed specificity for the two major
phospholipids present in bacterial membranes,
phosphatidylethanolamine and
phosphatidylglycerol. In order to relate their strong effects on membranes to their biological activity, the capacity of these compounds to inhibit the infectivity of the
viral haemorrhagic septicaemia rhabdovirus (VHSV), a negative
RNA enveloped virus, or the growth of Escherichia coli was tested.
Anthraquinone-loaded
liposomes showed a strong antimicrobial activity whereas these compounds in their free form did not. Both
anthraquinones showed
antiviral activity but only
emodin was a virucidal agent. In conclusion, a molecular mechanism based on the effect of these compounds on the structure of biological membranes is proposed to account for their multiple biological activities.
Anthraquinone-loaded
liposomes may suppose an alternative for antimicrobial, pharmaceutical or cosmetic applications.