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Abstract
A hypericin derivative ω,ω'-appended at the methyl groups with two polyethylene glycol moieties (about 23 units long) and capped with acetyl groups was synthesized starting from emodin. This derivative proved to be soluble in water and was investigated by means of spectrophotometric titrations and electrophoresis experiments. Deprotonation at thebay-region hydroxyl group was observed atpKa=1.6. This was followed by a second deprotonation step of aperi-hydroxyl group at apKa value of 9.4. This derivative could be protonated at the carbonyl group characterized by apKa value of −5.7. FrompKa determinations in water-ethanol mixtures the correspondingpKa values of hypericin itself determined in such mixtures were extrapolated to the aqueous phase. This resulted in estimatedpKa values of 1.8, 9.2, and −6.
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