The invention relates to a dextran functionalized by at least one hydrophobic alpha-amino acid radical, said alpha-amino acid being grafted or bonded to the dextran by a bonding arm and a functional group. A hydrophobic amino acid radical is understood as being the product of coupling between the amine of the amino acid and an acid carried by the bonding arm, said dextran being amphiphilic at neutral pH. In an embodiment, the hydrophobic amino acid is selected from tryptophan derivatives, such as tryptophan, tryptophanol, tryptophanamide, 2-indole ethyl-amine and their alkaline cation salts. The present invention relates also to a pharmaceutical composition comprising one of the dextrans according to the invention.