Dodecyl and dodecanoyl groups (C12) are attached at the chain ends of poly [N-5-(2-hydroxyethyl) (L)-glutamine] (PHEG), and association behavior of this both-ends hydrophobically modified water soluble polypeptide (C12-PHEG-C12) has been investigated by means of light scattering measurements. Water/ethylene glycol (EG) mixed solvents were used as selective solvent for PHEG block, and PHEG changed its structure from random-coil state to alpha-helix with increasing EG content in the mixed solvent (W-EG)- When W-EG is less than 0.5, flower-like micelle with C12 associated core and PHEG corona in loop conformation was suggested to be formed. Increase of WEG from 0.5 to 0.6 induced drastic increase of association number and size of the associate, in which many C12 associated cores may be connected by PHEG in bridge conformation. This structure change of associate is considered to be driven by the increase of helix content of PHEG with WEG, which enhances the possibility to form bridge conformation because of its rigidity. Solution preparation method, i.e. order of addition of solvent, was found to mfluence the structure of associate, although its effect on the helix content of PHEG was negligible. (c) 2005 Elsevier Ltd. All rights reserved.