Self-Association Behavior of Hydrophobically Modified Poly(aspartic acid) in Water Studied by Fluorescence and Dynamic Light Scattering Techniques

Abstract

Poly(succinimide) (PSI), derived from l-aspartic acid by polycondensation, is a reactive precursor for synthesis of various derivatives of poly(aspartic acid) (PASP), a commercially interesting polyelectrolyte because of its biodegradable properties. Hydrophobically modified PASP was synthesized by reacting PSI with dodecylamine, followed by hydrolysis. The self-association behavior of PASP having pendant dodecyl groups (DDA-PASP) in water was characterized by fluorescence and quasielastic light scattering (QELS) techniques as a function of the mole percent content of dodecyl groups in the polymer (fDod). For fluorescence studies, the polymers were labeled singly with pyrene (Py-DDA-PASP) or doubly with naphthalene and pyrene (Np-Py-DDA-PASP). The vibrational fine structure of pyrene fluorescence spectra and the pyrene fluorescence lifetime for an aqueous solution of Py-DDA-PASP at pH = 9 indicated that associations of polymer-bound dodecyl groups started to occur at a low fDod (near 3 mol %), and a significant fraction of pyrene labels were incorporated in hydrophobic microdomains formed. Intrapolymer nonradiative energy transfer from naphthalene to pyrene in Np-Py-DDA-PASP indicated that a significant contraction of the conformational dimension occurred in the region 20 < fDod < 50 mol % at pH = 9, this fDod region shifting to 10−30 mol % upon decrease in pH to 4 or upon addition of 0.05 M NaCl at pH = 9. QELS results at pH = 9 in the presence of 0.05 M NaCl indicated that DDA-PASP with fDod ≤ 31 mol % forms two kinds of aggregates with well-defined hydrodynamic radii (Rh), Rh ranging 3−4 nm for one type of the aggregates and 60−90 nm for the other, and they are coexisting together. The fraction of the larger aggregates increases with polymer concentration. In contrast, DDA-PASP with fDod = 48 mol % forms only small aggregates with Rh on the order of 4 nm independent of the polymer concentration studied (0.25−1.0 g/dL). The mean aggregation number of dodecyl groups in this small aggregate was determined to be ca. 68 using a time-resolved fluorescence method. Since this aggregation number agrees well with the number of dodecyl groups per polymer chain (ca. 64) calculated from the degree of polymerization along with fDod, this small aggregate is suggested to be a unimolecular micelle. Accordingly, all the small aggregates with Rh = 3−4 nm found for the polymers with fDod ≤ 31 mol % are also likely to be unimolecular micelles. Consequently, DDA-PASP exhibits a strong preference for intrapolymer hydrophobe self-associations to form a unimolecular micelle if the hydrophobe content is as high as 48 mol %.