Steady-state fluorescence and NMR study on self-association behavior of poly(methacrylamides) bearing hydrophobic amino acid residues

Self-association properties of poly(N-methacryloylphenylalanine) and poly(N-methacryloyltryptophan) (pMPhe and pMTrp, respectively) were investigated by several characterization techniques, including steady-state fluorescence and NMR. These characterization data revealed similarities and distinctions of their self-association properties.The pH dependencies of association properties of pMPhe and pMTrp are practically the same. Apparent pK_a values for pMPhe and pMTrp were determined to be 5.7 and 5.8, respectively, by potentiometric titration. Steady-state fluorescence measurements at varying pH using pyrene as fluorescence probe indicated that, at pH≈5 (< apparent pH), hydrophobic microdomains were formed, while, at pH≈7 and 9 (> apparent pK_a), hydrophobic microdomains were not formed significantly. ¹H NMR spectra for both the polymers measured in D₂O exhibited that a significant fraction of aromatic rings in amino acid residues were located close to the polymer main chain, and that, at pH≈5, the mobility of the polymer main chain and the aromatic ring was extremely restricted.The polymer concentration (C_p) dependencies of association properties of pMPhe and pMTrp at pH≈5 are distinct. Steady-state fluorescence data at varying C_p indicated that pMPhe was more hydrophobic microscopically than pMTrp. Dynamic light scattering data indicated that pMTrp had a stronger tendency for interpolymer association than pMPhe did at pH≈5. It is concluded that the distinction in C_p dependency of the self-association properties of pMPhe and pMTrp is due to the differences in the bulkiness and the hydrophobicity of the substituents of amino acid residues.