The synthesis, characterization, and evaluation of a novel polymer-protein conjugate are reported here. The covalent conjugation of high-molecular weight poly(acrylic acid) (PAA) to the lysine amino groups of met-hemoglobin (Hb) resulted in the covalent conjugation of Hb to PAA (Hb-PAA conjugate), as confirmed by dialysis and electrophoresis studies. The retention of native-like structure of Hb in Hb-PAA was established from Soret absorption, circular dichroism studies, and the redox activity of the iron center in Hb-PAA. The peroxidase-like activities of the Hb-PAA conjugate further confirmed the retention of Hb structure and biological activity. Thermal denaturation of the conjugate was investigated by differential scanning calorimetry and steam sterilization studies. The Hb-PAA conjugate indicated an improved denaturation temperature (T(d)) when compared to that of the unmodified Hb. One astonishing observation was that polymer conjugation significantly enhanced the Hb-PAA storage stability at room temperature. After 120 h of storage at room temperature in phosphate-buffered saline (PBS) at pH 7.4, for example, Hb-PAA retained 90% of its initial activity and unmodified Hb retained <60% of its original activity under identical conditions of buffer, pH, and temperature. Our conjugate demonstrates the key role of polymers in enhancing Hb stability via a very simple, efficient, general route. Water-swollen, lightly cross-linked, stable Hb-polymer nanogels of 100-200 nm were produced quickly and economically by this approach for a wide variety of applications.

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Protein-polymer interactions play a very important role in a number of applications, but details of these interactions are not fully understood. Chemical modification was introduced here to tune protein-polymer interactions in a systematic manner, where methemoglobin (Hb) and poly(acrylic acid) (PAA) served as a model system. Under similar conditions of pH and ionic strength, the influence of protein charge on Hb/PAA interaction was studied using chemically modified Hb by isothermal titration calorimetry (ITC). A small fraction of COOH groups of Hb were amidated with triethylenetetramine (TETA) or ammonium chloride to produce the corresponding charge ladders of Hb-TETA and Hb-ammonia derivatives, respectively. All the Hb/PAA complexes produced here are bioactive, entirely soluble in water, and indicated the retention of Hb structure to a significant extent. Binding of Hb to PAA was exothermic (ΔH < 0). The binding of Hb-TETA charge ladder to PAA indicated decrease of ΔH from -8 ± 0.2 to -89 ± 4 kcal/mol, at a rate of -3.8 kcal/mol per unit charge introduced via modification. The Hb-ammonia charge ladder, in contrast, showed a decrease of ΔH from -8 ± 0.2 to -17 ± 1.5 kcal/mol, at much slower rate of -1.0 kcal/mol per unit charge. Thus, the amine used for the modification played a strong role in tuning Hb/PAA interactions, even after correcting for the charge, synergistically. Charge clustering may be responsible for this synergy, and this interesting observation may be exploited to construct protein/polymer platforms for advanced biomacromolecular applications.

Artificial antenna complexes built via self-assembly are reported here, which indicated excellent energy transfer efficiency, macroscopic organization, unprecedented thermal stability, and ease of formation.Our system consists of four fluorescent donor-acceptor dyes, double-helical DNA and cationized bovine serum albumin, all self-assembled on cover glass slips to form functional materials. These captured radiation in the range of 330-590 nm, and excitation of any of the donor dyes resulted in efficient emission from the terminal acceptor. Excitation spectra provided unequivocal proof of energy transfer via jumper dyes, and transfer was interrupted when one of the jumper dyes was omitted, another direct evidence for cascade energy transfer. The entire assembly indicated unusually high thermal stability and continued to function efficiently even after exposure to 80 C for >169 days, an important consideration for field applications. These unusually stable, high efficiency, multichromophoric, artificial antennas are the first of their kind to demonstrate self-assembled 4-dye energy cascade, converting blue photons to red photons. † Electronic supplementary information (ESI) available: Including experimental methods, characterization of modied BSA, uorescence, circular dichroism, etc. See Scheme 1 Artificial antenna complexes constructed from donors, acceptors, cationized BSA (cBSA), and DNA.This journal is