pH- and thermal-dependent conformational transition of PGAIPG, a repeated hexapeptide sequence from tropoelastin

The secondary structure of PGAIPG (Pro-Gly-Ala-IIe-Pro-Gly), a repeated hexapeptide of tropoelastin, in buffer solution of different pH was determined by using attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. The thermal-dependent structural change of PGAIPG in aqueous solution or in solid state was also examined by thermal FTIR microspectroscopy. The conformation of PGAIPG in aqueous solution exhibited a pH-dependent structural characterization. A predominant peak at 1614 cm−1 (aggregated β-sheet) with a shoulder near 1560 cm−1 (β-sheet) appeared in pH 5.5-8.5 buffer solutions. A new broad shoulder at 1651 cm−1 (random coil and/or α-helix) with 1614 cm−1 was observed in the pH 4.5 buffer solution. However, the broad shoulder at 1651 cm−1 was converted to a maximum peak at 1679 cm−1 (β-turn/antiparallel β-sheet) when the pH shifted from 4.5 to 3.5, but the original pronounced peak at 1614 cm−1 became a shoulder. Once the pH was lowered to 2.5, the IR spectrum of PGAIPG was dominated by major absorption at 1679 cm−1 with a minor peak at 1552 cm−1 (α-helix/random coil). The result indicates that the pH was a predominant factor to transform PGAIPG structure from aggregated β-sheet (pH 8.5) to β-turn/intermolecular antiparallel β-sheet (pH 2.5). Moreover, a partial conformation of PGAIPG with minor α-helix/random coil structures was also explored in the lower pH buffer solution. There was no thermal-dependent structural change for solid-state PGAIPG. The thermal-induced formation of aggregated β-sheet for PGAIPG in aqueous solution was found from 28 to 30 °C, however, which might be correlated with the formation of an opaque gel that turned from clear solution. The formation of aggregated β-sheet structure for PGAIPG beyond 30 °C might be due to the intermolecular hydrogen bonded interaction between the hydrophobic PGAIPG fragments induced by coacervation.