Direct observation of the multistep helix formation of poly-L-glutamic acids

The helix formation dynamics of poly-l-glutamic acids (PGAs) were observed by the microsecond-resolved Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopies. The helix formation of 34-residue PGA from random coil at pH (or pD for FTIR) 8.0 was initiated by a pH jump to 4.9 using the rapid solution mixer whose mixing dead time is 50 μs. The amide I- line in the time-resolved FTIR spectra exhibited the fast (<100 μs) increase of the total helical content. The time-resolved CD spectra of the same process also showed the fast (<150 μs) formation of short helical segments (5 ± 1 residues), which was followed by the slower (<1 ms) elongation of the short helices to longer helices (>10 residues). Similar dynamics were observed for the same pH jump of ∼190-residue PGA, although there were additional steps that made the helix formation of ∼190-residue PGA more complex. The observed multistep helix formation is likely caused by the strong hydrogen-bonding interactions between the protonated side chains of PGAs.