Investigations of vibrational coherence in the low frequency region of ferric heme proteins

¹ NORTHEASTERN UNIVERSITY
² Northeastern University
³ Northeastern Univ.

^* To whom correspondence should be addressed. E-mail: champ{at}neu.edu.

Submitted on September 14, 2007
Revised on October 25, 2007
Accepted on 6 November 2007

	Abstract

Femtosecond coherence spectroscopy is applied to a series of ferric heme protein samples. The low frequency vibrational spectra that are revealed show dominant oscillations near 40cm^-1. MbCN is taken as a typical example of a histidine ligated, six-coordinate, ferric heme and a comprehensive spectroscopic analysis is carried out. The results of this analysis reveal a new heme photoproduct species, absorbing near 418nm, which is consistent with the photolysis of the His93 axial ligand. The photoproduct undergoes subsequent rebinding/recovery with a time constant of ~4ps. The photoproduct lineshapes are consistent with a photolysis quantum yield of 75-100%, although the observation of a relatively strong 6-coordinate heme coherence near 252cm^-1 (assigned to ₉ in the MbCN Raman spectrum) suggests that the 75% lower limit is much more likely. The phase and amplitude excitation profiles of the low frequency mode at 40cm^-1 suggest that this mode is strongly coupled to the MbCN photoproduct species and it is assigned to the doming mode of the transient penta-coordinated material. The absolute phase of the 40cm-1 mode is found to be /2 on the red side of 418nm and it jumps to 3/2 as excitation is tuned to the blue of 418nm. The absolute phase of the 40cm^-1 signal is not explained by the standard theory for resonant impulsive stimulated Raman scattering. New mechanisms that give a dominant momentum impulse to the resonant wavepacket, rather than a coordinate displacement, are discussed. The possibilities of heme iron atom recoil following photolysis, as well as ultrafast non-radiative decay, are explored as potential ways to generate the strong momentum impulse needed to understand the phase properties of the 40cm^-1 mode.

Key Words: coherence, heme proteins, low frequency modes, myoglobin cyanide, photolysis, ultrafast