Ultrafast relaxation in picosecond photolysis of nitrosylhemoglobin.

We report the successful observation of a picosecond transient difference spectrum in human nitrosylhemoglobin. The sample (23 degrees C) is excited with an approximately 8 ps, approximately 10 microJ pulse at 353 nm that generates a prompt transient having a two-component decay: the first is approximately exponential with tau = 17 +/- 4 ps, whereas the second is much weaker with an approximate tau = 100 ps. At slightly lower temperature (4 degrees C), the spectrum and time dependence are essentially unchanged. In contrast to our previous observations on carboxyhemoglobin and oxyhemoglobin, we find no long-lived photoproduct in nitrosylhemoglobin. We tentatively attribute the 17 +/- 4 ps decay to geminate recombination. These results, in conjunction with our previous work in HbO2 and HbCO, show that the rate of geminate recombination for 5 ns greater than tau greater than 5 ps increases through the series HbCO less than HbO2 less than HbNO. We note that trends are also seen for microsecond recombination rates HbCO less than HbO2 approximately equal to HbNO and for the kinetic co-operativity ratio HbCO greater than HbO2 greater than HbNO. A "critical on-barrier" model is presented that provides a consistent representation of these results. We suggest that spin-orbit effects could be a major contribution to the different recombination characteristics exhibited by the three ligands.

Different dissociation pathways and observation of an excited deoxy state in picosecond photolysis of oxy- and carboxymyoglobin.

Picosecond transient absorption spectra of Mb, MbCO, and MbO2 have been studied at time delays of up to 10 ns after excitation at 353 nm. Particular attention has been paid to the rapid spectral changes that occur in the Soret region during the first 50 ps in MbCO and MbO2. In MbCO both the bleaching of the Soret peak (feature I) and the appearance of new deoxy-like absorption (feature II) occur instantaneously, whereas in MbO2 feature II is delayed with respect to feature I. A short-lived (approximately 12 ps) feature near 455 nm (feature III) was much more intense in MbO2 than in MbCO and was also identified in the transient spectrum of Mb. No evidence of subnanosecond geminate recombination was found in either MbCO or MbO2. These observations are consistent with a scheme in which MbO2 photodissociates through an excited state of Mb, whereas MbCO under the same conditions produces ground state Mb directly. The results and conclusions are compared with those of previous picosecond studies on these molecules and related hemoglobin derivatives.

Role of self-phase modulation in stimulated Raman scattering from more than one mode.

Stimulated Stokes spectra obtained with picosecond pulses in organic liquids often exhibit transient gain in more than one mode. A theory is developed here to describe the important effect of self-phase modulation on such multiple-component spectra. The nonlinear refractive index generated by a two-component Stokes field produces additional spectral components, which are separated by the frequency spacing between the two initial components. Calculations indicate that the new components are observable at considerably lower intensities than that necessary to produce noticeable phase modulation of the pulse envelope and may be an intrinsic part of the excitation process in molecules exhibiting Stokes gain in more than one mode. A comparison between the calculated spectra and experimentally obtained stimulated Raman spectra in methanol confirms the theoretical predictions, and the consequences of the theory with regard to the stimulated Raman spectra of large molecules are discussed.