Ultrafast protein dynamics of hemoglobin as studied by picosecond time-resolved resonance Raman spectroscopy

Time-resolved resonance Raman spectroscopy on human adult hemoglobin (HbA) following ligand photolysis revealed that the frequency of the iron-histidine stretching [m(Fe-His)] mode exhibited a 2-cm(-1) downshift with a time constant of about 300 ps, suggesting a structural change in the heme pocket following the ligand photolysis. Low-frequency heme modes suggested that the primary metastable form of HbA has a more disordered orientation of propionates and a less strained environment than the deoxy form. The latter fact is consistent with the experimental observation that the m(Fe-His) frequency of the metastable form is higher than the deoxy form. The present study shows that HbA adopts a metastable structure within the instrument response time and remains little changed in the subnanosecond to nanosecond time regime. Characteristics of the primary protein response of HbA based on the comparison of the results of HbA with those of the isolated chains and myoglobin are discussed. (C) 2011 Elsevier B. V. All rights reserved.