Conformational dynamic and enthalpy changes associated with pH induced unfolding of apomyoglobin were studied using photoacoustic calorimetry and photothermal beam deflection methods. The transition between the native state and the I intermediate was induced by a nanosecond pH jump from o-nitrobenzaldehyde photolysis. Deconvolution of photoacoustic waves indicates two kinetic processes. The fast phase (τ < 50ns) is characterized by a volume expansion of 8.8 ml mol-1. This process is followed by a volume contraction of about -22 ml mol-1 (τ ∼ 500 ns). Photothermal beam deflection measurements do not reveal any volume changes on the time scale between ∼100 μs and 5 ms. We associate the volume contraction with structural changes occurring during the transition between the native state and the I intermediate. The lack of any processes on the ms time scale may indicate the absence of structural events involving larger conformational changes of apomyoglobin after the pH jump.
