Structural volume changes upon photoisomerization of the bilirubin-albumin complex: a laser-induced optoacoustic study

The Z --> E photoisomerization of the 1:1 complexes of human serum albumin (HSA) and several bilirubins (BR-IX alpha, -III alpha, meso-IX alpha and a mixture of -IX alpha, -III alpha and XIII alpha) affords in every case an almost negligible structural volume change (Delta V-R approximate to 0) within detection limits (i.e. less than 2-4 cm(3)/mol for this isomerization with very low quantum yield) as determined by laser-induced optoacoustic spectroscopy. Based on previous model studies of photoisomerizations in aqueous environment, this negligible small change is interpreted as indicating that the part of the molecule undergoing photoisomerization is not exposed to water but is located in a hydrophobic protein cavity that shields the molecule from the aqueous medium. The BR- protein interaction within this cavity seems to be very loose in view of the small structural volume change observed. The energy difference between the Z and E isomers of the BR-HSA complexes was estimated to be less than 4 kJ/mol, probably close to zero (Delta H approximate to 0).