Disentangling Electronic and Vibrational Coherence in the Phycocyanin-645 Light-Harvesting Complex

Energy transfer between chromophores in photosynthesis proceeds with near-unity quantum efficiency. Understanding the precise mechanisms of these processes is made difficult by the complexity of the electronic structure and interactions with different vibrational modes. Two-dimensional spectroscopy has helped resolve some of the ambiguities and identified quantum effects that may be important for highly efficient energy transfer. Many questions remain, however, including whether the coherences observed are electronic and/or vibrational in nature and what role they play. We utilize a two-color, four-wave mixing experiment with control of the wavelength and polarization to selectively excite specific coherence pathways. For the light-harvesting complex PC645, from cryptophyte algae, we reveal and identify specific contributions from both electronic and vibrational coherences and determine an excited-state structure based on two strongly coupled electronic states and two vibrational modes. Separation of the coherence pathways also uncovers the complex evolution of these coherences and the states involved.

Coherent Vibronic Coupling in Light-Harvesting Complexes from Photosynthetic Marine Algae

Observations of long-lived coherences in photosynthetic light-harvesting complexes utilize short pulses with broad spectral bandwidths to coherently excite multiple transitions and coherent superpositions. In order to identify the role that such quantum effects might play in efficient energy transfer, however, an alternative approach is required. We have developed a technique for two-color photon echo spectroscopy to selectively excite the pathway of interest and measure its evolution in the absence of any other excitation. We use this technique to excite a coherence pathway in phycocyanin-645 from cryptophyte algae and measure the dynamics of this coherence. A decoherence time of 500 fs was measured, and clear signatures for strong coupling between the electronic states and phonon modes were observed, allowing coherent coupling between otherwise nonresonant transitions. This provides detailed experimental evidence of the long-lived coherences and the nature of the quantum mechanical interactions between electronic states and phonon modes in phycocyanin-645 from cryptophyte marine algae.

Megakaryoblastic transformation of chronic granulocytic leukaemia. An electron microscopy and cytochemial study

Morphological, cytochemical, and ultrastructural electron microscopic (EM) studies were performed on blood and bone-marrow cells of case of Ph1-positive chronic ganulocytic leukaemia in megakaryocytic acute transformation. The entire leukaemic cell population was found to consist and of megakaryoblasts and megakaryocytes. Intermediate stages of maturation between blasts and micromegakaryocytes were observed at EM level.