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The knowledge of the vibrational properties is of key importance to understand their function of β-carotene molecules in the reaction centre of the light-harvesting complexes. We present polarisation-dependent Raman spectroscopy on β-carotene molecules in two different environments: first single dimeric photosystem II crystal, and second encapsulated β-carotene in single-wall carbon nanotube bundles. Both systems have nearly perpendicular or parallel orientated β-carotene molecules. We compare the dependence of their Raman spectra on the polarisation of incident and scattered light.

We have studied the resonance Raman spectra of β-carotene in solution and in protein-complexes using laser excitation at various wavelengths in the visible region. Main focus was laid on the v 1 -band, which shows an anomalous behaviour upon variation of the excitation wavelength resulting in a frequency shift of the peak maximum. This shift is due to different excitation profiles of two closely spaced modes as shown previously for β-carotene in solution. In this work, we have extended the studies to the antenna pigment in photosystems I and II (PS I and PS II).

In photosynthesis the sunlight is collected by antenna pigments and transmitted to special molecule structures called photosystem I (PS I) and photosystem II (PS II), where the energy of light is transformed into a separation of charge. In the PS II reaction centre (RC), there are two β-carotene molecules Car D1 and Car D2 , which are oriented perpendicular to each other. The role of these molecules in the photosynthesis process is still unknown. We use Raman spectroscopy to study the two β-carotenes. We present polarised Raman measurements of the β-carotene vibrational modes in single crystals of photosystem II (PS II) and discuss the possible origin of the observed polarisation dependence. These measurements are important for the basic understanding of the β-carotene vibrational properties and guide future investigations of PS II.