A cross peak in a 2D spectrum connecting two diagonal Epacadostat concentration peaks indicates coupling between the two states. When electronic coupling is sufficiently strong relative to the coupling to the bath, quantum coherence may be preserved long enough for observation, giving rise to coherent cross peaks. However, cross peaks may also arise as a result of irreversible, dissipative energy transfer, namely from higher to lower energy states. This type of cross peak can be observed in the energy funnelling processes of light harvesting. In reality, cross peaks in 2D spectra arise from multiple sources, and it may be difficult to distinguish

between the limits of coherent and incoherent signals. In the case of LH3, the early-time spectra show faint off-diagonal signals (both positive and negative), and strong cross peaks are first observed at ~2 ps. Fig. 5 The experimental and theoretical 2D spectra of the LH3 complex, corresponding to the real part of electric field at 77 K at population times T = 0 fs, 20 fs, 50 fs, 1 ps, 2 ps, and 5 ps. The B800 and B820 peaks appear at (ω τ  ~ 12450 cm−1, ω t  ~ 12450 cm−1) and (ω τ  ~ 12150 cm−1, ω t  ~ 12150 cm−1), respectively, in the T = 0 spectrum. All spectra are normalized to the absolute maximum; positive features correspond to “more light” and negative to “less light” (Zigmantas et al. 2006) www.selleckchem.com/products/z-vad-fmk.html LH3

is a low-light adapted variant of the more common LH2 peripheral antenna complex, containing 27 BChla Thiamine-diphosphate kinase pigments arranged in two parallel rings, known as B820 and B800, due to their absorption wavelengths. Note that the B820 ring of LH3 discussed here is different from the solubilized dimer subunit of LH1, also called B820, discussed earlier. The 18 BChls of the B820 ring are closely packed, resulting in nearest-neighbor coupling interactions of about 300 cm−1. In contrast, the nine BChls of the B800 ring are more widely spaced, coupled by only 30 cm−1. These interactions and resulting

dynamics are apparent in the 2D experimental spectral features. While no strong cross peaks are apparent at T = 0, the weak off-diagonal features, and in particular the above-diagonal negative signal, indicate coherent coupling in the LH3 complex. The effect of coherent coupling is more apparent in the lower energy (B820) peak, in that it is shifted further down off the diagonal relative to the B800 peak (as a result of interference with the above-diagonal negative feature) and it exhibits coherent dynamics within the first 50 fs, while the B800 peak remains unchanged. Still, the off-diagonal signal above the B800 peak shows that coherence effects are present even for the weakly coupled BChla ring: if coherent coupling were not present, the B800 peak would be perfectly centered on the diagonal. Thus, 2D spectra are exquisitely sensitive even to weak interactions between chromophores.