Our results suggest a striking influence of MP fluorescence on the macular AF lifetime. We demonstrated that macular
τm show a strong correlation to the amount of MP, independent of MT and subject's age. Due to the emission spectra of xanthophylls with a peak around 500 to 550 nm, the short-wavelength channel is more strongly influenced by this fluorescence than the long-wavelength channel. Lutein and zeaxanthin with 10 and 11 double bonds, respectively, emit fluorescence by relaxing directly from the S
2 (instead of S
1) to the S
0 state.
42–45 The autofluorescence lifetThe AF lifetime of xanthophylls in vitro has been demonstrated to be very short, within the range of approximately 200 to 250 fs.
28,30 We found longer macular AF lifetimes in vivo, where the macular fluorescence decayed in the lower picosecond range. The shortest average
τm that we found within the central foveal area C
S were 52 ± 12 ps (ch2: 105 ± 16 ps). These lifetime values clearly differ from those of xanthophylls in solution, which is probably due to the following: (1) The retina contains a composition of multiple fluorophores rather than a single component, and therefore its
τm is very likely a superposition. The same applies to area C
S, which presumably contains a large fraction of MP but to a certain extent also a fraction of longer-decaying fluorophores, such as lipofuscin or melanin. This is supported by the nonzero correlation of
τm across area C
S to the MPOD volume; (2) assuming that area C
S would contain only MP, the resulting
τm would still be longer compared to the in vitro decay time of a few hundred femtoseconds. This is due to the fact that FLIO's time resolution is limited to approximately 30 ps; (3) additionally, Billsten et al.
46 found that the lifetime of the nonradiative S
1 state of zeaxanthin is prolonged either when bound to the xanthophyll binding protein or in self-assembled aggregates.
47 This may also affect the lifetime of the radiative S
2 state, which, however, has not been demonstrated yet; and (4) the individual AF of the human lens may also have an additional but small impact on the fluorescence lifetimes measured in vivo.