Edited by James N. SiedowCyanobacteria possess a family of one-helix high light-inducible proteins (Hlips) that are homologous to light-harvesting antenna of plants and algae. An Hlip protein, high light-inducible protein D (HliD) purified as a small complex with the Ycf39 protein is evaluated using resonance Raman spectroscopy. We show that the HliD binds two different -carotenes, each present in two non-equivalent binding pockets with different conformations, having their (0,0) absorption maxima at 489 and 522 nm, respectively. Both populations of -carotene molecules were in all-trans configuration and the absorption position of the farthest blue-shifted -carotene was attributed entirely to the polarizability of the environment in its binding pocket. In contrast, the absorption maximum of the red-shifted -carotene was attributed to two different factors: the polarizability of the environment in its binding pocket and, more importantly, to the conformation of its -rings. This second -carotene has highly twisted -rings adopting a flat conformation, which implies that the effective conjugation length N is extended up to 10.5 modifying the energetic levels. This increase in N will also result in a lower S 1 energy state, which may provide a permanent energy dissipation channel. Analysis of the carbonyl stretching region for chlorophyll a excitations indicates that the HliD binds six chlorophyll a molecules in five non-equivalent binding sites, with at least one chlorophyll a presenting a slight distortion to its macrocycle. The binding modes and conformations of HliD-bound pigments are discussed with respect to the known structures of LHCII and CP29.In nature, photosynthetic organisms obtain their energy by collecting solar photons, using complex arrays of pigments known as antennas. Subsequently, the energy harvested by the antennas is transferred to reaction centers to be transduced into electrochemical potential. Finally after a cascade of molecular steps, this energy is stored as potential chemical energy that is easy to store and transport (1). To accomplish this process, these organisms have developed a large number of different protein antenna assemblies, a notable subgroup of which are the light-harvesting complexes (LHCs) 3 of green plants and algae. LHCs constitute a large family of proteins, which can exist as monomers, dimers, or trimers in the membrane, and have related amino acid sequences. It is hypothesized that in the evolution of photosynthesis there was, and continues to be, a selective advantage for the organism to make optimum use of low light conditions to drive the relatively slow downstream metabolic reactions of legacy biochemistry (2). Plants, algae, and cyanobacteria are oxygenic photosynthetic organisms that are exposed to random fluctuations in light intensity. In the case of low light requiring large antennas, during high light exposure, most of the energy absorbed is in excess of that required to drive downstream metabolism. If excess light energy is not dealt with properly, i...