Chlorophyll a/b proteins of Photosystem I

The chlorophyll a/b ‐protein complex (LHCPI) associated with Photosystem I (PS I) has been isolated from spinach thylakoids and further fractionated into two chlorophyll‐containing complexes by sucrose gradient centrifugation. The lighter fraction contains two polypeptides with relative molecular masses of 23 and 22 kDa and has been designated as LHCPIa. The denser fraction is enriched in a 20 kDa polypeptide and has been named LHCPIb. Both fractions have a chlorophyll a/b ratio of 3.5 ± 0.5. The absorption spectra and 77 K fluorescence emission spectra of the fractions show distinct characteristics with LHCPIb having a fluorescence maximum at 730 nm at 77 K while LHCPIa shows a maximum at 680 nm. The optical activities of the chlorophyll a/b complexes and the antenna‐depleted PS I have been examined by circular dichroism (CD) in the near‐UV and visible regions of the spectrum. All the LHCPI complexes show strong CD signals at 648, 485 and 340 nm which are absent in the antenna‐depleted PS I complex.