Spectral changes induced by alkaline pH and specific chemical modificationof amino acid residues in the light-harvesting II antenna complex from Ectothiorhodospira sp.

The absorption spectrum of the membrane-bound light-harvesting (LH)II anten na complex from Ectothiorhodospira sp. has two characteristic near-infrared bands at 797 (B800 band) and 857 (B850 band) nm. Alkaline pH induced a B85 0 band blue shift of 17-21 nm depending on experimental conditions. The blu e shift was totally reversible when the original experimental conditions we re re-established. No significant effect was observed, however, on the B800 band under the same experimental conditions. The intensity and shape of th e pigment circular dichroism signals were maintained with the exception of a blue shift of the signal from the B850 band concomitant with the blue shi ft of that absorption band. Specific chemical modification of the LHII comp lex with salicylaldehyde allowed correlation of the alkaline pH effect with the neutralization of a lysine positive charge. We propose that the observ ed blue shift of the B850 band is due to distortion of the bacteriochloroph yll domain as a consequence of electrostatic and probably hydrogen-bonding changes but not due to modification of the pigment excitonic interactions w ithin the pigment-protein complex.