Reversible wavelength shifts of chlorophyll induced by a point charge

Abstract

The recently supported point-charge hypothesis of shifts in the optical spectra of the retinal Schiff base in rhodopsin suggests a mechanism for the anomalous red shifts of chlorophyll (Chl) spectra in vivo. This idea is tested here by synthesis of a Chl model compound, 3-demethyl-3-(aminomethyl) Chl a, in which a point charge is reversibly induced on the periphery of the Chl macrocycle. Chl(3a-NH/sub 2/) is formed from Chl b by reductive amination of the formyl group with sodium cyanoborohydride. The structure is confirmed by the Chl a-like absorption spectrum (lambda/sub max/ = 661 nm, epsilon = 5.67 x 10/sup 4/M/sup -1/ cm/sup -1/ in CHCl/sub 3/), by /sup 1/H NMR, and by reversible changes in both absorption and /sup 1/H NMR on adjustment of pH (amine nitrogen pK/sub a/ = 7.9). Chl(3a-NH/sub 3//sup +/) displays a reversible blue shift in its red absorption maximum of 4 nm (90 cm/sup -1/) relative to Chl(3a-NH/sub 2/). Presumably, a negative charge in the same position would give a red shift of the same size; charges located differently on the macrocycle perimeter might give shifts of larger magnitude. Because such charges could be provided by polar amino acids of the Chl-binding proteins in vivo,more » our results lend support to a point-charge hypothesis of Chl spectral shifts.« less