Synthesis of N‐heteroaryl retinals and their artificial bacteriorhodopsins

Abstract

N-Heteroaryl retinals derived from indole, 1-indolizine and 3-indolizine (10a–c) have been synthesized after their UV/Vis red-shifted absorption properties had been predicted by time-dependent density functional theory (TD-DFT) computations. The three new analogues form artificial pigments upon recombination with bacterioopsin: indolyl retinal 10a undergoes fast and efficient reconstitution to form a species with a UV/Vis absorbance maximum similar to that ofwild-type bacteriorhodopsin, whilst the indolizinyl retinals 10b and 10c also reconstitute in significant proportion to give noticeably red-shifted, although unstable, pigments. Significant changes in the pKa values ofthese artificial bacteriorhodopsins are interpreted as arising from nonoptimal bindingsite occupancy by the chromophore due to steric constraints.