THE TRANS → CIS PHOTOISOMERIZATION OF PROTONATED RETINYL SCHIFF BASES

Abstract

Schiff bases were prepared from all-trans-retinal (I) and pyrrolidine perchlorate (11) and from I and n-butyl m i n e , protonated with anhydrous hydrogen chloride gas, 111. Initial quantum yields of rrunr cis photoisomerization (+'PI) were determined and primary photoproducts and product ratios were measured in aerated methanol. +'PI of all-fruns-111 is independent of excitation energy. All analyses were made using high-pressure liquid chromatographic (LC) methods. It was necessary to hydrolize the Schiff bases to corresponding retinals prior to LC analysis.

The role of intersection topography in bond selectivity ofcis-transphotoisomerization

“…However, this is not the case in either protein or solution environments. Especially relevant in the present context are the solution results, where a clear preference for the 11-cis photoproduct is observed (6)(7)(8). One possible explanation is the existence of barriers (of quite different heights) between the Franck-Condon geometry and the minimal energy CIs.…”

“…In contrast, illumination of the all-trans chromophore in solution results in several photoproducts with 11-cis being the most dominant (6)(7)(8). The isomerization quantum yield is greater than 50% in proteins (23), but rarely exceeds 20% in solution (6)(7)(8)(9)(10). Finally, the time scale for isomerization in solution has been measured to be 10 ps (9), while it is faster than 2 ps in protein environments (24,25).…”

“…In the protein environment, isomerization occurs exclusively around a single bond, e.g., 11-cis 3 all-trans in rhodopsin and all-trans 3 13-cis in bacteriorhodopsin. In contrast, illumination of the all-trans chromophore in solution results in several photoproducts with 11-cis being the most dominant (6)(7)(8). The isomerization quantum yield is greater than 50% in proteins (23), but rarely exceeds 20% in solution (6)(7)(8)(9)(10).…”

THE VISUAL PROCESS: PHOTOPHYSICS and PHOTOISOMERIZATION OF MODEL VISUAL PIGMENTS and THE PRIMARY REACTION

“…It should first be said that despite some earlier studies of photoisomerization yields and lifetimes, no proof of isomerization was obtained (nor was there identification of any geometric isomers produced). This was only done very recently for the Schiff bases Freedman and Becker, 1986; and protonated Schiff bases (references as above and Donahue and Waddell, 1984 for the all-trans isomer). Isomerization yield data on all of the isomers is only very recently available Freedman and Becker, 1986).…”

“…It is worth noting that it was found that dark/ thermal isomerization can occur in the synthesis or in a hydrolysis step of the Schiff bases (Freedman and B'ecker, 1986) which affects the value of the extinction coefficients previously reported, as well as the QpI values. Others (Donahue and Waddell, 1984) believed that hydrolysis and extraction did not cause significant isomerization and so if this is true, the dark isomerization, 7-17%, would occur in the synthesis step (even though done at W C in basic solution). QpI data correcting for this fact have been obtained by Freedman and Becker (1986) and Freedman et al (1980).…”