J . Phys. Chem. 1986, 90, 2813-2816
2813
Time-Resolved Luminescence Study of Protonated Schiff Bases D. Huppertt and P. M. Rentzepis* Department of Chemistry, University of California. Irvine, Irvine, California 9271 7 (Received: April I I, 1986)
Picosecond time-resolved emission studies of protonated Schiff bases of all-trans- and 11-cis-retinals suggest that the excited state relaxes within 7 ps. Studies of the effect of temperature and exchange of the Schiff base proton with deuterium on the emission kinetics suggest that these model compounds behave quite similarly to visual chromophore rhodopsin.
Introduction Retinal derivatives are considered to be the chromophores of visual pigment and photosynthetic bacteria. The retinal in the case of the visual pigment is bound to opsin (an apoprotein) via a Schiff base linkage and to a lysine residue. When rhodopsin is irradiated with visible light the visual transduction process proceeds through several intermediate steps before its completion which involves cleavage. The first intermediate species, bathorhodopsin, is known to be formed within 6 ps.' Cis-trans isomerization of the 11-cis-retinal is an important feature of the visual p r o c e s ~at ~ !room ~ temperature regardless if it is the only cause of the first intermediate. Proton transfer has also been proposed as the primary photoprocess preceding or in conjunction with the structural deformation process?*s Because of the complexity of the rhodopsin whose structure is not known yet, retinal isomers, retinal Schiff bases, and protonated Schiff bases have been studied extensively possibly because they are simple model compounds of the rather complicated visual pigment. Studies of these model systems are expected to contribute to the basic understanding of the complex photochemical process of vision and the events leading to and responsible for the bacteria rhodopsin proton pump process. The value of a kinetic approach to illucidate the structure and dynamics of the transient species has first been shown by picosecond time-resolved differential absorption measurements in rhodopsin itself' where the kinetics of formation and decay of the first intermediate, bathorhodopsin, were measured. Subsequently, fluorescence lifetime studies were also performed in several model systems,6-8 which are simpler and are thought to imitate the primary events of the visual chromophore. Typical of these studies have been the time-resolved fluorescence of retinal measured by Kropf et aL6 and of Everaert and Rentzepis9 who used picosecond emission spectroscopy to study the relaxation decay channels of the excited n-butylamine Schiff base al[-trans-retinal. In the later case, samples were excited with a single 355-nm, 25-ps pulse, to 360 nm). The its first strongly absorbing electronic state (A, lifetime dependence on temperature and the spectral characteristics support a vibrational-torsional deactivation model of the excited singlet state of two conformers. In addition, strong quenching in alcoholic solutions was observed and attributed to molecular reorientation. Protonated Schiff base (PSB) retinal exhibits large 490 nm) compared to the red-shifted absorption band (A,, nonprotonated Schiff base which has its absorption maximum at 360 nm. Similarly, the rhodopsin absorption maximum is also red shifted to 540 nm compared to the 390-nm absorption of the nonprotonated analogue. In fact the protonated Schiff base of rhodopsin has been widely considered as the cause of the observed absorption shift to 540 nm.lo In this paper we report a picosecond time-resolved emission study of the protonated Schiff bases of all-trans-retinal and 11cis-retinal. We present experimental data on the temperaturedependent kinetics from 4 to 293 K. W e also present data and discuss the effect of isotopic exchange of the Schiff base proton with deuterium on the emission spectra and the energy relaxation mechanism and henceforth comment on the effect of the hydrogen in the visual process based on experimental data rather than conceptual models.
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Department of Chemistry, Tel Aviv University, Tel Aviv, Israel.
0022-3654/86/2090-28 13$01.50/0
Experimental Section 1 1 4 s - and all-trans-retinal samples were obtained from Prof. R. S. Becker, University of Houston, Houston, TX. The n-butylamine protonated Schiff bases were prepared, as described previously by Becker, by using trichloroacetic acid as the protonating agent. Spectrograde solvents were dried and kept over 3-,& molecular sieves and then used without further purification. The temperature of experiments ranged from 4 to 300 K and was controlled within f l K by a Helitran cryotip system (Air Products CO.) . The optical system providing the picosecond excitation pulse and the time-resolved fluorescence detection system were similar to those described previously by us.ll They briefly consist of a Spectra Physics, Model 171, mode-locked argon laser which synchronously pumps a tunable rhodamine 6G ring dye laser (Spectra Physics Model 380) emitting in the 560-620-nm range. The resulting laser pulses had at a repetition rate of 246 MHz, a temporal width of
