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Visual Pigments. 111. Determination and Interpretation of the Fluorescence Quantum Yields of Retinals, Schiff Bases, and Protonated Schiff Bases' Walter H. Waddell, Arnold M. Schaffer, and Ralph S. Becker" Contribution from the Department of Chemistry, University of Houston, Houston, T e x a s 77004. Received May 5 , 1973
Abstract: The fluorescence quantum yields ( 4 ~of) all-trans-, g-cis-, and 13-cis-retinal, their corresponding nbutylamine Schiff bases, the methyl Schiff base of all-trans-retinal, and the protonated methyl and n-butyl Schiff bases of all-trans-retinal have been determined at 77°K. No fluorescence is observed from any of the foregoing molecules a t room temperature, +F < 0.001. The dependence of the quantum yield upon the excitation wavelength was calculated from the fluorescence excitation spectrum for the isomeric retinals and Schiff bases. While +F showed only a small dependence upon the exciting wavelength for the Schiff bases, there was a large variation when exciting into the long wavelength absorption region (400-440 nm) in the retinals. Utilizing the known data on +ISC and + p ~a n analysis of the fate of the absorbed quanta is carried out for several of the molecules. The differences in the spectroscopic behavior of retinals and Schiff bases are interpreted primarily o n the basis of the existence of a 1(1i,a*) state at energies comparable t o the two lowest energy l(a,n*) states in the retinals.
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reviously, we have examined the emission spectral properties of the all-trans, 9-cis, 11-cis, and 13-cis isomers of retinal.l.2 Since the linkage to retinal in rhodopsin h a s been shown to be a Schiff base linkage cia a p r i m a r y amino g r o u p of lysine, we h a v e extended this study to include t h e n-butylamine Schiff bases of t h e retinal isomers. We also h a v e investigated the methylamine Schiff base of all-trans-retinal, as well as the p r o t o n a t e d m e t h y l and n-butyl trans Schiff bases. In a d d i t i o n , we h a v e reinvestigated a l l - t r a m r e t i n a l and h a v e quantitative data on t h e 9-cis- and 13-cis-retinals. Fluorescence h a s been r e p o r t e d from t h e t r y p t o p h a n s ? of r h o d o p s i n as well a s r h o d o p s i n 5 itself. A w e a k emission, I#JF = 0.005, centered around 575 nm w a s f o u n d a t b o t h 276 and 77°K;; however, more recent a t t e m p t s to observe this emission from r h o d o p s i n were u n s u ~ c e s s f u l . ~ Several of t h e intermediates i n t h e bleaching process of rhodopsin7 have been observed to fluoresce.* A l t h o u g h no fluorescence w a s observed from prelumirhodopsin, emission was observed from l u m i r h o d o p s i n a n d both m e t a r h o d o p s i n I and I1 at 77 OK. Emission from N-retinylidene opsin* resembled that of all-trans-retinal. l , ? T h o m p s o n g h a s r e p o r t e d emission f r o m t h e isomeric retinols a t room t e m p e r a t u r e a n d a series of comp o u n d s c o n t a i n i n g t h e retinyl c h r o m o p h o r e , including t h e methyl Schiff base of all-trans-retinal at 77°K. A fluorescence has been detected for the n-butyl Schiff base of all-trans-retinal. T h e emission maximized a t 486 nm a t 77°K in both 3MP and EPA. W h e n a (1) Paper 11: R. S. Becker, I
