These authors observed self-association of a proline-containing tripeptide in dimethyl sulfoxide solution, but not in deuterium oxide at solute concentrations of -0.2 M.(27) We assumed all bond lengths of the proline ring to be 1.52 A, the average of bond lengths found in a number of crystal structures. Four angles are needed to specify a particular conformation.** As before, three contiguous bond angles and a dihedral angle were used for this purpose.'2 (28) (31) Accepting Cavanaugh's values Jgauehe = 2.6 Hz and r/tranr = 13.6 H z ,~' we estimate that the major form of iI consists of 21 % Va. 33% Vb. and 46% Vc. The minor structure is calculated to be a mixture of 2% Va, 24%Vb. and 74 % Vc. Cavanaugh's constants may not be transferable to this system, but probably give population estimates that are qualitatively correct.(32) (a) C. W. Haigh and R.Abstract: Kinetics of the photoisomerization of retinal (RET), a retinylidene Schiff base (SB), and a protonated Schiff base (PSB) have been studied using nanosecond laser photolysis techniques. Two paths of isomerization were observed, one occurring over 10's of nanoseconds and one occurring instantaneously in the time frame of the experiment. Data are presented identifying the former process as being mediated through the triplet state. Isomerization is the net result of a delicate balance between competing processes, and the relative amounts of isomerization via the two paths vary with the compound and with the solvent. A model is presented which allows an adequate and consistent interpretation of the experimental results. The significance of our results to visual pigments is discussed. The protonated Schiff base, which is the closest chemical analogue to rhodopsin we investigated, isomerizes through a singlet mechanism. The extremely rapid rate we observe for this isomerization is consistent with prelumirhodopsin being photogenerated from rhodopsin in picoseconds.

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The quenching of 8-methoxypsoralen (8-MOP) fluorescence by protons was observed to occur at the diffusion controlled rates in aqueous solutions at room temperature. Enhanced basicity of 8-MOP in the excited state compared to the ground state is expected on theoretical grounds. The fluorescence yield. which we determined as 6.3 x at pH 1 is surprisingly low and indicative of extremely fast radiationless decay pathways. The fluorescence lifetime of 8-MOP in neutral aqueous solution is on the order of 1-2 ns.

1977General Procedure for Second Cycloaddition. The appropriate monocycloaddition product was dissolved in enough toluene to make the solution -0.10 M. The solution was degassed by bubbling argon through for -5 min. The solution was heated in a sealed tube at 240 OC for 3-5 h. The solution was concentrated and the residue chromatographed on silica gel using ether-hexane as the solvent. 1,2,4a,4b,7,8,8a,9a-Octahydro-7,8-dimethy1-3,9H-fluorene-3,9dione (lla): yield 0.16 g (35% overall); IR (film) 1720, 1745, 1450 cm-I; N M R (CDCI3) 6 1.1-1.35 (m, 6 H), 2.0-3.1 (broad m, 12 H), 1,2,4a,4b,7,8,8a,9a-Octahydro-7-methyl-8-phenyl-3-tert-butyldimethylsilyloxy-9H-fluoren-9-one (llb): yield 0.26 g (32% overall); N M R (CC14) 6 0.20 (s, 6 H), 0.70 (d, 3 H, J = 7 Hz), 1.0 (s, 9 H), 5.05(m,1 H),6.20(m,2H),7.5(m,5H);IR(film)cm-i1735,1648, 1250, 1190, 835, 780; high-resolution mass spectrum C26H3602Si requires 408.248 47, measured 408.248 56.1,2,4a,4b,7,9a-Hexahydro-7-methyl-8-phenyl-3-tert-butyldimethylsilyloxy-9H-fluoren-9-one (13a): yield 0.24 g (30% overall); NMR (CC14) 6 0.2 (s, 6 H), 1.0 (s, 9 H), 5.1 (m, 1 H), 5.85 (m, 1 H), 6.0 (m, 1 H), 7.4 (m, 5 H); IR (film) cm-I 1720, 1660, 1250, 1190, 832; high-resolution mass spectrum C26H3402Si requires 406.232 8 1, measured 406.232 70. 1,2,4a,4b,7,9a-Hexahydro-7,9a-dimethyl-8-phenyl-3-tert-butyldimethylsilyloxy-9H-fluoren-9-one (13b): yield 0.235 g (28% overall); NMR (CC14) 6 0.20 (s, 6 H), 1.0 (s, 9 H), 5.0 (m, 1 H), 5.90 (m, 1 H), 6.05 (m, 1 H), 7.50 (m, 5 H); IR (film) cm-' 1720,1640,1460,1190, 830; high-resolution mass spectrum C27H3602Si requires 420.248 46, measured 420.248 38. 7-Methyl-3-tert-butyIdimethylsilyloxy-9~-fluoren-9-one (16). To a stirred solution of 22 mg of 15 in 2 mL of toluene was added 30 mg of DDQ. The reaction mixture was refluxed for 24 h. The solution was diluted with ether, filtered, and concentrated: yield 0.21 g (100%); N M R (CDCI3) 6 0.28 (s, 6 H), 1.02 (s, 9 H), 2.36 (s, 3 H), 7.45 (m, 5.6-5.9 (bs, 2 H). 6H); IR(CHCI3)cm-' 1700,1600, 1255, 1215,833,791;high-resolution mass spectrum C20H2402Si requires 324.1 54 56, measured 324.151 14; UV (MeOH) 256,278 nm.Abstract: The protonation constant (~K B H + ) of 8-methoxypsoralen was determined from measurements of its absorption in various concentrations of sulfuric and perchloric acids using Hammett acidity functions. Proton nuclear magnetic resonance spectra of highly acidic solutions indicated protonation of the exocyclic oxygen atom. With D2S04. proton exchange with the furan side of the molecule (4'3' positions) was observed. The intensity of fluorescence of the conjugated acid increases markedly with increasing acid concentration in solutions where absorption measurements indicate that protonation is nearly complete. This unexpected behavior is discussed and a mechanism is proposed.

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