GLYCYLGLYCINE PI-]PI-ASTERISK ACID CHARGE-TRANSFER TRANSITION-MOMENT ORIENTATIONS - NEAR-RESONANCE RAMAN SINGLE-CRYSTAL MEASUREMENTS

We have grown a novel crystal of hydrated gycylglycine (Gly-Gly), dete rmined its crystal structure, and measured its Raman tensor at 488, an d 244 nm, close to resonance. This crystal has 8 molecules of (Gly-Gly ) and 12 molecules of water per unit cell. The Gly-Gly hydrogen-bonds in a beta-sheet-like structure where all of the amide planes are paral lel. We report here the development of a general method to use prereso nance single-crystal Raman measurements to determine the direction of the molecular electronic transition moments. We utilize the UV (244 nm ) excited Raman tensor and relationships derived here to accurately de termine the transition moment orientation of the amide and carboxylate group pi-->pi transitions and the orientation of a charge transfer t ransition from the carboxylate group to the amide group. The amide pi- ->pi (NV'(l)) transition moment is found to be in the peptide plane a t an angle of -46 degrees +/- 3 degrees to the amide carbonyl bond. Th e carboxylate pi-->pi (NV'(l)) transition moment is oriented almost p arallel to a line connecting the carboxylate oxygens (4.6 degrees off) , as theoretically expected. The charge transfer band from the carboxy late to the amide chromophore is found at an angle of -83 degrees +/- 3 degrees to the amide carbonyl bond, almost along the line of interse ction of the amide and carboxylate planes.