Glycylglycine π→π* and Charge Transfer Transition Moment Orientations:  Near-Resonance Raman Single-Crystal Measurements

Abstract

We have grown a novel crystal of hydrated gycylglycine (Gly-Gly), determined its crystal structure, and measured its Raman tensor at 488, and 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 β-sheet-like structure where all of the amide planes are parallel. We report here the development of a general method to use preresonance 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 determine the transition moment orientation of the amide and carboxylate group π→π* transitions and the orientation of a charge transfer transition from the carboxylate group to the amide group. The amide π→π* (NV1) transition moment is found to be in the peptide plane at an angle of −46° ± 3° to the amide carbonyl bond. The carboxylate π→π* (NV1‘) transition moment is oriented almost parallel to a line connecting the carboxylate oxygens (4.6° off), as theoretically expected. The charge transfer band from the carboxylate to the amide chromophore is found at an angle of −83° ± 3° to the amide carbonyl bond, almost along the line of intersection of the amide and carboxylate planes.

Conformational analysis of tetrapeptides by exploiting the excitonic coupling between amide I modes

“…Finally, we neglected the possibility that like amide III 23 the intrinsic frequency of amide I might be dependent on the dihedral angles owing to the admixture of C˛H in-plane bending, as suggested by theoretical calculations. 19 In order to formulate the Raman tensor of the uncoupled amide I mode, we made use of the work of Pajcini et al 24 for diglycine combined with some recent results obtained for trialanine. 12 Figure 2.…”

“…peptide or to an electronic transition of the adjacent amino acid residue. 22,24 The parameters in Eqn (9) are not independent. The tensor element d can be obtained as a function of a and b by rotating S 1 into the principal axis system, in which the Raman tensor is diagonal.…”

“…The tensor element d can be obtained as a function of a and b by rotating S 1 into the principal axis system, in which the Raman tensor is diagonal. For diglycine, Pajcini et al 24 found that the major axis of the principal axis system forms an angle of 33.3°with the peptide's carbonyl bond for visible excitation. The transformation between S 1 and the principal axis system is given by 11…”

UV Resonance Raman Investigation of a 310-Helical Peptide Reveals a Rough Energy Landscape

“…The AmII vibration involves C-N stretching with some N-H bending (61, 62). The trough in the 1420-1500 cm -1 region is due to the 1483 cm -1 COO -stretching band from the Trp monomer which was subtracted from the original peptide spectra (64,65).A weak 1355 cm -1 band (AmIII 1 /residual W7) appears as a shoulder on the 1390 cm -1 C R -H bending band. The C R -H bending band intensity is derived from non-R-helical peptide bond conformations (61,66).…”

Correlation between Raman and X-ray crystallography data of (Pro-Pro-Gly)10

“…The amide I bands of (PPG) 10 crystals and powders are significantly different, not only for the intensity distribution of the crystal spectrum (depending on the orientation of the polarization of the exciting laser beam with respect to the main axes of the Raman tensor in the respective unit cell [30]), but also for the frequency of the Raman peaks. Analogously to (PPG) 10 crystals, spectra of (PPG) 10 powders are characterized by three distinct well-resolved amide I bands (1638, 1655, 1690 cm -1 ), suggesting only one conformation (φ and ψ pair) for each residue of the repeating triplet.…”