Preresonance Raman Single-Crystal Measurements of Electronic Transition Moment Orientations in N-Acetylglycinamide Vasil Pajcini and Sanford A. Asher* Contribution from the Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed February 10, 1999. ReVised Manuscript ReceiVed September 17, 1999 Abstract: We have examined electronic coupling between the two amide electronic transitions in a dipeptide and have found strong excitonic interactions in a case where the amide planes are almost perpendicular. We compared the absorption and resonance Raman spectra of N-methylacetamide (NMA) and acetamide (AM) to that of the dipeptide N-acetylglycinamide (NAGA), which is composed of linked primary and secondary amides. We measured the transition moment magnitudes of each of these species and also determined the orientation of the preresonance Raman tensor of NAGA in a single crystal. From these single-crystal tensor values, we calculated the NAGA diagonal Raman tensor orientations and compared them to those expected for unperturbed primary and secondary amides oriented as in the NAGA crystal. Because the primary and secondary amide III vibrations are vibrationally uncoupled and nonoverlapping, we can use their intensities to determine the contributions to their resonance enhancement from the coupled NAGA electronic transitions. The Raman tensor major axes of the primary and secondary amide III and amide I vibrations do not lie in their corresponding amide planes, indicating excitonically coupled states which mix the primary and secondary amide transitions. These results are relevant to the understanding of amide coupling in peptides and proteins; the NAGA crystal conformation is similar to that of a type I -turn in peptides and proteins, with the amide planes nearly perpendicular to each other (dihedral angle 85°). Introduction The electronic properties of macromolecules can either result from the uncoupled or coupled electronic responses of individual molecular fragments. 1,2 This coupling could derive from a delocalization of the electronic transitions between linked chromophores in a through-bond manner, similar to that which occurs for the conjugated π network of a polyene, or the interaction may occur through space, through excitonic interac- tions between the transition dipoles of the linked fragments with similar electronic transition energies. In proteins and peptides, the conventional understanding of the backbone electronic excited states and transitions is that the backbone linked amide fragments interact only through excitonic interactions without any through-bond mixing of their excited states. 3,4 In R-helical peptides, for example, these through-space excitonic interactions are proposed to result in two electronic transitions oriented parallel and perpendicular to the helix axis. 3,5,6 However, this established view of peptide transitions completely neglects the possibility of through-bond interactions of the excited states which could lead to delocalization of the excited states of the amide fragments. We recently examined the electronic transitions and excited states of the simplest dipeptide, glycyl-glycine (Gly-Gly), and discovered intimate electronic coupling between the amide and carboxylate groups. 7 This coupling results in a new charge- transfer transition at ∼200 nm, which involves electron transfer from a nonbonding carboxylate orbital to the π* orbital of the amide group. 7 We also measured the direction of this charge- transfer transition moment as well as the orientations of the amide and carboxylate NV 1 (π f π*) transition moments in a hydrated crystal of Gly-Gly. 8 The charge-transfer transition moment was found to be oriented along the axis connecting the carboxylate and amide groups. Thus, in this case we found intimate interactions between the amide and carboxylate groups, which results in a new electronic transition, which is in fact, the lowest energy allowed electronic transition of dipeptides as well as of the carboxylate penultimate ends of peptides and proteins. A number of theoretical papers have recently appeared which verify the existence of charge-transfer transitions both in dipeptides and now also in polypeptides with linked amide groups. 9,10 The existence of these types of transitions should impact our understanding of peptide electronic excited states, peptide spectroscopy, and the phenomenology of electron transfer in peptides and proteins. To further probe electronic interactions between coupled amides, we have examined electronic coupling between the linked primary and secondary amide groups of N-acetylglyci- * To whom correspondence should be addressed. Phone: 412-624-8570. Fax: 412-624-0588. E-mail: asher+@pitt.edu. (1) Murrell, J. N. The Theory of the Electronic Spectra of Organic Molecules; Chapman and Hall Ltd.: New York, 1971; pp 47-269. (2) Scholes, G. D.; Ghiggino, K. P. J. Phys. Chem. 1994, 98, 4580- 4590. (3) Moffit, W. J. Chem. Phys. 1956, 25, 467-478. (4) Woody, R. W. Circular Dichroism and the Conformational Analysis of Biomolecules; Fasman, G. D., Ed.; Plenium Press: New York, 1996; pp 25-67 and references therein. (5) Moffit, W. Proc. Natl. Acad. Sci. U.S.A. 1956, 42, 736-746. (6) Brahms, J.; Pilet, H.; Damany, H.; Chandrasekharan, V. Proc. Natl. Acad. Sci. U.S.A. 1968, 60, 1130-1137. (7) Chen, X. G.; Li, P.; Holtz, J. S. W.; Chi, Z.; Pajcini, V.; Asher, S. A.; Kelly, L. A. J. Am. Chem. Soc. 1996, 118, 9705-9715. (8) Pajcini, V.; Chen, X. G.; Bormett, R. W.; Geib, S. J.; Li, P.; Asher, S. A.; Lidiak, E. G. J. Am. Chem. Soc. 1996, 118, 9716-9726. (9) Serrano-Andre ´s, L.; Fu ¨lscher, M. P. J. Am. Chem. Soc. 1996, 118, 12200-12206. (10) Serrano-Andre ´s, L.; Fu ¨lscher, M. P. J. Am. Chem. Soc. 1998, 120, 10912-10920. 10942 J. Am. Chem. Soc. 1999, 121, 10942-10954 10.1021/ja990429u CCC: $18.00 © 1999 American Chemical Society Published on Web 11/11/1999