Photophysical Properties of Dipeptides Containing Substituted 3-(Quinoxalin-6-yl) Alanine. Spectroscopic Studies and Theoretical Calculations

The photophysical properties of excited states of two hybrid dipeptides [N-(3-(2,3-diphenylquinoxaline-6-ylo)alanylo) glycine], Pe-DPhQ, and [N-(3-(2,3 (pirydine-2-ylo) quinoxaline-6-ylo)alanylo) glycine], Pe-DPiQ, have been investigated by a combined solution-state study (absorption, emission) and quantum-mechanical (ab initio, DFT) calculations. The RHF and DFT B3LYP/6-31G (d,p) computations of the ground-state isomers of Pe-DPiQ dipeptide (open, half-closed, and closed) indicate that the most stable is the "open"-type structure with approximately equal (-44.43 degrees, -43.05 degrees) dihedral angles describing rotation of the aromatic side rings with respect to the quinoxaline framework. This agrees with the literature findings that synthetic peptides are mostly unfolded. The experiments show that emission of Pe-DPiQ dipeptide is strongly temperature dependent, and at ambient and elevated temperatures the fluorescence is prevailing while the phosphorescence dominated emission spectra are observed at 77 K. On the basis of the decay curves that in the broad temperature range (rt-77 K) are biexponential (2 and 9 ns), it was concluded that at least its two major excited-state conformations may interconvert on the nanosecond time scale. The third component, of a small amplitude (10%) and a long time constant (25 ns), appears only in a new fluorescence band (570 nm) that grows up with the temperature increase. Analysis of the CIS/6-3IG(d,p) results of the excited-state isomers of Pe-DPiQ supports the interpretation of experimental emission spectra and enables one to assign two excited-state conformations, demonstrating a tendency to keep one of their two side rings coplanar relative to the central quinoxaline plane, as Pe-DPiQ-I* (41.9 degrees, 6.3 degrees) and Pe-DPiQ-II* (40.1 degrees, 4.5 degrees) isomers contributing to the room temperature (403 nm) and 363 K (570 nm) fluorescence bands, respectively. The calculations also explain the electronic character of the corresponding transitions and show that the state ordering of Pe-DPiQ resembles that of other diazines where the first singlet is of the n pi* character while the S-2 and T-1 are the pi pi* states. The reason for a strong phosphorescence is assigned to an effective spin orbit coupling of appropriate singlet and triplet states that leads to ISC transitions and in result to population of the T, state and a phosphorescence from the T-1 state. From the present study, it was concluded that incorporation of quinoxaline moiety into the model peptides does not change the useful spectroscopic properties of the fluorophore and allows one to design its new analogues with improved activity and specificity.