Environment-Controlled Interchromophore Charge Transfer Transitions in Dipeptides Probed by UV Absorption and Electronic Circular Dichroism Spectroscopy

Abstract

Charge transfer (CT) transitions between the C-terminal carboxylate and peptide group have been investigated for alanyl-X and X-alanine dipeptides by far-UV absorption and electronic circular dichroism (ECD) spectroscopy (where X represents different amino acid residues). The spectra used in the present study were obtained by subtracting the spectrum of the cationic species from that of the corresponding zwitterionic peptide spectrum. These spectra displayed three bands, e.g., band I between 44 and 50 kK (kK = 10(3) cm(-1)), band II at 53 kK, and band III above 55 kK, which were, respectively, assigned to a n(COO-) --> pi* CT transition, a pi(COO-) --> pi* CT transition, and a carboxylate pi --> pi* (NV1) transition, respectively By comparison of the intensity, bandwidth, and wavenumber position of band I of some of the investigated dipeptides, we found that positive charges on the N-terminal side chain (for X = K), and to a minor extent also the N-terminal proton, reduce its intensity. This can be understood in terms of attractive Coulomb interactions that stabilize the ground state over the charge transfer state. For alanylphenylalanine, we assigned band I to a n(COO-) --> pi* CT transition into the aromatic side chain, indicating that aromatic side chains interact electronically with the backbone. We also performed ECD measurements at different pH values (pH 1-6) for a selected subset of XA and AX peptides. By subtraction of the pH 1 spectrum from that observed at pH 6, the ECD spectrum of the CT transition was obtained. A titration curve of their spectra reveals a substantial dependence on the protonation state of the aspartic acid side chain of AD, which is absent in DA and AE. This most likely reflects a conformational transition of the C-terminus into a less extended state, though the involvement of a side chain --> peptide CT transition cannot be completely ruled out.

Circular dichroism eigenspectra of polyproline II and β‐strand conformers of trialanine in water: Singular value decomposition analysis

“…As can be seen in the inset of Figure 4a, the negative peak in the difference CD spectrum (pH 7 spectrum 2 pH 2 spectrum) appears in between 200 and 210 nm. This spectral line shape is in fact quite similar to those of Ac-Ala and Ala-Ala in reference 41 (see Fig. 6 in this reference), which indicates that the charge transfer transition mechanism applies to the present system.…”

“…Such a small change of J rules out the possibility that the backbone conformation changes with respect to pH. It is believed that such a strong pH dependency of trialanine CD spectrum is likely related to the charge transfer transitions 41,[64][65][66][67] between the C-terminal carboxylate group and the neighboring peptide group as discussed by Dragomir et al 41 , where they examined the far-UV absorption and electronic CD spectra of Ala-X and X-Ala (where X represents different amino acid residues) and considered the difference spectra obtained by subtracting the spectrum of the cationic species from that of the corresponding zwitterionic one. In Figure 4a inset, the difference CD spectrum of trialanine obtained by subtracting the pH 2 spectrum from the pH 7 spectrum at 208C is plotted.…”

“…This difference CD spectrum is quite similar to those of Ac-Ala and Ala-Ala measured by Schweitzer-Stenner and coworkers. 41 This suggests that the charge transfer transition is an important factor for describing the pH dependency of trialanine CD spectrum. An alternative possibility for describing such a pH dependency could be an internal field effect on the involved electronic transition frequencies of the C-terminal peptide group.…”

“…Quite a number of spectroscopic studies to ultimately determine solution structures of trialanine in water have been performed. 5,[15][16][17] They include vibrational spectroscopic studies with IR absorption and isotropic and anisotropic Raman scattering, 11,12,17,[24][25][26][27] vibrational and electronic circular dichroism (CD), 9,[41][42][43][44][45][46][47][48] resonance Raman scattering, two-dimensional IR spectroscopy, 19,20 NMR, 30,31,[41][42][43][44][45][46][47][48][49] and classical and semiquantum mechanical molecular dynamics (MD) simulations. 38,39,50,51 Despite these experimental and theoretical efforts, the conclusions drawn by a number of workers using different spectroscopic techniques have been found to be often inconsistent with each other.…”

pH-Independence of Trialanine and the Effects of Termini Blocking in Short Peptides: A Combined Vibrational, NMR, UVCD, and Molecular Dynamics Study

“…This band has been previously reported by Pajcini et al 88 for glycylglycine and by Dragomir et al for AX and XA peptides, and is assignable to a n coo ->π* transition. 89 Dragomir et al showed that the frequency position of this CT band correlates well with the positive dichroic maxima of pPII in the respective CD spectrum.…”