<p>We investigate the UV absorption spectra of a series of cationic GxG (where
x denotes a guest residue) peptides in aqueous solution and find that the
spectra of a subset of peptides with x = A, L, I, K, N, and R (and, to a lesser
extent, peptides with x = D and V) vary as a function of temperature. To
explore whether or not this observation reflects conformational dependencies,
we carry out time-dependent density functional calculations for the polyproline
II (pPII) and β-strand conformations of a limited set of tripeptides (x = A, V,
I, L, and R) in implicit and explicit water. We find that the calculated CD
spectra for pPII can qualitatively account for the experimental spectra irrespective
of the water model. The reproduction of the <i>β</i>-strand
UV-CD spectra, however, requires the explicit consideration of water. Based on
the calculated absorption spectra, we explain the observed temperature
dependence of the experimental spectra as being caused by a reduced dispersion
(larger spectral density) of the overlapping NV<sub>2</sub> band and the
influence of water on electronic transitions in the β-strand conformation.
Contrary to conventional wisdom, we find that both the NV<sub>1</sub> and NV<sub>2</sub>
band are the envelopes of contributions from multiple transitions that involve
more than just the HOMOs and LUMOs of the peptide groups. A natural transition
orbital analysis reveals that some of the transitions with significant
oscillator strength have a charge-transfer character. The overall manifold of
transitions, in conjunction with their strengths and characters, depends on the
peptide’s backbone conformation, peptide hydration, and also on the side chain
of the guest residue. It is particularly noteworthy that molecular orbitals of
water contribute significantly to transitions in <i>β</i>-strand conformations. Our results reveal that peptide groups,
side chains, and hydration shells must be considered as an entity for a physically
valid characterization of UV absorbance and circular dichroism. </p>
Water-Mediated Electronic Structure of Oligopeptides Probed by Their UV Circular Dichroism, Absorption Spectra, and Time-Dependent DFT Calculations
