Amide-I and -II Vibrations of the Cyclic β-Sheet Model Peptide Gramicidin S in the Gas Phase

In the condensed phase, the peptide gramicidin S is often considered
as a model system for a β-sheet structure. Here, we investigate
gramicidin S free of any influences of the environment by measuring
the mid-IR spectra of doubly protonated (deuterated) gramicidin S
in the gas phase. In the amide I (i.e., CO stretch) region,
the spectra show a broad split peak between 1580 and 1720 cm−1. To deduce structural information, the conformational space has
been searched using molecular dynamics methods and several structural
candidates have been further investigated at the density functional
level. The calculations show the importance of the interactions of
the charged side-chains with the backbone, which is responsible for
the lower frequency part of the amide I peak. When this interaction
is inhibited via complexation with two 18-crown-6 molecules, the amide
I peak narrows and shows two maxima at 1653 and 1680 cm−1. A comparison to calculations shows that for this complexed ion,
four CO groups are in an antiparallel β-sheet arrangement.
Surprisingly, an analysis of the calculated spectra shows that these
β-sheet CO groups give rise to the vibrations near 1680
cm−1. This is in sharp contrast to expectations
based on values for the condensed phase, where resonances of β-sheet
sections are thought to occur near 1630 cm−1. The
difference between those values might be caused by interactions with
the environment, as the condensed phase value is mostly deduced for
β-sheet sections that are embedded in larger proteins, that
interact strongly with solvent or that are part of partially aggregated
species.