The FTIR spectra of a selection of cyclic and linear peptides were mea
sured in DMSO and TFE, The backbone conformation in DMSO of the cyclic
models has been inferred from ROESY-based interproton connectivities
and the temperature coefficients of the NH protons. The FTIR measureme
nts give support to the following assignment of low-frequency amide I
bands: > 1645 cm(-1), open (weakly H-bonded) beta- and gamma-turns; ap
proximate to 1640 cm(-1), beta-turns (1 <-- 4 H-bonded); approximate t
o 1625 cm(-1), gamma-turns (1 <-- 3 H-bonded); < 1620 cm(-1), beta- an
d gamma-turns with bifurcated H-bondings, Solvent-dependence studies o
n the diamide models Ac-Xxx-NHCH3 (8, Xxx = Pro, Ala and Gly) suggest
that it is the inverse gamma-turn (C-7(eq)) structure which is capable
of forming a strong 1 <-- 3 intramolecular H-bond (band at approximat
e to 1625 cm(-1)). DMSO destroys 1 <-- 3 IHBs of gamma-turns and bifur
cated turn systems even in cyclic peptides but does not affect 1 <-- 4
IHBs. TFE has a stabilizing effect an both 1 <-- 4 and 1 <-- 3 H-bond
ings which gives rise to mixtures of beta- and gamma-turn conformers.
Contrary to vibrational spectroscopy, circular dichroism can different
iate between type I(III) and type II beta-turns showing class C and cl
ass B CD spectra, respectively. Based on the above findings, a class C
CD spectrum measured in TFE can reflect the predominance of type I(II
I) beta-turn conformation, but it may also be a composite of subspectr
a of H-bonded beta-turn and gamma-turn as well as open conformers. (C)
1997 Elsevier Science B.V.