Is β-homo-proline a pseudo-γ-turn forming element of β-peptides? An IR and VCD spectroscopic study on Ac-β-HPro-NHMe in cryogenic matrices and solutions.
In order to test the pseudo-γ-turn forming capability of β-homo-proline (β(3)-HPro) 2-[(2S)-1-acetylpyrrolidin-2-yl]-N-methylacetamide (Ac-β(3)-HPro-NHMe) was synthesized and its potential energy landscape was investigated by infrared (IR) and vibrational circular dichroism (VCD) spectroscopy combined with density functional calculations. Based upon a comparison between experimental and computed spectra three different pseudo-γ-turn-like trans conformers and a cis conformer were identified in low-temperature Ar and Kr matrices. The computations in agreement with the observations reveal that, in contrast to its α-Pro analogue, the room-temperature abundance of the cis conformer is significant, falling above 10% in the isolated phase. Furthermore, solution-phase vibrational spectra and computations show that the cis conformer is predominant in polar solvents. This result indicates that β(3)-HPro is significantly less apt to form pseudo-γ-turns when compared to the γ-turn forming tendency of α-proline. The present study also shows that the interpretation of solution-phase VCD spectra of flexible molecules should be done with extra caution.
Góbi
Sándor
S
Laboratory of Molecular Spectroscopy, Institute of Chemistry, Eötvös University, PO Box 32, H-1518, Budapest 112, Hungary.
Knapp
Krisztina
K
Vass
Elemér
E
Majer
Zsuzsanna
Z
Magyarfalvi
Gábor
G
Hollósi
Miklós
M
Tarczay
György
G
D002942:Circular Dichroism; D010455:Peptides / Q000737:chemistry*; D011392:Proline / Q000031:analogs & derivatives* / Q000737:chemistry; D011759:Pyrrolidines / Q000138:chemical synthesis / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry*; D013055:Spectrophotometry, Infrared
D010455:Peptides; D011759:Pyrrolidines; D012996:Solutions; D011392:Proline; C010983:homoproline