Secondary structure of Ac-Alan_n-LysH+^+ polyalanine peptides (nn=5,10,15) in vacuo: Helical or not?

The polyalanine-based peptide series Ac-Ala_n-LysH+ (n=5-20) is a prime example that a secondary structure motif which is well-known from the solution phase (here: helices) can be formed in vacuo. We here revisit this conclusion for n=5,10,15, using density-functional theory (van der Waals corrected generalized gradient approximation), and gas-phase infrared vibrational spectroscopy. For the longer molecules (n=10,15) \alpha-helical models provide good qualitative agreement (theory vs. experiment) already in the harmonic approximation. For n=5, the lowest energy conformer is not a simple helix, but competes closely with \alpha-helical motifs at 300K. Close agreement between infrared spectra from experiment and ab initio molecular dynamics (including anharmonic effects) supports our findings.Comment: 4 pages, 4 figures, Submitted to JPC Letter

Picosecond infrared spectra and structure of locally excited and charge transfer excited states of isotope-labeled 4- (dimethylamino)benzonitriles

Infrared spectra of the ground, charge transfer (CT), and locally excited (LE) states of isotope-labeled 4- (dimethylamino)benzonitriles (DMABNs) in the region between 1700 and 900 cm(-1) are reported. The isotopomers measured are normal DMABN, NC-C6H4-(NMe2)-N-15 (dimethylamino nitrogen labeled DMABN), and NC-C6H2D2-NMe2 (3,5-dideuterated DMABN). The infrared spectrum of the excited CT state of DMABN-d(2) is consistent with the previous band assignments for normal DMABN and DMABNs isotopically labeled on dimethylamino group. For the LE state of normal DMABN, three bands are observed at 1481, 1415, and 1399 cm(-1). This is in contrast with a previously reported transient infrared spectrum, where positions of bands due to the transient do not shift from the ground state ones. The band at ca. 1481 cm(-1) is observed for normal and N-15 labeled DMABN, but not for DMABN-d(2). Except for this point, the band positions are almost identical for the three isotope- labeled species. The vibrational transitions observed at ca. 1415 and 1398 cm(-1) are hence attributed to modes with atomic displacements localized on methyl groups and/or the part of the benzonitrile moiety adjacent to the cyano group or the cyano group itself, Quantum chemical calculations of the vibrational spectra for the CT and LE states of DMABN at present do not correctly reproduce the experimental spectra, which means that more accurate calculations are needed for a reliable analysis of these spectra

Picosecond infrared spectra and structure of locally excited and charge transfer excited states of isotope-labeled 4- (dimethylamino)benzonitriles

Infrared spectra of the ground, charge transfer (CT), and locally excited (LE) states of isotope-labeled 4- (dimethylamino)benzonitriles (DMABNs) in the region between 1700 and 900 cm(-1) are reported. The isotopomers measured are normal DMABN, NC-C6H4-(NMe2)-N-15 (dimethylamino nitrogen labeled DMABN), and NC-C6H2D2-NMe2 (3,5-dideuterated DMABN). The infrared spectrum of the excited CT state of DMABN-d(2) is consistent with the previous band assignments for normal DMABN and DMABNs isotopically labeled on dimethylamino group. For the LE state of normal DMABN, three bands are observed at 1481, 1415, and 1399 cm(-1). This is in contrast with a previously reported transient infrared spectrum, where positions of bands due to the transient do not shift from the ground state ones. The band at ca. 1481 cm(-1) is observed for normal and N-15 labeled DMABN, but not for DMABN-d(2). Except for this point, the band positions are almost identical for the three isotope- labeled species. The vibrational transitions observed at ca. 1415 and 1398 cm(-1) are hence attributed to modes with atomic displacements localized on methyl groups and/or the part of the benzonitrile moiety adjacent to the cyano group or the cyano group itself, Quantum chemical calculations of the vibrational spectra for the CT and LE states of DMABN at present do not correctly reproduce the experimental spectra, which means that more accurate calculations are needed for a reliable analysis of these spectra