Microhydration of Phenyl Formate: Gas-Phase Laser Spectroscopy, Microwave Spectroscopy, and Quantum Chemistry Calculations.

Herein, we have investigated the structure of phenyl formate⋅⋅⋅water (PhOF⋅⋅⋅H2 O) dimer and various non-covalent interactions present there using gas-phase laser spectroscopy and microwave spectroscopy combined with quantum chemistry calculations. Two conformers of PhOF⋅⋅⋅H2 O (C1 and T1), built on the two cis/trans conformers of the bare molecule, have been observed in the experiment. In cis-PhOF, there is an nCO → π A r * ${{{\rm \pi }}_{{\rm A}{\rm r}}^{{\rm {^\ast}}}}$ interaction between the lone-pair orbital of the carbonyl oxygen atom and the π* orbital of the phenyl ring, which persists in the monohydrated C1 conformer of PhOF⋅⋅⋅H2 O according to the NBO and NCI analyses. On the other hand, this interaction is absent in the trans-PhOF conformer as the C=O group is away from the phenyl ring. The C1 conformer is primarily stabilized by an interplay between O-H⋅⋅⋅O=C hydrogen bond and O-H⋅⋅⋅π interactions, while the stability of the T1 conformer is primarily governed by the O-H⋅⋅⋅O=C hydrogen bond. The most important finding of the present work is that the conformational preference of the PhOF monomer is retained in its monohydrated complex.

Sequence dependent folding motifs of the secondary structures of Gly-Pro and Pro-Gly containing oligopeptides.

Folding motifs of the secondary structures of peptides and proteins are primarily based on the hydrogen bonding interactions in the backbone as well as the sequence of the amino acid residues present. For instance, the ß-turn structure directed by the Pro-Gly sequence is the key to the ß-hairpin structure of peptides/proteins as well as a selective site for the enzymatic hydroxylation of pro-collagen. Herein, we have investigated the sequence dependent folding motifs of end-protected Gly-Pro and Pro-Gly dipeptides using a combination of gas phase laser spectroscopy, quantum chemistry calculations, solution phase IR and NMR spectroscopy and single crystal X-Ray diffraction (XRD). All three observed conformers of the Gly-Pro peptide in the gas phase have been found to have extended ß-strand or polyproline-II (PP-II) structures with C5-C7 hydrogen bonding interactions, which correlates well with the structure obtained from solution phase spectroscopy and XRD. On the other hand, we have found that the Pro-Gly peptide has a C10/ß-turn structure in the solution phase in contrast to the C7-C7 (i.e. 27-ribbon) structure observed in the gas phase. Although the lowest energy structure in the gas phase is not C10, we find that C7-C7 is an abundantly found structural motif of Pro-Gly containing peptides in the Cambridge Structural Database, indicating that the gas phase conformers are not sampling any unusual forms. We surmise that the role of the solvent could be crucial in dictating the preferential stabilization of the C10 structure in the solution phase. The present investigation provides a comprehensive picture of the folding motifs of the Gly-Pro and Pro-Gly peptides observed in the gas phase and condensed phase weaving a fine interplay of the intrinsic conformational properties, solvation, and crystal packing of the peptides.

Observation of an Unusually Large IR Red-Shift in an Unconventional S-H···S Hydrogen-Bond.

The S-H···S non-covalent interaction is generally known as an extremely unconventional weak hydrogen-bond in the literature. The present gas-phase spectroscopic investigation shows that the S-H···S hydrogen-bond can be as strong as any conventional hydrogen-bond in terms of the IR red-shift in the stretching frequency of the hydrogen-bond donor group. Herein, the strength of the S-H···S hydrogen-bond has been determined by measuring the red-shift (∼150 cm-1) of the S-H stretching frequency in a model complex of 2-chlorothiophenol and dimethyl sulfide using isolated gas-phase IR spectroscopy coupled with quantum chemistry calculations. The observation of an unusually large IR red-shift in the S-H···S hydrogen-bond is explained in terms of the presence of a significant amount of charge-transfer interactions in addition to the usual electrostatic interactions. The existence of ∼750 S-H···S interactions between the cysteine and methionine residues in 642 protein structures determined from an extensive Protein Data Bank analysis also indicates that this interaction is important for the structures of proteins.

Observation of a weak intra-residue C5 hydrogen-bond in a dipeptide containing Gly-Pro sequence.

Specific folded structures of peptides and proteins depend on the sequence of various amino acid residues as well as different types of noncovalent interactions induced by the backbone as well as side-chains of those residues. In general, secondary structures of peptides and proteins are stabilized by C6 (δ-turn), C7 (γ-turn), C10 (ß-turn), C13 (α-turn), and C15 (π-turn) hydrogen-bonded rings formed through inter-residue interactions. However, it has been reported recently that an intraresidue C5 hydrogen-bond, which is relatively weak in strength, can contribute significantly to the stability of peptides and proteins. The C5 hydrogen-bond is mostly present in the ß-sheet structures of peptides and proteins along with other inter-residue noncovalent interactions. In this work, we have studied structures and conformational preferences of a dipeptide Z-Gly-Pro-OH (Z = benzyloxycarbonyl) using mass-selected vibrationally resolved electronic spectroscopy and IR-UV double resonance spectroscopy coupled with quantum chemistry calculations. Two conformers of the peptide are observed in the experiment. One of the conformers has an extended ß-strand type structure stabilized by C5 hydrogen-bonding, while the other one is folded through O-H ⋯ π interaction. The noncovalent interactions present in the two observed structures of the peptide are validated by natural bond orbital and noncovalent interaction calculations.