Because of both experimental and computational challenges, protonated tryptophan has remained the last aromatic amino acid for which the intrinsic structures of low-energy conformers have not been unambiguously solved. The IR-IR-UV hole-burning spectroscopy technique has been applied to overcome the limitations of the commonly used IR-UV double resonance technique and to measure conformer-specific vibrational spectra of TrpH(+), cooled to T = 10 K. Anharmonic ab initio vibrational spectroscopy simulations unambiguously assign the dominant conformers to the two lowest-energy geometries from benchmark coupled-cluster structure computations. The match between experimental and ab initio spectra provides an unbiased validation of the calculated structures of the two experimentally observed conformers of this benchmark ion. Furthermore, the vibrational spectra provide conformer-specific signatures of the stabilizing interactions, including hydrogen bonding and an intramolecular cation-π interaction.

The intrinsic structure of an opioid peptide [Ala2, Leu5]-leucine enkephalin (ALE) has been investigated using first-principles based vibrational self-consistent field (VSCF) theory and cold ion spectroscopy. IR-UV double resonance spectroscopy revealed the presence of only one highly abundant conformer of the singly protonated ALE, isolated and cryogenically cooled in the gas phase. High-level quantum mechanical calculations of electronic structures in conjunction with a systematic conformational search allowed for finding a few low-energy candidate structures. In order to identify the observed structure, we computed vibrational spectra of the candidate structures and employed the theory at the semi-empirically scaled harmonic level and at the first-principles based anharmonic VSCF levels. The best match between the calculated "anharmonic" and the measured spectra appeared, indeed, for the most stable candidate. An average of two spectra calculated with different quantum mechanical potentials is proposed for the best match with experiment. The match thus validates the calculated intrinsic structure of ALE and demonstrates the predictive power of first-principles theory for solving structures of such large molecules.

The tremendous isomeric diversity of carbohydrates enables awide range of their biological functions but makes the identification and study of these molecules difficult. We investigated the ability of intermolecular interactions to communicate structural specificity of carbohydrates to protonated aromatic molecules in non-covalent complexes,i solated and cooled in the gas phase.Our study revealed that small structural differences between carbohydrate isomers of any type,i ncluding enantiomers,a re accurately communicated by these interactions to aromatic molecules as detectable changes in their electronic excitation spectra. The specific response of the aromatics to the isomers of carbohydrates is fine-tuned by the interplay of the various involved non-covalent bonds.These findings enable the gas-phase identification and relative quantification of any isomers of oligosaccharides in their solution mixtures using the 2D UV-MS fingerprinting technique.Carbohydrates are by far the most abundant organic molecules present in living organisms. [1] Existing in myriads of structural forms,m any of which are isomeric,c arbohydrates may function as messenger and recognition molecules in signaling the type and the state of living cells in complex chains of biological processes. [2] Based on this,cancer cells can be identified, for instance,b yd etecting their specific membrane glycoproteins, [3] which may differ from those of the healthy cells by the isoform of the glycan only,w hile viruses and bacteria adhere to appropriate cells for invasion by selective binding to specific membrane glycans, [4] distinguishing them from many other structurally similar (for example, isomeric) saccharides.T hese and many other examples illustrate the importance of comprehensive structural studies of carbohydrates,i ncluding the identification of their isoforms. [5] Them onomeric structural units of carbohydrates exist in av ariety of stereoisomeric forms,s uch as d/l-enantiomers, epimers,a nd a/b-anomers.F or instance,acyclic aldohexo-pyranose has 5stereogenic centers,which implies an existence of 2 5 = 32 stereoisomers (for example, a-d-glucose, b-lgalactose,e tc.), although not all of them are essential in nature.I nc ontrast to amino acids and nucleotides,t hese isomeric units can interconnect through different OH groups, forming regioisomers,but also at multiple points,assembling into linear and branched structural isomers.N atural modifications (for example, N-acetylation) of different units further multiply the number of possible isoforms.Human milk alone, for instance,c ontains at least 200 different oligosaccharides, many of which are essential not only for supplying energy to newborns,but also for their antibacterial defense. [6] Theenormous isomeric diversity of carbohydrates makes their identification and study exceptionally challenging.Most common analytical methods of isomeric identifications of carbohydrates,such as chromatography [7] and, more recently, ion mobility, [8] demonstrate ah igh capability in separating small is...