Introduction

Figure 1. Two essential amino acids, phenylalanine and tryptophan, and some of their important 'products' made in the body.

Despite their familiarity and importance, the conformational landscapes of most of the natural amino acids are unknown territory. In aqueous solution or in crystalline media, they adopt charge-separated, zwitterionic structures, but it is generally agreed that they all exist as neutral molecules when isolated in the gas phase, as do their residues in peptide chains. In some cases this has been confirmed experimentally, e.g. through the analysis of the microwave and millimetre wave spectra of glycine and a-alanine (see Fig. 2).
 

Figure 2.  Lowest energy conformations of Gly and Ala (from Godfrey: JACS 1995, 117, 2019 and JACS 1993, 115, 9687; Stepanian: JPCA 1998, 102, 1041 and 4623): conformer I benefits from a (bifurcated) hydrogen bonded interaction between the amino and carbonyl groups, and from the preferred syn-configuration of the carboxylic acid group. In conformer II, the syn-conformation is sacrified to allow the stronger hydrogen-bonded interaction between the acidic proton and the amino group. In conformer III (not detected in the gas phase), the syn-conformation is recovered, permiting a weak, bifurcated hydrogen bond between the amino and hydroxy groups.

In our group the neutral conformational structures of monomeric Phe and Trp have been determined in the gas phase using the powerful combination of ultraviolet and infrared laser spectroscopy coupled with mass spectrometric detection, and high level ab initio computation (see Fig. 3). Ab initio calculations lead to the same conclusion as the experiments: the zwitterionic structure of phenylalanine, for example, is predicted to lie ca. 90 kJ/mol above the global minimum on the neutral potential energy hypersurface.

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(b)

Figure 3. (a) Low energy conformers of Phe: the energy ordering of the aliphatic amino acids is reversed for the aromatic amino acids. The most stable structures show an additional stabilising interaction between the (polarised) amino group and the p-electrons on the aromatic ring to generate a co-operative 'daisy-chain' of hydrogen bonds, OH···NH2···p (MP2/6-311G** optimised structures, zero-point correction from B3LYP/6-31+G*). (b) R2PI (top) and UV-UV holeburn (bottom) spectra of Phe, showing the 6 conformer origins labelled A-E and X, corresponding to the structures shown on the left. They were assigned via IR-ion depletion spectroscopy, probed on these origin bands.

A start has been made to investigate the structural effects of hydration on the known, bare structures of aromatic aminoacids. For Trp this has resulted in a series of experiments and calculations on the Trp·W1-3 clusters (see Fig. 4).
 

(a)	(b)
Figure 4. (a) R2PI spectrum of Trp.Wn recorded in the Trp.Wn+ mass channel, with underneath UV-holeburn spectra recorded in the W1 and monomer channels.  (b) IR-UV ion dip (inverted) spectrum on band 'P', with frequency predictions of calculated structures.  (c) Ab-initio calculated structures by Romano T. Kroemer (MP2/6-31+G*// B3LYP/6-31+G*, z.p. corr. (B3LYP) included).	(c)

The time-of-flight mass spectrum of hydrated Trp generated under laser ablation conditions, reflecting the population of the different clusters, differs strikingly from the equivalent mass spectrum generated under thermal (oven) evaporation conditions: the laser ablated sample shows a  propensity for generating signal in the Trp·W3⁺ mass channel, indicating a uniquely stable cluster ion, either formed directly or through efficient cluster fragmentation of larger clusters. Encouraged by this observation and by the results of the ab initio computation, further infrared ion dip experiments are in progress to explore their possible association with triply hydrated zwitterionic structures, stabilised by hydration in the gas phase.

Our current projects on amino acids and peptides

References

A spectroscopic and computational exploration of tryptophan–water, L.C. Snoek , R.T. Kroemer, and J.P. Simons, Phys. Chem. Chem. Phys., 2002, 4, 2130.

Conformational landscapes in amino acids: infrared and ultraviolet ion dip spectroscopy of tryptophan, L.C. Snoek, R.T. Kroemer, M.R. Hockridge and J.P. Simons, Phys. Chem. Chem. Phys., 2001, 3, 1819-1826.

Conformational landscapes in amino acids: infrared and ultraviolet ion-dip spectroscopy of phenylalanine in the gas phase, L.C. Snoek, E.G. Robertson, R.T. Kroemer and J.P. Simons, Chem. Phys. Lett., 2000, 321, 49-56.