Conformer‐selective Photodynamics of TrpH⁺−H₂O

Abstract

Supporting information for this article is given via a link at the end of the document.

Revealing the Structure of Tryptophan in Microhydrated Complexes by Cold Ion Spectroscopy

“…Such structures were earlier identified for singly hydrated TrpH + . 36 Similar conformers were also found among those calculated herein for n = 1 and 2 structures with low free energy (named 1B 1 and 2B 1 ; Figure S5), although the very low level of the broadband UV absorption did not allow us to measure their IR spectra. These structures originate from the first excited conformer of bare TrpH + , which differ from the most stable one, mainly by the position of carboxyl/amine moieties, relative to the reminder of the molecule.…”

“…Similar to the most stable conformer of TrpH + (H 2 O) 1 , the main common structural feature of conformer 2A 1–3 is the insertion of one water molecule between the N-terminus and the indole ring (Figure ). As was discussed above, the insertion interrupts the proton−π coupling that allows fast proton transfer to the indole ring. , The structures of the three TrpH + (H 2 O) 2 conformers validated herein provide a rational explanation of this phenomenon. In these structures the positive charge of the NH 3 + group is solvated by two H-bonds, leaving one hydrogen atom of the N-terminus free of noncovalent interactions.…”

“…As was discussed above, the insertion interrupts the proton−π coupling that allows fast proton transfer to the indole ring. 12,36 The structures of the three TrpH + (H 2 O) 2 conformers validated herein provide a rational explanation of this phenomenon. In these structures the positive charge of the NH 3 + group is solvated by two Hbonds, leaving one hydrogen atom of the N-terminus free of noncovalent interactions.…”

“…This lengthens the lifetime of the excited state and sharpens UV transitions in the respective conformer. 12,36 Retaining a second water on TrpH + increases the intensity of the sharp transitions, 12 although no full conformational analysis of this complex has been done so far.…”

“…Here we explore this approach with protonated amino acid tryptophan (TrpH + ). It is a benchmark aromatic biomolecule known for its high yield of UV fluorescence in aqueous solutions. , Structure and photophysical properties of the gas-phase protonated bare and singly hydrated Trp have been the subject of numerous studies. ,,− Briefly, the most stable conformers of isolated TrpH + exhibit a proton−π interaction between the protonated N-terminus and the nearby indole ring. The coupling enables ultrafast barrierless transfer of the proton to the ring in the electronic excited state, which results in broadening of UV transitions. , In one of the two conformers of singly hydrated TrpH + , the water molecule sticks between the two groups, blocking the proton transfer.…”