IR–UV spectroscopy of jet-cooled 1-indanol: Restriction of the conformational space by hydration

Highlights

• 1-Indanol and its water clusters were characterized by gas phase IR spectroscopy.

• The monomer conformational population is sensitive to the expansion conditions.

• Hydration induces a conformational locking of 1-indanol.

• 1-Indanol(H₂O) favors a cooperative H-bond pattern with –OH⋯O(H)–H⋯π topology.

• 1-Indanol(H₂O)₂ forms a cyclic H-bond network with three OH⋯O interactions.

Abstract

The effect of hydration on a flexible amphiphilic molecule has been studied on the example of 1-hydroxyindan (1-indanol). Studies in jet-cooled conditions by means of resonance-enhanced two-photon ionization and IR–UV double resonance experiments show that the mono-hydrate 1-indanol(H₂O) is formed in a dominant isomer, as well as the di-hydrate 1-indanol(H₂O)₂. 1-Indanol(H₂O) favors a cooperative hydrogen bond pattern with –OH⋯O(H)–H⋯π topology, while 1-indanol(H₂O)₂ forms a cyclic hydrogen bond network with three OH⋯O interactions. The single conformation observed for the hydrates contrasts with the bare molecule which shows two dominant conformations, with the hydroxyl in axial or in equatorial position, respectively. Hydration therefore results in a restriction of the conformational space and conformational locking.

Introduction

Solvation plays a key role in the activity of biologically-relevant molecules, either by stabilising ionic forms relative to neutral ones or by making active functions more or less accessible. In a more subtle way, solvation may also play a role by changing the conformation of whole or part of a molecule. In this respect, water is ubiquitous as a solvent and hydration plays a major role in life-related molecules.

Complex molecules are often composed of several sub-units, either hydrophobic or hydrophilic, which both interact with the environment. Study of hydrates in supersonic expansions elegantly remedies the lack of solubility of the hydrophobic part and allows studying its interaction with water. FTIR spectroscopy [1] or microwave experiments [2], [3], [4] give valuable information on the structure of small hydrated molecules. Recently, the use of VUV lasers has allowed studying size-controlled hydrates of model molecules deprived of UV chromophore [5]. Thanks to its conformational selectivity, IR–UV double resonance spectroscopy has been especially applied to the interaction of water with the elementary bricks of life like amino-acids, neurotransmitters, sugars or peptides [6], [7], [8] and flexible molecules like arylalcohols [9], [10]. This method has proven to be very useful for studying the hydration-induced changes in the ground-state structures [11], [12], [13], their modification upon electronic excitation [14], or the changes in photophysical processes in the excited states of simple molecules [15], [16].

Especially interesting is the conformer-selective effect of hydration. Preferential solvation of one or a few conformers, sometimes not the most stable in the bare entity, results in a restriction of the conformational space which can be referred to as conformational locking [3], [12], [17], [18], [19]. Hydration can even lead to the stabilisation of metastable conformations which do not correspond to minima of the potential energy surface of the bare molecule. This behavior has been observed in flexible systems like peptides [20], or molecules displaying low-frequency motions like cycle inversion [21], [22], [23]. Hydration can also play a role in conformational selection in the early part of the expansion, by forming transient complexes, the energy content of which allows isomerisation of the solute. This intriguing behavior has raised questions as to the mechanisms controlling the formation of supersonically cooled molecules and complexes [15], [23], [24], [25].

With the aim of addressing the effect of hydration on the conformational space of a molecule with puckering motion, we have undertaken the study of the conformational preference of jet-cooled 1-hydroxyindan (1-indanol) hydrates by IR–UV double resonance experiments. Like indan, 1-indanol is built from a benzene ring fused with a 5-membered aliphatic ring. The alicyclic ring shows puckering motion, which results in two identical conformations for indan, or two different conformers in substituted indan [26], [27], [28]. While the electronic spectrum of its hydrates is already well documented [29], no direct experimental determination of their structure has been reported. Moreover, there was a controversy as to the presence of a second conformer of 1-indanol. Indeed, a single conformer has been detected in REMPI or microwave experiments [29], [30] using argon as carrier gas. In contrast, two conformers have been observed in IR–UV fluorescence dip measurements [31] or FTIR and Raman spectroscopy in helium supersonic expansions [32]. We will show how hydration selects one of the two conformers of 1-indanol. The scheme of the molecule is presented in Fig. 1.