IR−UV Ion-Dip Spectroscopy of N-Benzylformamide Clusters:  Stepwise Hydration of a Model Peptide

Fluorescence excitation, resonant two-photon ionization (R2PI) and IR−UV ion dip spectroscopy have been used to study conformers of N-benzylformamide (NBFA) and associated clusters including hydrates with up to n = 3 water molecules. The most stable conformer has a trans arrangement of the HNCO atoms. It is distinguished from the cis conformer by a higher frequency for the NH stretch (3478 cm-1, compared to 3443 cm-1) and lower frequency for the amide I overtone (3435 cm-1, compared to 3465 cm-1). The cis conformer forms cyclic H-bonded structures with one or two water molecules, binding via strong H-bonds to the neighboring CO and NH groups. With the addition of a third water molecule, the cyclic water trimer binds to both these groups in preference to a linear chain of three waters. For trans-NBFA, a single water binds to the carbonyl group and is further stabilized by dispersive CH···Owater interactions. Two water molecules bind to the NH group instead and form a bridge to the π-system of the aromatic ring. A heterodimer species is also observed, composed of cis- and trans-NBFA. It is stabilized by NHtrans···OCcis and NHcis···πtrans H-bonds, which give rise to shifts in the NH stretch frequency of −102 and −28 cm-1, respectively. Flexibility of the amide side chain plays a key role in promoting additional CH···Owater interactions in these clusters. When compared to the unsolvated conformers, some of the clusters exhibit considerable distortion in the dihedral angle τ1 (C2C1CαN) and in τ2 (C1CαNC), equivalent to the Ramachandran angle φ in proteins. Solvation also affects the photophysics of NBFA, as the clusters show normal fluorescence behavior while the S1 states of the isolated molecules are affected by a competing, nonfluorescent decay process.