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 io n dip spectroscopy have been used to study conformers of N-benzylformamide (NBFA) and associated clusters including hydrates with up to n = 3 water mo lecules. The most stable conformer has a trans arrangement of the HNCO atom s. 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 confor mer forms cyclic H-bonded structures with one or two water molecules, bindi ng via strong H-bonds to the neighboring C=O and NH groups. With the additi on of a third water molecule, the cyclic water trimer binds to both these g roups in preference to a linear chain of three waters. For trans-NBFA, a si ngle water binds to the carbonyl group and is further stabilized by dispers ive CH . . .O-water interactions. Two water molecules bind to the NH group instead and form a bridge to the, pi -system of the aromatic ring. A hetero dimer species is also observed, composed of cis- and trans-NBFA. It is stab ilized by NHtrans. . .O=C-cis and NHcis. . . pi (trans) H-bonds, which give rise to shifts in the NH stretch frequency of -102 and -28 cm(-1), respect ively. Flexibility of the amide side chain plays a key role in promoting ad ditional CH . . .O-water interactions in these clusters. When compared to t he unsolvated conformers, some of the clusters exhibit considerable distort ion in the dihedral angle tau (1) (C2C1CalphaN) and in tau (2) (C1CalphaNC) , equivalent to the Ramachandran angle phi in proteins. Solvation also affe cts the photophysics of NBFA, as the clusters show normal fluorescence beha vior while the SI states of the isolated molecules are affected by a compet ing, nonfluorescent decay process.