UVRR SPECTROSCOPY OF THE PEPTIDE-BOND .2. CARBONYL H-BOND EFFECTS ON THE GROUND-STATE AND EXCITED-STATE STRUCTURES OF N-METHYLACETAMIDE

Ultraviolet resonance Raman (UVRR) spectra of N-methylacetamide (NMA) are markedly solvent dependent. When non-H-bonding solvents are compared with water, the amide I band is seen to intensify and shift up in frequency, while the amide II, III, and S bands weaken and shift down in frequency. The amide I frequency is linearly correlated with the solvent acceptor number (H-bond donating tendency), while the N-H stretching infrared frequency is linearly correlated with the solvent donor number (H-bond accepting tendency), but there is no cross-correlation. Thus, the ground-state structure of the amide bond is sensitive to C = O but not to N-H H-bonding., Raman excitation profiles (EPs) in water show maxima at the first allowed absorption maximum, 188 nm, for amide II, III, and S but not for amide I, which shows only preresonance enhancement from a transition at ca. 165 nm. In acetonitrile, the intensity order is amide I > III > II > S, and the EPs all rise strongly toward the blue-shifted first absorption maximum. These dramatic changes are suggested to arise from stabilization of the C = O pi*-fragment orbital by H-bonding, resulting in an altered orbital pattern and lowered energy for the first pi-pi* excited state. In stationary samples, a 1495-cm-1 band grows in with increasing laser power. It is assigned to the amide II band of the cis-amide isomer by comparison with the UVRR spectrum of the cyclic cis-amide, caprolactam. At laser powers sufficient to convert 20% of the NMA to the cis isomer, the Stokes/anti-Stokes intensity ratio of an 831-cm-1 probe band of deuterated acetonitrile revealed a temperature rise within the scattering volume of 25-degrees-C, sufficient to account for an increase of only 1% in the cis isomer fraction via heating. Most of the isomerization is therefore induced by photoexcitation. MINDO/3 calculations of the potential with respect to twisting about the C-N bond show a minimum energy for the S1 state at 90-degrees, suggesting a facile pathway for photoisomerization. The photoisomerization yield is much higher in water than in acetonitrile, an effect attributable to a deeper excited-state potential well, due to the same orbital changes that account for the altered EPs. The yield also decreases with steric bulk of substituents on the amide C and N atoms, as expected from steric clashes in the 90-degrees twisted geometry. Photoisomerization is seen for glycylglycine but is barely detectable for glycyllysine and alanylalanine. For hexaglycine, the photoisomerization yield, per amide bond, is only about 40% of that seen for glycylglycine, suggesting that only the outer two of the five amide bonds are affected. Consequently, photoisomerization is expected to be unimportant for polypeptides and proteins except for terminal glycine residues. The overtone of the amide V out-of-plane N-H bend is not observed in NMA UVRR spectra, even directly on resonance at 192 nm. The low enhancement is suggested to reflect a relatively shallow slope for the S1 twisting potential or a small degree of twist in the amide V normal coordinate.