Characterization of the conformational probability of N-acetyl-phenylalanyl-NH2 by RHF, DFT, and MP2 computation and AIM analyses, confirmed by jet-cooled infrared data.
Computational and experimental determinations were carried out in parallel on the conformational probability of N-Acetyl-Phenylalanine-NH2 (NAPA). Ab initio computations were completed at the BLYP/6-311G(df,p), B3LYP/6-31G(d), B3LYP/6-31G(d,p), and B3LYP/6-31+G(d) levels of theory, labeled L/61fp, B/6, B/6p, and B/6+, respectively. Three experimentally identified conformers were compared with theoretical data, confirming their identities as the betaLanti, gammaLgauche+, and gammaLgauche- (BACKBONESIDECHAIN) conformers. Evidence comes from matching experimental and theoretical data for all three constituent N-H stretches of NAPA, with a Delta(Experimental-Theoretical) = approximately 1-3 cm(-1), approximately 0-5 cm(-1), and approximately 1-6 cm(-1), at the L/61fp and B/6+ levels, respectively. Corrected-ZPE relative energies were computed to be 0.14, 0.00, 0.26 and 0.00, 0.67, 0.57 (kcal*mol(-1)) for the betaLanti, gammaLgauche+, and gamma(Lgauche- conformers, respectively, at the L/61fp and B/6+ levels, respectively. The MP2/6-31+G(d) level of theory was subsequently found to give similar relative energies. Characterization of the intramolecular interactions responsible for red and blue shifting of the N-H stretches showed the existence of the following intramolecular interactions: C=O[i]- - -HN[i], (Ar[i])-Cgamma- - -HN[i+1], (Ar[i])-Cdelta-H- - -O=C[i-1] for betaLanti; C=O[i-1]- - -HN[i+1], (Ar[i])-Cgamma- - -HN[i+1], (Ar[i])-C-H- - -O=C[i] for gammaLgauche+; and C=O[i-1]- - -HN[i+1] for gammaLgauche-. Each of these interactions were further investigated and subsequently characterized by orbital population and Atoms-In-Molecules (AIM) analyses, with the identity of overlap and bond critical points (BCP) serving as 'scoring criteria', respectively. Experimental and theoretical carbonyl stretches were also compared and showed good agreement, adding further strength to the synergy between experiment and theory.
Chass
Gregory A
GA
Global Institute of COmputational Molecular and Materials Science (GIOCOMMS), 2-158 Major Street, Toronto, Ontario M5S 2K2, Canada. gchass@giocomms.org
Mirasol
Rei S
RS
Setiadi
David H
DH
Tang
Ting-Hua
TH
Chin
Wutharath
W
Mons
Michel
M
Dimicoli
Iliana
I
Dognon
Jean-Pierre
JP
Viskolcz
Bela
B
Lovas
Sandor
S
Penke
Botond
B
Csizmadia
Imre G
IG
D000409:Alanine / Q000031:analogs & derivatives / Q000737:chemistry; D000577:Amides / Q000737:chemistry; D003080:Cold Temperature; D003198:Computer Simulation; D004583:Electrons; D005998:Glycine / Q000031:analogs & derivatives / Q000737:chemistry; D006860:Hydrogen Bonding; D007259:Infrared Rays; D008958:Models, Molecular; D008968:Molecular Conformation; D010649:Phenylalanine / Q000031:analogs & derivatives* / Q000737:chemistry; D013055:Spectrophotometry, Infrared
D000577:Amides; C410804:acetylphenylalanylamide; C089248:N-acetylalaninamide; C061328:N-acetylglycinamide; D010649:Phenylalanine; D000409:Alanine; D005998:Glycine