We report in this paper the results of an ab initio gauge-including atomic orbital study of the influence of hydrogen-bonding on the carbonyl carbon ( 13 Cʹ) chemical shift in peptides and protein model systems. For N-methylacetamide (NMA) interacting with formamide, the experimentally observed trends of the 13 Cʹ shielding tensor elements on hydrogen bond distance in peptides are moderately well reproduced. Shielding computations were also performed on SCF-optimized helical and β-turn N-formylpentaalaninamide structures. Here, we find the calculated helix—sheet chemical shift difference to be 4.9 ppm, with the helical site deshielded, in good agreement with experimental trends observed in proteins, where alanine 13 Cʹ helical sites are typically deshielded by ~4.6 ppm when compared with sheet or sheetlike residues. The well-known 13 Cʹ helix—sheet chemical shift separation is therefore attributable to hydrogen bond formation, since Φ, Ψ effects alone (in model dipeptides) result in small upfield shifts for 13 Cʹ in helical sites. Unlike the situation with C α , C β , N H , and 19 F shielding in proteins, ab initio geometry optimization of hydrogen-bonded systems appears to be essential in order to reproduce experimental shift patterns.
ASJC Scopus subject areas
- Colloid and Surface Chemistry