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 (C-13’) chemical shift in peptides and protein model systems. For N-methylacetamide (NMA) interacting with formamide, the experimentally observed trends of the C-13’ Shielding tenser elements on hydrogen bond distance in peptides are moderately well reproduced. Shielding computations were also performed on SCF-optimized helical and beta-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 C-13’ helical sites are typically deshielded by similar to 4.6 ppm when compared with sheet or sheetlike residues. The well-known C-13’ helix-sheet chemical shift separation is therefore attributable to hydrogen bond formation, since phi, psi effects alone (in model dipeptides) result in small upfield shifts for C-13’ in helical sites. Unlike the situation with C-alpha, C-beta, N-H, and F-19 shielding in proteins, ab initio geometry optimization of hydrogen-bonded systems appears to be essential in order to reproduce experimental shift patterns.