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This article compares several quantum mechanical approaches to the computation of chemical shielding tensors in peptide fragments. First, we describe the effects of basis set quality up to the complete basis set (CBS) limit and level of theory (HF, MP2, and DFT) for four different atoms in trans N-methylacetamide. For both isotropic shielding and shielding anisotropy, the MP2 results in the CBS limit show the best agreement with experiment. The HF values show quite a different tendency to MP2, and even in the CBS limit they are far from experiment for not only the isotropic shielding of carbonyl carbon but also most shielding anisotropies. In most cases, the DFT values differ systematically from MP2, and small basis-set (double- or triple-zeta) results are often fortuitously in better agreement with the experiment than the CBS ones. Second, we compare the mixed basis set and ONIOM methods, combined with CBS extrapolation, for chemical shielding calculations at a DFT level using various model peptides. From the results, it is shown that the mixed basis set method provides better results than ONIOM, compared to CBS calculations using the nonpartitioned full systems. The information studied here will be useful in guiding the selection of proper quantum chemical models, which are in a tradeoff between accuracy and cost, for shielding studies of peptides and proteins.

We propose a new computational model to predict amide proton chemical shifts in proteins. In addition to the ring-current, susceptibility and electrostatic effects of earlier models, we add a hydrogen-bonding term based on density functional calculations of model peptide-peptide and peptide-water systems. Both distance and angular terms are included, and the results are rationalized in terms of natural bond orbital analysis of the interactions. Comparison to observed shifts for 15 proteins shows a significant improvement over existing structure-shift correlations. These additions are incorporated in a new version of the SHIFTS program.

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Discoloration characteristics of 3 major muscles (LD, Longissimus dorsi; PM, Psoas major; SM, Semimemebranosus) from Korean native cattle (Hanwoo) were monitored during 7 d of cold storage at 4• C. The muscles were obtained from 12 Hanwoo carcasses at 24 h postmortem. Meat color (CIE L*, a*, b*), myoglobin (Mb) concentration, chemical form, metmyoglobin (MetMb) reducing ability (MRA), mitochondria concentration, and thiobarbituric acid reactive substance (TBARS) were measured at 1, 3, 5, and 7 d of storage. Although there were no significant differences in CIE a* and b*-values between the 3 muscles at day 1, the values of PM muscle were significantly (P < 0 .05) lower than those of LD and SM muscles at day 5 and 7. PM muscle showed a rapid decrease in the oxymyoglobin (OxyMb) and an increase in MetMb, which resulted in a significantly (P < 0.05) higher percentage of MetMb in PM muscle compared to LD and SM muscles. Also, the Mb and mitochondria concentration of PM muscle was significantly (P < 0.05) higher than those of LD and SM muscles. However, there were no significant differences in MRA, pH, or TBARS between the 3 muscles during 7 d of cold storage. It was concluded that rapid discoloration (that is, MetMb accumulation) in PM muscle of Hanwoo could be due to its higher contents of Mb and mitochondria.