A simple method to adjust inconsistently referenced 13C and 15N chemical shift assignments of proteins.

Inconsistent 13C and 15N chemical shift referencing is a continuing problem associated with protein chemical shift assignments deposited in BioMagResBank (BMRB). Here we describe a simple and robust approach that can quantitatively determine the 13C and 15N referencing offsets solely from chemical shift assignment data and independently of 3D coordinate data. This novel structure-independent approach permitted the assessment and determination of 13C and 15N reference offsets for all protein entries deposited in the BMRB. Tests on 452 proteins with known 3D structures show that this structure-independent approach yields 13C and 15N referencing offsets that exhibit excellent agreement with those calculated on the basis of 3D structures. Furthermore, this protocol appears to improve the accuracy of chemical shift-derived secondary structural identification, and has been formally incorporated into a computer program called PSSI (http//www.pronmr.com).