BIOPSEL : NMR spectrum simulation

The theoretical ¹³C NMR spectrum is simulated for each structure and the deviation from the experimental spectrum is stored. To calculate the theoretical spectrum the program breaks the repeating unit into separate residues, then simulates subspectra for each one with substitution effects added and combines subspectra into the whole spectrum again. Here is the general scheme of theoretical spectrum calculation:

The most accurate residue subspectrum is obtained if the exact fragment is found in the spectral database, otherwise the program applies average substitution effects to the spectrum of the terminal residue.

The subspectrum simulation implies the following sequence of procedures:

1. The spectral data for unsubstituted (terminal) residue are searched for in the spectral database
2. If the current residue has no non-carbohydrate substituents, the same part of the spectral database is scanned for the chemical shifts of the residue substituted the way revealed. If the residue is bisubstituted at neighboring positions, the theoretical calculation of substituion effects loses additivity due to the spatially-caused conformational changes in the trimeric unit. In this case program tries to find the spectral data for similar fragments varying the type (-OH/-NH₂/deoxy) and orientation (axial/equatorial/unoriented) of the group at C2 of both substituting residues. Each of them may be with varied type, or orientation, or both, forming 4*4=16 kinds of similar fragments. If none of them is described in the spectral database, the program considers the chemical shifts search failed and utilizes theoretical effects in additive approximation.
3. If the chemical shifts search failed or the residue is substituted by at least one non-carbohydrate residue, the program simulates the spectrum for this residue using data for unsubstituted monomer and substitution effects read from the effect database.
4. The obtained spectrum of the residue is modified considering the fact that this residue itself substitutes another residue too. This includes adding effects on C2 and C5 for cyclic sugars, and on C1 for any residues. The effects on C2 and C5 have fixed values for each sugar configuration, effects on C1 are stored in the spectral database together with the chemical shift data.

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