13C shielding effects at γ-carbon atoms in the side-chains of α-aminoacids

Abstract

The ¹³C shielding contributions at the γ-carbons from the α-carbon substituents in the side-chains of α-aminoacids have been estimated using data from 1-aminocyclohexanecarboxylic acid derivatives and amino-acids where the rotamer populations are known from studies of three-bond spin coupling constants. It is possible to use these values to estimate rotamer populations in amino-acids and in some cases to provide information not available from measuring J_{α–CH,β–CH} spin coupling constants. However, the estimated errors using the chemical shift method are somewhat larger than those encountered in the methods using spin coupling constants. For peptides and proteins additional errors will arise if there are shielding contributions at the γ-carbons from sources other than the immediate α-carbon substituents (such as ring current effects and electric field effects from neighbouring residues). In these cases the method will only become useful if corrections can be made for these other sources of shielding.