Terms proton shielding constant

The proton chemical shifts are dominated, in diamagnetic systems, by the electron population pis of the Is orbital. The magnetic shielding constant for a nucleus is mainly expressed in terms of the so-called local... 

As discussed earlier for protons, there are a number of factors which affect chemical shifts of atoms. The extent of shielding which a atom experiences can be considered in terms of the shielding constant (t which is made up of a number of contributing components ... 

In RNase the protonated histidyl residues and the cystyl residues are apparently the most reactive sites in reactions with the hydrated electron. This was concluded from measurements of the absolute reaction rates of amino acids, peptides, and proteins (I, 2) and from an analysis of the reactivity of proteins in terms of the reactivities of the individual amino acids. At a pH of about 9 the histidyl residues are dissociated and have a low reactivity. The absolute reaction rate constant of RNase at this pH is 5 X 109 M-1 sec.-1 (2) and can only be explained on the assumption that the four disulfide bridges inside the protein molecule contribute to its reactivity. This contribution, however, is less than would be expected for cystyl residues which show a rate constant of about Ke-aq = 2 X 1010 M 1 sec.-1. The lack of a full contribution from the disulfide bonds was ascribed to the shielding of these reactive groups by the unreactive peptide chains. 

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