CD and 1 H‐n.m.r. studies on the side‐chain conformation of tyrosine derivatives and tyrosine residues in di‐ and tripeptides

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Abstract
Tyrosine, tyrosine peptides and derivatives, in total 11 species, were selected as models for the study of optical properties (1Lb band of phenolic group) and side-chain arrangement (rotamers around C.alpha..sbd.C.beta. bond) of tyrosine as a function of chemical structure and pH effects. Circular dichroism spectra between 240 and 320 nm and NMR spectra were recorded for the different ionization states. Results are discussed in terms of charge effects from N- and C-terminal groups and local conformation influence on 1Lb band of the phenolic chromophore and on distribution of rotamer populations in side-chains of tyrosine. Fractions of rotamer populations were estimated from .alpha.-.beta. proton-proton coupling constants and, in the cases of tyrosine and n-acetyl-tyrosine, from 15N-.beta. nitrogen-proton coupling constants, which allow the stereospecific assignment of the .beta. and .beta.'' protons. The rotamer populations of tyrosine, averaged from all the data of the samples in solution, were then compared with their statistical distribution in the solid state. Interestingly, agreement is excellent when referring to crystals of tyrosine, tyrosine derivatives or small peptides (31 samples) and poor in the case of proteins. The validity of using statistical distributions of rotamers in proteins as reference for rotamer preferences inside small peptides in solution and the choice of the appropriate Jg and Jt values in Pachler''s approach are discussed. The possible existence of a correlation between ellipticity and rotamer populations for such samples is examined.

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