Influence of sequence on the fragmentation of serine- and threonine-containing peptides in 252Cf-plasma desorption mass spectrometry.

The molecular weights of four linear synthetic peptides, fragments of a snake alpha-neurotoxin, were measured by 252Cf-plasma desorption mass spectrometry. The fragmentation phenomenon observed at the level of serine and/or threonine residue with a concomitant ion/fragment association is reported for a group of two peptides (B and D) in contrast with the group (A and C) in spite of the high ratio of serine and threonine, namely peptide A. The propensity for specific fragmentation of peptide D seems to be correlated to the repetitive sequence, (Gly-Ser)2. Finally, based on the m/z of the daughter-ions measured, we propose an overall mechanism as an N----O acyl shift analogous to that observed for serine- and threonine- containing peptides in solution chemistry.

Conformation and mobility of tyrosine side chain in tetrapeptides. Specific effects of cis- and trans-proline in Tyr-Pro- and Pro-Tyr-segments.

We examined the properties of tyrosine in four free tetrapeptides: Ala-Ala-Tyr-Ala (AATA), Ala-Pro-Tyr-Ala (APTA), Ala-Tyr-Ala-Ala (ATAA) and Ala-Tyr-Pro-Ala (ATPA) by CD, n.m.r. and energy calculations. Experimental data (the aromatic 1Lb signal, rotamer populations around the C alpha-C beta bond (chi 1), rotations around C beta-C gamma(chi 2), chemical shifts of ortho- and meta-protons in the phenolic ring (in aqueous and Me2SO solutions), NH proton temperature coefficients and vicinal coupling constants 3JNH-C alpha H in the backbone (Me2SO solution) were compared with calculated minimum energy conformations. We find qualitative agreement between the results of the different techniques with respect to global tendencies of conformational behaviour: we present experimental evidence showing that the presence of proline in the sequence has a more pronounced effect on the side chain organization of the residues preceding it than on one succeeding it. This steric influence of proline on its immediate neighbor is even stronger in the cis isomer than in the more common trans isomer. The strong preference for Rotamer II (chi 1 = 180 degrees) over Rotamer I (chi 1 = -60 degrees) in ATPA (cis-form) concomitant with a noticeable deviation of chi 2 is a striking example.

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

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-C beta bond) of tyrosine as a function of chemical structure and pH effects. Circular dichroism spectra between 240 and 320 nm and n.m.r. 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-chain 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 th solid state. Interestingly, agreement is excellent when we refer to crystal of tyrosine, tyrosine derivatives or small peptides (31 samples) and poor in the case of proteins. This leads to a discussion on both 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. The possible existence of a correlation between ellipticity and rotamer populations for such samples is examined.