Pentapeptide, protonated

For this FAB experiment, a sample of the pentapeptidic enkephalin, Tyr.Gly.Gly.Phe.Leu., dissolved in glycerol was bombarded by xenon atoms. The resulting mass spectrum shows abundant protonated molecular ions at m/z 556. 

Firstly, the (negative) values of the NOE for residues of the unstructured N-terminus that do not interact with the DPC micelle surface are larger. This result is most probably due to increased saturation transfer from the water and results from increased exchange of amide protons at the used pH of 6.0 compared to that used in the absence of DPC (pH 3.1). Secondly, the values for residues from the C-terminal pentapeptide are negative in the case of NPY free in solution whereas they are positive in the micelle-bound form. This clearly indicates that the C-terminal pentapeptide is significantly rigidified upon binding to the micelle. The result is supported by the structure calculation that displays rather low RMSD values for that part... 

Urry, D. W., Cunningham, W. D., and Ohnishi, T. (1974). Studies on the conformation and interactions of elastin. Proton magnetic resonance of the repeating pentapeptide. Biochemistry 13, 609-616. 

Metaldi ef o . 112) nthesized a cyclic pentapeptide, Cyclo Qly-Ala-Gly-Gly Pro), and invest ted the 220-MHz NMR spectra in DMSO-d. Each proton gave two sets of resonance signals, so that the existence of two different conformations was considered, a nu r conformation being represented by M and a minor conformation by m. Hie chemical shift and the temperature dependence of the resonance signal, and die oinqparison with model peptides suggested the structures shown in Fig. 19... 

They also obtained the sequence of z-pentapeptides (carbamic acid) as well as the protonated molecules of these nonvolatile compounds. The presence of leucine in these compounds could be differentiated from that of isoleucine to some extent, but reproducibility of measurements remains a problem. 

Figure 5.49. Sections from the DQF-COSY spectrum of the pentapeptide Leu-enkephalin 5.4. The 2D crosspeaks and the f2 ID trace taken through these correspond to correlations within the tyrosine (Y) residue. The upper trace is taken from the conventional ID spectrum in which the P-proton resonances partially overlap with those of phenylalanine (F). The original 2D data had an f2 resolution of 1.8 Hz/pt but the ID trace was treated as described in the text to yield a final resolution of 0.4 Hz/pt.

The efficient photodecarboxylation of the keto acids (77) has been studied. The reactions involve the formation of the carbanions (78). Aqueous solutions of fenofibric acid (79) at pH 7.4 show the formation of two intermediates when subjected to laser excitation. The study has indicated that the triplet state of the acid in water is of a jtji type. Photoionization is an important process in the aqueous medium. New photoreactive phenylalanine analogues (80) and (81) have been prepared. These were incorporated into position 5 of the pentapeptide, thymopentin. The resultant derivatives were photolabile and underwent decomposition on irradiation at 365 nm. Computational methods have been used to analyse the photoreactivity of the tryptophan derivative (82). The calculations were directed towards an understanding of the quenching of the fluorescence. The results indicate that hydrogen transfer alone does not quench the fluorescence, but that an aborted decarboxylation path is involved. Proton transfer... 

The mixture of peptides referred to as gramicidins or gramicidin D is composed of a mixture of gramicidin A (e.g., valine-gramicidin A (58)), B, and C-type linear pentapeptides and other cyclic peptides. Gramicidins form dimeric complexes that create channels that allow protons, K, and other monovalent cations to rapidly transverse mitochondrial membranes and other lipid bilayers. ... 

Figure 231. The CPMG sequence performed on the pentapeptide Leu-enkephalin 2.2 in DMSO. Hie faster decay of the amide protons (left) relative to the aromatic protons of the tyrosine residue (right) results from the amide protons coupling to quadmpolar (Section 2.5). The very fast decay of the highest frequency amide proton occurs because this is in rapid chemical exchange with dissolved water, broadening the resonance significantly. The numbers show the total T2 relaxation period 2xn.

V. Renugopalakrishnan, M.A. Khaled, and D.W. Urry, Proton Magnetic Resonance and Conformational Energy Calculations of Repeat Peptides of Tfopoelastin The Pentapeptide. J. Chem. Soc., Perkin II, 111-119,1978. 

Boelens R, Ganadu ML, Verheyden P and Kaptein R (1990) Two-dimensional NMR studies on des-pentapeptide-insulin. Proton resonance assignments and... 

As with electrophoresis and NMR spectroscopy, mass spectrometry is also helping in medical research - to both identify and research the amino acid sequences in proteins. It can be used to analyse the whole protein molecule or the peptides left after breaking down the protein with specific enzymes. Figure 29.28 shows the mass spectrum of a pentapeptide that is made into a charged compound by the addition of a proton, hence the MH peak. 

---