Torgils Fossen

Colour and stability of pure anthocyanins influenced by pH including the alkaline region

This study on anthocyanin colour variation (intensity, λmax, e) over the pH range 1–9 during 60 days of storage, was conducted on petunidin 3-[6-O-(4-O-E-p-coumaroyl-O-α-l-rhamnopyranosyl)-β-d-glucopyranoside]-5-O-β-d-glucopyranoside (petanin) and cyanidin 3-O-β-d-glucopyranoside (cy3glc) at 10 and 23°C. Compared to cy3glc, petanin afforded higher colour intensity and higher or similar stability throughout the whole pH range. At pH 4.0, 84% of petanin was intact after 60 days storage at 10°C, while the corresponding solution of cy3glc was totally degraded. At pH 8.1 the colour intensity of petanin was even higher than at the lowest pH values. The visible λmax absorption of petanin after 5 days at pH 8.1 at 10°C was similar or higher than the corresponding absorptions of the fresh solutions of cy3glc at any pH. The use of anthocyanins like petanin as food colorants in slightly alkaline products (bakery, milk, egg, etc.) should therefore be considered—at least in products with limited storage time kept in a refrigerator..

Colour and stability of the six common anthocyanidin 3-glucosides in aqueous solutions

Abstract This study on anthocyanin stability and colour variation ( λ max , e ) in the pH range 1–12 during a period of 60 days storage at 10 and 23°C, was conducted on the 3-glucosides of the six common anthocyanidins. It was mostly in the alkaline region that differences in colour and stability became significant. Although it has been generally accepted that anthocyanins are stable only at low pH values, this study revealed that, for some of the anthocyanin 3-glucosides (e.g. malvidin 3-glucoside), the bluish colours were rather intense and stability relatively high in the alkaline region. Thus, they can be regarded as potential colorants for some slightly alkaline food products.

Flavonoids from red onion (Allium cepa)

Abstract Quercetin 3,7,4′-O-β-triglucopyranoside together with quercetin, quercetin 4′-O-β-glucopyranoside and quercetin 3,4′-O-β-diglucopyranoside were isolated from the pigmented scales of Allium cepa var. ‘Red Baron’. The former flavonol has previously been reported to be formed when cell cultures of a Vitis hybrid is fed with quercetin. Minor amount of taxifolin 4′-O-β-glucopyranoiside, a rare dihydroflavonol, was also detected. The structures were established on the basis of acid hydrolysis, chromatography (TLC and HPLC) and homo- and heteronuclear NMR spectroscopic techniques.

Flavonoids from blue flowers of Nymphaèa caerulea

Abstract Seven flavonols including the novel 3-(2″-acetylrhamnosides) of myricetin and quercetin ( 2 and 6 ), the rare kaempferol 3-(2″-acetylrhamnoside) and quercetin 3-(3″-acetylrhamnoside), in addition to the 3-rhamnosides of kaempferol, quercetin and were isolated from blue flowers of the African water lily Nymphaea caerulea (= Nymphaea capensis ). Their structures were elucidated by a combination of chromatography and homo- and heteronuclear two-dimensional NMR techniques and electrospray MS for compound 2 .

Solution Structure of the Human Immunodeficiency Virus Type 1 p6 Protein

The human immunodeficiency virus type 1 p6 protein represents a docking site for several cellular and viral binding factors and fulfills major roles in the formation of infectious viruses. To date, however, the structure of this 52-amino acid protein, by far the smallest lentiviral protein known, either in its mature form as free p6 or as the C-terminal part of the Pr55 Gag polyprotein has not been unraveled. We have explored the high resolution structure and folding of p6 by CD and NMR spectroscopy. Under membranous solution conditions, p6 can adopt a helix-flexible helix structure; a short helix-1 (amino acids 14–18) is connected to a pronounced helix-2 (amino acids 33–44) by a flexible hinge region. Thus, p6 can be subdivided into two distinct structural and functional domains; helix-2 perfectly defines the region that binds to the virus budding factor AIP-1/ALIX, indicating that this structure is required for interaction with the endosomal sorting complex required for transport. The PTAP motif at the N terminus, comprising the primary late assembly domain, which is crucial for interaction with another cellular budding factor, Tsg101, does not exhibit secondary structure. However, the adjacent helix-1 may play an indirect role in the specific complex formation between p6 and the binding groove in Tsg101. Moreover, binding studies by NMR demonstrate that helix-2, which also comprises the LXXLF motif required for incorporation of the human immunodeficiency virus type 1 accessory protein Vpr into budding virions, specifically interacts with the Vpr binding region, indicating that under the specific solution conditions used for structure analysis, p6 adopted a functional conformation.

Structural characterization of the HIV-1 Vpr N terminus: evidence of cis/trans-proline isomerism.

The 96-residue human immunodeficiency virus (HIV) accessory protein Vpr serves manifold functions in the retroviral life cycle including augmentation of viral replication in non-dividing host cells, induction of G2 cell cycle arrest, and modulation of HIV-induced apoptosis. Using a combination of dynamic light scattering, circular dichroism, and NMR spectroscopy the N terminus of Vpr is shown to be a unique domain of the molecule that behaves differently from the C-terminal domain in terms of self-association and secondary structure folding. Interestingly, the four highly conserved proline residues in the N terminus are predicted to have a high propensity for cis/trans isomerism. Thus the high resolution structure and folding of a synthetic N-terminal peptide (Vpr1–40) and smaller fragments thereof have been investigated. 1H NMR data indicate Vpr1–40 possesses helical structure between residues 17–32, and for the first time, this helix, which is bound by proline residues, was observed even in aqueous solution devoid of any detergent supplements. In addition, NMR data revealed that all of the proline residues undergo a cis/ trans isomerism to such an extent that ∼40% of all Vpr molecules possess at least one proline in a cis conformation. This phenomenon of cis/trans isomerism, which is unprecedented for HIV-1 Vpr, not only provides an explanation for the molecular heterogeneity observed in the full-length molecule but also indicates that in vivo the folding and function of Vpr should depend on a cis/trans-proline isomerase activity, particularly as two of the proline residues in positions 14 and 35 show considerable amounts of cis isomers. This prediction correlates well with our recent observation (Zander, K., Sherman, M. P., Tessmer, U., Bruns, K., Wray, V., Prechtel, A. T., Schubert, E., Henklein, P., Luban, J., Neidleman, J., Greene, W. C., and Schubert, U. (2003) J. Biol. Chem. 278, 43170–43181) of a functional interaction between the major cellular isomerase cyclophilin A and Vpr, both of which are incorporated into HIV-1 virions.

Structural characterization and oligomerization of PB1-F2, a proapoptotic influenza A virus protein.

Recently, a novel 87-amino acid influenza A virus protein with proapoptotic properties, PB1-F2, has been reported that originates from an alternative reading frame in the PB1 polymerase gene and is encoded in most known human influenza A virus isolates. Here we characterize the molecular structure of a biologically active synthetic version of the protein (sPB1-F2). Western blot analysis, chemical cross-linking, and NMR spectroscopy afforded direct evidence of the inherent tendency of sPB1-F2 to undergo oligomerization mediated by two distinct domains located in the N and C termini, respectively. CD and 1H NMR spectroscopic analyses indicate that the stability of structured regions in the molecule clearly depends upon the hydrophobicity of the solvent. In aqueous solutions, the behavior of sPB1-F2 is typical of a largely random coil peptide that, however, adopts α-helical structure upon the addition of membrane mimetics. 1H NMR analysis of three overlapping peptides afforded, for the first time, direct experimental evidence of the presence of a C-terminal region with strong α-helical propensity comprising amino acid residues Ile55-Lys85 connected via an essentially random coil structure to a much weaker helix-like region, located in the N terminus between residues Trp9 and Lys20. The C-terminal helix is not a true amphipathic helix and is more compact than previously predicted. It corresponds to a positively charged region previously shown to include the mitochondrial targeting sequence of PB1-F2. The consequences of the strong oligomerization and helical propensities of the molecule are discussed and used to formulate a hypothetical model of its interaction with the mitochondrial membrane.

Anthocyanins from a Norwegian potato cultivar

Abstract The main anthocyanins of purple sprouts of a Norwegian potato cultivar, Solanum tuberosum L., were isolated from a purified methanolic extract by preparative HPLC. Their structures were determined to be the novel anthocyanins, petunidin 3- O -[6- O -(4- O - E -caffeoyl- O -α-rhamnopyranosyl)-β-glucopyranoside]-5- O -β-glucopyranoside ( 1 ) (10%) and peonidin, 3- O -[6- O -(4- O - E -caffeoy1- O -α-rhamnopyranosyl)-β-glucopyranoside]-5- O -β-glucopyranoside ( 2 ) (6%) in addition to petunidin, 3- O -[6- O -(4- O-E -p- coumaroyl -O- α-rhamnopyranosyl)-β-glucopyranoside]-5- O -β-g1ucopyranoside, petanin ( 3 ) (37%) and peonidin, 3- O -[6- O -(4- O - E - p -coumaroyl- O -α-rhamnopyranosyl)-β-glucopyranoside]-5- O -β-glucopyranoside, peonanin ( 4 ) (25%). The same major anthocyanins, however, in other proportions (4, 54, and 32%, for 1 , 3 , and 4 , respectively), were also found in the thin violet zone located in the flesh 0.5–1 cm from the surface of the tubers. This is the first report on anthocyanins from S . tuberosum, which are acylated with caffeic acid.

Anthocyanins from tubers and shoots of the purple potato, Solanum tuberosum

Summary The anthocyanin content of tubers and shoots of the purple potato, Solanum tuberosum cv. Congo has been determined using various chromatographic and spectroscopic techniques to be petanin and the novel 3-O-[6-(4-ferulyl-O-α-rhamnopyranosyl)-β-glucopyranoside]-5-O-β-glucopyranosides of petunidin and malvidin. Acylated pigments constitute more than 98% of the total anthocyanin content both in tubers and shoots.

Anthocyanins with an unusual acylation pattern from stem of Allium victorialis

Abstract Two novel anthocyanins have been isolated from the stem of Allium victorialis . By means of chemical degradation and spectroscopy, especially homo- and hetero-nuclear two-dimensional NMR techniques, the structures were determined to be cyanidin 3- O -(3″,6″- O -dimalonyl-β-glucopyranoside) (76.6%) and cyanidin 3- O -(3″, O -malonyl-β-glucopyranoside) (13.8%). This is the first report of acylation of the 3-position in the sugar moiety of any anthocyanin. The stability of malonyl substitution in the 3″-position on glucose is higher than the corresponding 6″-malonylation.