ABSTRACT). We hypothesize that upper and lower bounds for phloem flow velocity may exist: when phloem flow velocity is too high, parietal organelles may be stripped away from sieve tube walls; when sap flow is too slow or is highly variable, phloem-borne signalling could become unpredictable.

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Monolayers of a series of 1-alkynes, from 1-dodecyne to 1-octadecyne, have been prepared on the hydrogen-terminated Si(100) surface via a thermal reaction of the organic compound with this Si surface. An efficient procedure is presented for the synthesis of 1-alkynes from the corresponding 1-alkenes. The resulting monolayers were characterized by water contact angle measurements, ATR infrared spectroscopy, and X-ray reflectivity. The results show that these 1-alkynes give well-ordered, covalently bonded monolayers, which are at least as ordered as those of the corresponding 1-alkenes. The exact binding geometry of the 1-alkyne to the Si surface was investigated. The results from IR spectroscopy and X-ray reflectivity measurements indicate that the 1-alkynes form two Si−C bonds to the surface per reacting molecule. Quantum mechanical calculations confirm that this formation of two Si−C bonds is not only chemically possible but also energetically much more favorable than formation of only one Si−C bond per reacting molecule.

This article presents a new formalism to perform a quantitative fluorescence analysis using the Stokes shift of AEDANS-labeled cysteine mutants of M13 major coat protein incorporated in lipid bilayers. This site-directed fluorescence spectroscopy approach enables us to obtain the topology of the bilayer-embedded transmembrane alpha-helix from the orientation and tilt angles, and relative bilayer location. Both in pure dioleoylphosphatidylcholine and dioleoylphosphatidylcholine/dioleoylphosphatidylglycerol (4:1 mol/mol) bilayers, which have a similar bilayer thickness, the tilt angle of the transmembrane helix of the coat protein turns out to be 23 degrees +/- 4. Upon decreasing the hydrophobic thickness on going from dieicosenoylphosphatidylcholine to dimyristoylphosphatidylcholine, the tilt angle and orientation angle of the transmembrane alpha-helix change. The protein responds to an increase of hydrophobic stress by increasing the tilt angle so as to keep much of its hydrophobic part inside the bilayer. At the same time, the transmembrane helix rotates at its long axis so as to optimize the hydrophobic and electrostatic interactions of the C-terminal phenylalanines and lysines, respectively. The increase of tilt angle cannot completely keep the hydrophobic protein section within the bilayer, but the C-terminal part remains anchored at the acyl-chain/glycerol backbone interface at the cost of the N-terminal section. In addition, our analysis results in the profile of the dielectric constant of the hydrophobic domain of the bilayer. For all phospholipid bilayers studied the profile has a concave shape, with a value of the dielectric constant of 4.0 in the center of the bilayer. The dielectric constant increases on approaching the headgroup region with a value of 12.4 at the acyl-chain/glycerol backbone interface for the various phosphatidylcholines with different chain lengths. For dioleoylphosphatidylcholine/dioleoylphosphatidylglycerol (4:1 mol/mol) bilayers the value of the dielectric constant at the acyl-chain/glycerol backbone interface is 18.6. In conclusion, the consistency of our analysis shows that the applied cysteine-scanning mutagenesis method with AEDANS labeling of a helical transmembrane protein in combination with a quantitative formalism offers a reliable description of the lipid bilayer topology of the protein and bilayer properties. This also indicates that the spacer link between the protein and AEDANS label is long enough to monitor the local polarity of the lipid environment and not that of the amino-acid residues of the protein, and short enough to have the topology of the protein imposing on the fluorescence properties of the AEDANS label.

A flavodoxin from Azotobacter vinelandii is chosen as a model system to study the folding of a/P doubly wound proteins. The guanidinium hydrochloride induced unfolding of apoflavodoxin is demonstrated to be reversible. Apoflavodoxin thus can fold in the absence of the FMN cofactor. The unfolding curves obtained for wild-type, C69A and C69S apoflavodoxin as monitored by circular dichroism and fluorescence spectroscopy do not coincide. Apoflavodoxin unfolding occurs therefore not via a simple two-state mechanism. The experimental data can be described by a three-state mechanism of apoflavodoxin equilibrium unfolding in which a relatively stable intermediate is involved. The intermediate species lacks the characteristic tertiary structure of native apoflavodoxin as deduced from fluorescence spectroscopy, but has significant secondary structure as inferred from circular dichroism spectroscopy. Both spectroscopic techniques show that thermally-induced unfolding of apoflavodoxin also proceeds through formation of a similar molten globule-like species. Thermal unfolding of apoflavodoxin is accompanied by anomalous circular dichroism characteristics: the negative ellipticity at 222 nM increases in the transition zone of unfolding. This effect is most likely attributable to changes in tertiary interactions of aromatic side chains upon protein unfolding. From the presented results and hydrogen/deuterium exchange data, a model for the equilibrium unfolding of apoflavodoxin is presented.Keywords: apoflavodoxin; circular dichroism; equilibrium unfolding; fluorescence; molten globule-like species; NMR; protein stability; three-state model In contrast to most protein folds, the flavodoxin-like fold is shared by many (i.e., nine) superfamilies (Brenner, 1997 a broad range of unrelated proteins with different functions like catalases, chemotactic proteins, lipases, esterases, and flavodoxins. They are all characterized by a five-stranded parallel P-sheet surrounded by a-helices at either side of the sheet. Of these proteins, a flavodoxin is chosen by us as a model system to study protein folding and stability. General rules governing protein folding are only beginning to emerge and there is a strong need for additional well-characterized protein systems to obtain a better understanding of the fundamental rules describing protein folding. By studying the folding of flavodoxin, we extend the number of proteins under investigation and expect to learn the rules by which other, if not all, proteins with a flavodoxin-like topology fold.Flavodoxins are a group of small flavoproteins that function as low-potential one-electron carriers and contain a noncovalently bound FMN cofactor (Mayhew & Tollin, 1992). The protein investigated by us is flavodoxin 11 from Azotobacter vinelandii (strain ATCC 478), henceforth designated flavodoxin. Its gene has been cloned in Escherichia coli and brought to high expression in our laboratory. The protein consists of 179 amino acid residues and belongs to the class of "long-chain" flavodoxins (Ta...