Identification of peptides with 5-dimethylaminonaph-thalenesulfonyl chloride

The dansyl chloride procedure commonly used in amino acid analysis has been modified and adapted to peptide analysis. Two dansylation procedures are described. Procedure A is the simpler and more suitable for peptides not containing histidine, tyrosine, or N-terminal glutamic acid and glutamine. Procedure B is applicable to all peptides. The latter procedure employs (1) a more basic buffer which permits dansylation of the tyrosine hydroxyl and prevents the conversion of dansylglutamic and glutamine to dansylpyroglutamic acid and (2) a formic acid step which hydrolyzes the unstable imidazole-dansyl moiety of histidine. The techniques have been evaluated in a study of model peptides. They have also been applied to an examination of the purity of synthetic peptides and the characterization of a naturally occurring peptide.     

Association studies of N-acetyl-amino acid N,N-dimethylamides in carbon tetrachloride

The self-association of N-acetylglycine N,N-dimethylamide, N-acetyl-L -valine N,N-dimethylamide, and N-acetyl-L -phenylalanine N,N-dimethylamide in carbon tetrachloride was investigated by using ir and ¹H-nmr methods. It was concluded from ir measurements that the associated species is the dimer formed as a result of the simultaneous formation of two intermolecular hydrogen bonds. This is supported by the results of ¹H-nmr measurements. Thermodynamic quantities for the association were determined from the temperature and concentration dependence of the NH proton chemical shifts of the sample solutions. Compared with the Gly derivative, L -Val and L -Phe derivatives have larger values of ?ΔH for association, which shows good correlation with Δv_NH values, the difference between the maxima of the monomer and dimer bands, obtained from ir spectra. This is due to the less stable monomer conformation and to the stronger intermolecular hydrogen bonding of the dimers in L -Val and L -Phe derivatives. The line shapes of both methyl proton resonances of L -Val residue and methylene proton resonances of L -Phe residue were found to vary with concentration and temperature of the sample solutions. These data indicate that the rotation about the C^α—C^β bond is restricted by the steric hindrance present in the associated dimers. All these experimental results can be related to the fact that L -Val and L -Phe derivatives have a warped framework because of the bulky side chains, whereas the Gly derivative has a planar framework.     

Effect of N-bromosuccinimide-modification of tyrosine side chains of cardiotoxin II of the Indian cobra on biological activity

The essential role of tyrosine residue(s) of cardiotoxin II in the biological activity of the toxin was evaluated using N-bromosuccinimide. N-bromosuccinimide effected oxidation of the tyrosine residues in cardiotoxin II with enhancement in absorbance at 260 nm. The influence of various solvent media such as acetate-formate buffer (pH 4.0), 0.01 N H₂SO₄ (pH 2.0) and Tris-HCl buffer (pH 8.5) on oxidation of tyrosine residues was exa mined. In comparison with 0.01 N H₂S O₄, acetate-formate buffer could prevent secondary oxidations as revealed by lower consumption of oxidant, N-bromosuccinimide, to achieve oxidation. In Tris-HCl buffer oxidation of tyrosine did not take place effectively. N-iodo-succinimide caused only limited oxidation as evident from minor increase in absorbance at 260 nm. N-chlorosuccinimide was completely ineffective. Oxidation of cardiotoxin II with 3.75 equivalents of N-bromosuccinimide tyrosine residue led to complete loss of lethal activity. However, the derivative retained the ability to protect bacterial protoplasts from lysis in solutions of low tonicity. Unlike cardiotoxin II oxidized with N-chlorosuccinimide (50 equivalents/mol of toxin) which retained lethal activity as well as the ability to protect protoplasts from lysis, performic acid-oxidized toxin had lost both the activities.     

o-Phthalaldehyde–N-acetylcysteine polyamine derivatives: formation and stability in solution and in C18 supports

A comparative study of different derivatization procedures has been performed in order to improve the stability of the reaction products o-phthalaldehyde–N-acetylcysteine (OPA–NAC) polyamines. Procedures such as solution derivatization, solution derivatization followed by retention on a packing support, derivatization on different packing supports and on-column derivatization, have been optimized and compared. The degradation rate constant (k) of the derivative was dependent on the procedure used and on the analyte. For the spermine (the most unstable isoindol tested) k was 8±2×10⁻² min⁻¹ in solution versus 7.7±1.1×10⁻⁴ min⁻¹ on the (C₁₈) solid support. The results obtained showed that forming the derivative on the packing support (C₁₈) gave the best results following this procedure: conditioning the cartridges with borate buffer (1 ml, 0.5 M, pH 8), retention of the analyte, addition of 0.8 ml of OPA–NAC reagent, 0.2 ml borate buffer 0.8 M (pH 8) and elution of the isoindol with 3 ml of MeOH–borate buffer (9:1). The different derivatization procedures have been used to study the stability of the reaction products OPA–NAC polyamines formed in urine matrix using spermine as model compound. Similar results were obtained for standard solutions and urine samples.     

The tyrosine <Emphasis Type="Italic">O</Emphasis>-prenyltransferase SirD catalyzes <Emphasis Type="Italic">O</Emphasis>-, <Emphasis Type="Italic">N</Emphasis>-, and <Emphasis Type="Italic">C</Emphasis>-prenylations

Recently, the prenyltransferase SirD was found to be responsible for the O-prenylation of tyrosine in the biosynthesis of sirodesmin PL in Leptosphaeria maculans. In this study, the behavior of SirD towards phenylalanine/tyrosine and tryptophan derivatives was investigated. Product formation has been observed with 12 of 19 phenylalanine/tyrosine derivatives. It was shown that the alanine structure attached to the benzene ring and an electron donor, e.g., OH or NH₂, at its para-position are essential for the enzyme activity. Modifications were possible both at the side chain and the benzene ring. Enzyme products from seven phenylalanine/tyrosine derivatives were isolated and characterized by MS and NMR analyses including HSQC and HMBC and proven to be O- or N-prenylated derivatives at position C4 of the benzene rings. K _M values of six selected derivatives were found in the range of 0.10–0.68 mM. Catalytic efficiencies (K _cat/K _M ) were determined in the range of 430–1,110 s⁻¹·M⁻¹ with l-tyrosine as the best substrate. In addition, 7 of 14 tested tryptophan analogs were also accepted by SirD and converted to C7-prenylated derivatives, which was confirmed by comparison with products obtained from enzyme assays using a 7-dimethylallyltryptophan synthase 7-DMATS from Aspergillus fumigatus.     

Carbon dioxide reversibly inhibits meiosis of Xenopus laevis oocyte and the appearance of the maturation promoting factor

Full-grown Xenopus laevis oocytes were incubated in NaHCO₃ buffer equilibrated with carbon dioxide (5 to 100%). Germinal vesicle breakdown never occurred in spite of the appearance of the characteristic white spot at the animal pole. The effect of carbon dioxide was analyzed during progesterone-induced maturation. Carbon dioxide did not inhibit the early steps of maturation whereas it inhibited germinal vesicle breakdown even when applied 4 hr after the initial hormonal trigger. When oocytes were treated transiently in NaHCO₃ buffer equilibrated with carbon dioxide and further incubated in Tris buffer, drastic delay in the kinetic of germinal vesicle breakdown was observed. Inhibition of progesterone-induced maturation by carbon dioxide treatment is coincident with the time of maturation promoting factor appearance (MPF). On the basis of microinjection experiments of MPF into recipient oocytes, it was also shown that MPF expression is not inhibited by carbon dioxide and thus indicates that the late phase of MPF formation and/or MPF amplification is a carbon dioxide-sensitive period.     

Structure-function studies of N-acyl derivatives of tryptophan that function as specific cholecystokinin receptor antagonists

Benzotript, N-p-chlorobenzoyl-L-tryptophan, is a specific cholecystokinin receptor antagonist. In the present study we used dispersed pancreatic acini to examine tryptophan as well as several different N-acyl derivatives of tryptophan for their abilities to function as cholecystokinin receptor antagonists. L-Tryptophan, D-tryptophan as well as each acyl derivative tested inhibited cholecystokinin-stimulated amylase secretion and outflux of ⁴⁵Ca and there was a good correlation between the ability of a particular agent to inhibit the action of cholecystokinin on acinar function and its ability to inhibit binding of ¹²⁵I-labeled cholecystokinin to pancreatic acini. Results with butyloxycarbonyl-L-tryptophan indicated that the inhibition of the action of cholecystokinin caused by L-tryptophan and various acyl derivatives is specific, competitive and fully reversible. In functioning as a cholecystokinin receptor antagonist the relative potencies of the agents tested were: carbobenzoxyl-L-tryptophan >benzotript >benzoyl-L-tryptophan = butyloxycarbonyl-L-tryptophan >acetyl-L-tryptophan >L-tryptophan. In inhibiting the actions of cholecystokinin, native as well as N-acyl derivatives of D-tryptophan were equipotent with the corresponding compound containing L-tryptophan. Although L-tryptophan inhibited the actions of cholecystokinin, L-phenylalanine, L-methionine or L-aspartic acid, even when tested at concentrations as high as 3 mM, did not alter the action of cholecystokinin on pancreatic acini. The antagonism of the actions of cholecystokinin was not restricted to N-acyl derivatives of L-tryptophan because butyloxycarbonyl-L-methionine and butyloxycarbonyl-L-phenylalanine but not butyloxycarbonyl-L-aspartic acid also antagonized the actions of cholecystokinin. These results demonstrate that both the nature of the N-Prmacyl group and the amino acid residue are important determinants of the affinity of the antagonist for the cholecystokinin receptor. For derivatives of L-tryptophan, the more hydrophobic the N-acyl moiety, the greater the affinity of the derivative for the cholecystokinin receptor.     

Synthesis of new β-amidodehydroaminobutyric acid derivatives and of new tyrosine derivatives using copper catalyzed C–N and C–O coupling reactions

Several β-amidodehydroaminobutyric acid derivatives were prepared from N,C-diprotected β-bromodehydroaminobutyric acids and amides by a copper catalyzed C–N coupling reaction. The best reaction conditions include the use of a catalytic amount of CuI, N,N′-dimethylethylenediamine as ligand and K₂CO₃ as base in toluene at 110 °C. The stereochemistry of the products was determined using NOE difference experiments and the results obtained are in agreement with an E-stereochemistry. Thus, the stereochemistry is maintained in the case of the E-isomers of β-bromodehydroaminobutyric acid derivatives, but when the Z-isomers were used as substrates the reaction proceeds with inversion of configuration. The use of β-bromodehydrodipeptides as substrates was also tested. It was found that the reaction outcome depend on the stereochemistry of the β-bromodehydrodipeptide and on the nature of the first amino acid residue. The products isolated were the β-amidodehydrodipeptide derivatives and/or the corresponding dihydropyrazines. The same catalytic system (CuI/N,N′-dimethylethylene diamine) was used in the C–O coupling reactions between a tyrosine derivative and aryl bromides. The new O-aryltyrosine derivatives were isolated in moderate to good yields. The photophysical properties of two of these compounds were studied in four solvents of different polarity. The results show that these compounds after deprotection can be used as fluorescence markers.     

Preventing the N-terminal processing of human interferon α-2b and its chimeric derivatives expressed in Escherichia coli

We have previously shown that human interferon α-2b (IFN) produced in Escherichia coli (E. coli) is heterogeneous at the N-terminal, with three major species (Ahsan et al., 2014). These are: (a) the direct translation product of the gene retaining the N-terminal methionine, (b) a species from which the methionyl residue has been removed by E. coli methionyl aminopeptidase to give the native interferon α-2b and (c) in which the N-terminal Cys residue of the latter contains an acetyl group. In this paper we overcome this heterogeneity, using engineered interferon derivatives with phenylalanine residue directly downstream of the N-terminal methionine (Met-Phe-IFN). This modification not only prevented the removal of the N-terminal methionine by E. coli methionyl aminopeptidase but also the subsequent N-acetylation. Critically, Met-Phe-IFN had enhanced activity in a biological assay. N-terminal stabilization was also achieved by fusing human cytochrome b₅ at the N-terminal of interferon (b₅-IFN-chimera). In this case also, the protein was more active than a reciprocal chimera with cytochrome b₅ at the C-terminal of interferon (Met-IFN-b₅-chimera). This latter protein also had a heterogeneous N-terminal but addition of phenylalanine following Met, (Met-Phe-IFN-b₅-chimera), resolved this problem and gave enhanced biological activity.     

A Functional Screening of Adenosine Analogues at the Adenosine A2B Receptor: A Search for Potent Agonists

Abstract

Various adenosine analogues were tested at the adenosine A_2B receptor. Agonist potencies were determined by measuring the cyclic AMP production in Chinese Hamster Ovary cells expressing human A_2B receptors. 5′-.N-Substituted carboxamidoadenosines were most potent. 5′-N-Ethylcarboxamidoadenosine (NECA) was most active with an ECso value of 3.1 μM. Other ribose modified derivatives displayed low to negligible activity. Potency was reduced by substitution on the exocyclic amino function (N⁶) of the purine ring system. The most active N⁶-substituted derivative N⁶-methyl-NECA was 5 fold less potent than NECA. C8-and most C2-substituted analogues were virtually inactive. 1-Deaza-analogues had a reduced potency, 3-and 7-deazaanalogues were not active.     

Solid-phase synthesis of tyrosyl H-phosphonopeptides and methylphosphonopeptides

Phosphopeptides are a useful tool for the investigation of phosphorylation as a reversible post-translational modification. There is a growing interest in using mimics of phosphoamino acids involved in phosphorylation in order to study the enzymes concerned in these processes. These mimics should contain a non-hydrolysable or isoelectrically modified phosphate moiety to be used as a specific inhibitor of phosphatases and kinases. We introduce solid-phase synthesis of H- and methylphosphonopeptides as a new class of mimics of phosphotyrosyl peptides. The peptides were synthesized on solid phase using the standard fluorenyl-methyloxycarbonyl (Fmoc) strategy. Tyrosine residues were incorporated as allyl-protected derivatives, which were selectively deprotected on the resin by treatment with Pd(PPh₃)₄. The peptide resin carrying the side-chain unprotected tyrosine of the model peptide Gly-Gly-Tyr-Ala was phosphonylated with di-tert-butyl-N,N-diethyl-phosphoramidite in the presence of ¹H-tetrazole, yielding H-phosphonopeptides after trifluoroacetic acid (TFA) cleavage. Alternatively, phosphonylation of the unprotected tyrosine with O-tert-butyl-N,N-diethyl-P-methylphosphonamidite catalysed by ¹H-tetrazole and followed by oxidation led to the methylphosphonopeptides after TFA cleavage. We obtained both the H-phosphonopeptides and the methylphosphonopeptides of the tetrapeptide in high yields and purities above 90%, according to reversed-phase high-performance liquid chromatography (RP-HPLC). To investigate the general applicability of our new methodology, we synthesized phosphonopeptides up to 13 amino acids long, corresponding to recognition sequences of tyrosine kinases. After cleavage and deprotection, all phosphonopeptides were obtained in high yields and purities of about 90%, as shown by mass spectrometry. The only by-product found was the unmodified peptide. © 1997 European Peptide Society and John Wiley & Sons, Ltd.     

The effect of MII α-conotoxin and its N-terminal derivatives on Ca<Superscript>2+</Superscript>- and Na<Superscript>+</Superscript>-signals induced by nicotine in SH-SY5Y neuroblastoma cells

Nicotinic acetylcholine receptors (nAChRs) are involved in the regulation of intracellular Ca²⁺-dependent processes both in normal and pathological states. α-Conotoxins from the venom of Conus marine mollusks are a valuable tool for the investigation of the pharmacological action and functional role of nAChRs. Analogues of α-conotoxin MII labeled by Bolton-Hunter reagent (BH-MII) or fluorescein isothiocyanate (FITC-MII) on the N-terminal glycine residue have been synthesized in the present work. Fluorescence microscopy studies of SH-SY5Y neuroblastoma cells loaded with Ca²⁺ indicator Fura-2, or by both Ca²⁺ indicator Fluo-4 and Na⁺ indicator SBFI, were used to test the effect of MII modification on its ability to block Ca²⁺ and Na⁺ signals induced by nicotine. Measurements in SH-SY5Y cells showed that kinetics of the increase and recovery of the concentration of free Ca²⁺ ([Ca²⁺] _i ) upon nicotine application and washout was different from that for free Na⁺ ([Na⁺] _i ), this being evidence of differences in the mechanism of Ca²⁺ and Na⁺ homeostasis regulation. MII suppressed the nicotine-induced increase of [Ca²⁺] _i and [Na⁺] _i in a concentration-dependent manner. An additional tyrosine residue added to the N-terminus of one of the MII derivatives caused a significant decrease in the inhibitory action of MII; this decrease was even more pronounced when a large FITC label was introduced into MII. The BH-MII derivative had an inhibitory effect similar to that of unmodified α-conotoxin. MII and its iodinated derivatives are promising tools for radioligand assays.     

Structure of 2-hydroxy-5-nitrobenzylated carboxypeptidase A.

Bovine pancreatic carboxypeptidase A (EC 3.4.12.2) was treated with dimethyl (2-hydroxy-5-nitrobenzyl)sulfonium chloride at pH 7.5, resulting in a preparation which consisted primarily of a monohydroxynitrobenzylated derivative of the enzyme. Samples of the hydroxynitrobenzylated enzyme were subjected to tryptic digestion and to cyanogen bromide cleavage, and resulting peptides were isolated chromatographically. One tryptic hydroxynitrobenzyl-containing peptide was isolated; its amino acid composition was that of the N-terminal tryptic segment of carboxypeptidase Agamma (residues 8--35). Likewise, CNBr cleavage of the hydroxynitrobenzylated enzyme revealed that the hydroxynitrobenzyl group resided in the N-terminal fragment, FN (residues 8--22). Neither of these hydroxynitrobenzylated peptides contains Trp, the amino acid residue which is characteristically the site of hydroxynitrobenzylation in proteins, and each was found to contain approximately one less Asx than the corresponding native peptide. Both dansylation and automated Edman degradation procedures revealed that the N-terminal Asn of carboxypeptidase Agamma had been modified by hydroxynitrobenzylation of the enzyme. Thus the sulfonium salt reacts with carboxypeptidase A in the same manner as that established earlier for 2-hydroxy-5-nitrobenzyl bromide (Radhakrishnan, T.M., Bradshaw, R.A., Deranleau, D.A. and Neurath, H. (1970) FEBS Lett. 7, 72--76). Such reactivity of the alpha-amino group presumably reflects its unique location with respect to Trp residues in the tertiary structure of the enzyme.     

Chemo-enzymatic synthesis of a bioactive peptide containing a glutamine-linked oligosaccharide and its characterization

A bioactive peptide containing a glutamine-linked oligosaccharide was chemo-enzymatically synthesized by use of the solid-phase method of peptide synthesis and the transglycosylation activity of endo-β-N-acetylglucosaminidase. Substance P, a neuropeptide, is an undecapeptide containing two l-glutamine residues. A substance P derivative with an N-acetyl-d-glucosamine residue attached to the fifth or sixth l-glutamine residue from the N-terminal region was chemically synthesized. A sialo complex-type oligosaccharide derived from a glycopeptide of hen egg yolk was added to the N-acetyl-d-glucosamine moiety of the substance P derivative using the transglycosylation activity of endo-β-N-acetylglucosaminidase from Mucor hiemalis, and a substance P derivative with a sialo complex-type oligosaccharide attached to the l-glutamine residue was synthesized. This glycosylated substance P was biologically active, although the activity was rather low, and stable against peptidase digestion. The oligosaccharide moiety attached to the l-glutamine residue of the peptide was not liberated by peptide-N⁴-(N-acetyl-β-d-glucosaminyl) asparagine amidase F.     

Role of the N-terminal region of the a chain in β1-bungarotoxin from the venom ofBungarus multicinctus (Taiwan-banded krait)

The N-terminal α-amino groups of β₁-bungarotoxin (β₁-Bgt) fromBungarus multicinctus venom were modified with trinitrobenzene sulfonic acid and the modified derivative was separated by high performance liquid chromatography. The trinitrophenylated (TNP) derivative contained two TNP groups at the α-amino groups of A chain and B chain and showed a marked decrease in enzymatic activity. Methionine residues at positions 6 and 8 of the A chain were oxidized with chloramine T or cleaved with cyanogen bromide to remove the N-terminal octapeptide. Oxidation of methionine residues and removal of the N-terminal octapeptide caused a precipitous decrease in enzymatic activity, whereas antigenicity remained unchanged. The presence of dihexanoyllecithin influenced the interaction between β₁-Bgt and 8-antilinonaphthalene sulfonate (ANS) and revealed that β₁-Bgt consists of two types of ANS-binding sites, one at the substrate binding site of the A chain and the other might be at the B chain. The modified derivatives still retained their affinity for Ca²⁺ and ANS, indicating that the N-terminal region is not involved in Ca²⁺ and substrate binding. A fluorescence study revealed that the α-amino group of the A chain was in the vicinity of substrate binding site and that the TNP α-amino groups were in proximity to Trp-19 of the A chain. In addition, the study showed that the N-terminal region is important for stabilizing the architectural environment of Trp-19. The results, together with the proposal that Trp-19 of the A chain is involved in substrate binding, suggest that the N-terminal region of the A chain plays a crucial role in maintaining a functional active site for β₁-Bgt.     

Incorporation of N‐amidino‐pyroglutamic acid into peptides using intramolecular cyclization of α‐guanidinoglutaric acid

N‐terminal modification of peptides by unnatural amino acids significantly affects their enzymatic stability, conformational properties and biological activity. Application of N‐amidino‐amino acids, positively charged under physiological conditions, can change peptide conformation and its affinity to the corresponding receptor. In this article, we describe synthesis of short peptides, containing a new building block—N‐amidino‐pyroglutamic acid. Although direct guanidinylation of pyroglutamic acid and oxidation of N‐amidino‐proline using RuO₄ did not produce positive results, N‐amidino‐Glp‐Phe‐OH was synthesized on Wang polymer by cyclization of α‐guanidinoglutaric acid residue. In the course of synthesis, it was found that literature procedure of selective Boc deprotection using TMSOTf/TEA reagent is accompanied by concomitant side reaction of triethylamine alkylation by polymer linker fragment. It should be mentioned that independently from cyclization time and coupling agent (DIC or HCTU), the lactam formation was incomplete. Separation of the cyclic product from the linear precursor was achieved by HPLC in ammonium formate buffer at pH 6. HPLC analysis showed N‐amidino‐Glp‐Phe‐OH stability at acidic and physiological pH and fast ring opening in water solution at pH 9. The suggested method of N‐amidino‐Glp residue formation can be applied in the case of short peptide chains, whereas synthesis of longer ones will require fragment condensation approach. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.     

Derivatization and high-performance liquid chromatographic analysis of pentaazapentacosane pentahydrochloride

A rapid high-performance liquid chromatographic method for determination of the dansyl derivative of pentaazapentacosane (PAPC) pentahydrochloride has been developed. The chromatographic system uses a reversed-phase C₈ column, a mobile phase of acetic acid buffer and acetonitrile and UV detection. The dansylation conditions were optimized with a pH of 11.0 and a 20-fold dansyl chloride excess. The yield of dansyl PAPC increased 10-fold as the reaction pH was changed from 9.5 to 10.5. Under derivatization conditions of pH 8.5–11.0 and 1–30-fold excess dansyl chloride only perdansyl PAPC was found.     

1-Deoxy-d-galactonojirimycins with dansyl capped N-substituents as β-galactosidase inhibitors and potential probes for GM1 gangliosidosis affected cell lines

Two simple and reliably accessible intermediates, N-carboxypentyl- and N-aminohexyl-1-deoxy-d-galactonojirimycin were employed for the synthesis of a set of terminally N-dansyl substituted derivatives. Reaction of the terminal carboxylic acid of N-carboxypentyl-1-deoxy-d-galactonojirimycin with N-dansyl-1,6-diaminohexane provided the chain-extended fluorescent derivative. Employing bis(6-dansylaminohexyl)amine, the corresponding branched di-N-dansyl compound was obtained. Partially protected N-aminohexyl-1-deoxy-d-galactonojirimycin served as intermediate for two additional chain-extended fluorescent 1-deoxy-d-galactonojirimycin (1-DGJ) derivatives featuring terminal dansyl groups in the N-alkyl substituent. These new compounds are strong inhibitors of d-galactosidases and may serve as leads en route to pharmacological chaperones for GM1-gangliosidosis.     

Preparation of an isomorphous heavy-atom derivative of tobacco mosaic virus by chemical modification with 4-sulpho-phenylisothiocyanate

The reaction of the vulgare and U2 strains of tobacco mosaic virus with 4-sulpho-phenylisothiocyanate has been investigated. The coat protein of the U2 strain has a proline residue at its N-terminus and a lysine residue at position 53. Whereas both residues could be reacted with 4-sulpho-phenylisothiocyanate in the isolated coat protein, only proline-1 was modified during treatment of the intact virus with the same reagent, thereby showing that the loss of reactivity of the ?-amino group of lysine-53 is a consequence of the virus structure. The 4-sulpho-phenylthiocarbamoyl derivative of amino groups shows considerable tautomerism and, as a consequence, it proved possible to prepare a heavy-atom derivative of the intact U2 strain in which methyl mercury nitrate was bound by the modified N-terminal residue of the coat protein.On the other hand, when the intact vulgare strain was treated with 4-sulphophenylisothiocyanate, little or no modification of the ?-amino groups of the two lysine residues (positions 53 and 68) per polypeptide chain was observed. Taking into account previous studies on the reactivity of the amino groups of the coat protein in tobacco mosaic virus vulgare and assuming that all strains and mutants have closely similar three-dimensional structures, these experiments suggest that the N-terminal residue is more exposed (i.e. probably nearer the virus “surface”) than the side-chain of lysine-68, which in turn is more accessible than the side-chain of lysine-53. This interpretation is readily compatible with the results of X-ray diffraction analysis carried out on these chemically modified viruses (Mandelkow &; Holmes, 1974) and lends support to the hope that such methods of preparing heavy-atom derivatives of proteins will be of general use.