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PubMed1997

The specificity of antibodies raised against a T cell peptide is influenced by peptide amidation.

Peptides used for immunization are designed on the basis of combination of B and T cell epitopes. They are sometimes acetylated and amidated in order to mimic the protein insertion of the B cell epitope, but to our knowledge the effect of modifying the N- and C-termini is not clearly identified. In this paper, we have investigated in detail the influence of amidation and acetylation on the immunogenic properties of the T cell epitope 24-36 which is derived from a snake neurotoxin. Acetylation enhanced the capacity of the peptides to bind to I-Ed and to stimulate specific T cells in vitro but both modifications did not influence in vivo the T cell priming ability of the peptides. However, amidation of the peptides 24-36 provoked a dramatic effect on the antibody specificity they elicited, whereas acetylation did not. Antibodies recruited by amidated peptides weakly recognized the non amidated ones, while the latter elicited antibodies which hardly bind to the former. These results show how a subtle chemical change of a peptide immunogen modifies the reactivity of the elicited antibodies in an unrelated manner from the peptide MHC II binding ability and T cell stimulating capacity. We thus amplify the previously described polarity of chimeric TB peptides that raise antibodies mainly against their C-terminal part. Finally, these results may also facilitate the choice of the status of N and C termini of the peptides designed for immunization which at present have their extremities indifferently free or modified by acetylation and/or amidation.

Acetylation↗
PubMed1996

Mass spectrometric determination of isotopic exchange rates of amide hydrogens located on the surfaces of proteins.

The rates at which peptide amide hydrogens in folded proteins undergo isotopic exchange are reduced by factors of 10(0)-10(-8) relative to exchange rates at the same peptide linkages in unfolded proteins. To measure the isotopic exchange rates of the most rapidly exchanging peptide amide hydrogens in proteins, a flow-quench deuterium exchange-in step has been added to the protein fragmentation/mass spectrometry method (Zhang, Z.; Smith, D. L. Protein Sci. 1993, 2, 522-531). Isotopic exchange rates in eight short segments spanning the entire backbone of cytochrome c have been determined for exchange-in times of 0.2-120 s. These results show that the isotopic exchange rates of 10 of the peptide amide hydrogens in cytochrome c are similar to those expected for unfolded cyt c, while the exchange rates for 33 other non-hydrogen-bonded amide hydrogens are much less than expected for unfolded cyt c. Since the isotopic exchange rates of the most rapidly exchanging amide hydrogens in folded proteins are a direct measure of their access to the aqueous solvent, the ability to determine these isotopic exchange rates points to the possibility of using quenched-flow amide hydrogen exchange and mass spectrometry as a tool for identifying protein surfaces involved with binding.

Amides↗
PubMed2004

Dynamics of urokinase receptor interaction with Peptide antagonists studied by amide hydrogen exchange and mass spectrometry.

Using amide hydrogen exchange combined with electrospray ionization mass spectrometry, we have in this study determined the number of amide hydrogens on several peptides that become solvent-inaccessible as a result of their high-affinity interaction with the urokinase-type plasminogen activator receptor (uPAR). These experiments reveal that at least six out of eight amide hydrogens in a synthetic nine-mer peptide antagonist (AE105) become sequestered upon engagement in uPAR binding. Various uPAR mutants with decreased affinity for this peptide antagonist gave similar results, thereby indicating that deletion of the favorable interactions involving the side chains of these residues in uPAR does not affect the number of hydrogen bonds established by the main chain of the peptide ligand. The isolated growth factor-like domain (GFD) of the cognate serine protease ligand for uPAR showed 11 protected amide hydrogens in the receptor complex. Interestingly, a naturally occurring O-linked fucose on Thr(18) confers protection of two additional amide hydrogens in GFD when it forms a complex with uPAR. Dissociation of the uPAR-peptide complexes is accompanied by a correlated exchange of nearly all amide hydrogens on the peptide ligand. This yields bimodal isotope patterns from which dissociation rate constants can be determined. In addition, the distinct bimodal isotope distributions also allow investigation of the exchange kinetics of receptor-bound peptides providing information about the local structural motions at the interface. These exchange experiments therefore provide both structural and kinetic information on the interaction between uPAR and these small peptide antagonists, which in model systems show promise as inhibitors of intravasation of human cancer cells.

Amides↗
PubMed2006

RNA-Binding affinities and crystal structure of oligonucleotides containing five-atom amide-based backbone structures.

Among the hundreds of nucleic acid analogues that have been studied over the last two decades only very few exhibit backbones with linkers between residues that are either shorter or longer than the four-atom linker O3'-P-O5'-C5' connecting sugar ring moieties in DNA and RNA. 2'-Deoxyribonucleoside dimers connected by a five-atom linker O3'-CH(CH(3))-CO-NH-CH(2) (*designates a chiral center) were reported to lead to only a slight destabilization of RNA-DNA hybrids in which the DNA strand contained one or several of these amide-linked dimers (De Napoli, L., Iadonisi, A., Montesarchio, D., Varra, M., and Piccialli, G. (1995) Synthesis of thymidine dimers containing a new internucleosidic amide linkage and their incorporation into oligodeoxyribonucleotides, Bioorg. Med. Chem. Lett. 5, 1647-1652). To analyze the influence of various chemistries of such five-atom amide linkers on the RNA-binding affinity of modified DNA strands, we have synthesized five different amide-linked dimers, including structures with homochiral linkers of the type X3'-C*H(CH(3))-CO-NH-CH(2) (X = O, CH(2)) as well as the corresponding analogues carrying methoxy groups at the 2'-position of the 3'-nucleosides. We have conducted a detailed thermodynamic analysis of duplex formation between the modified DNA and RNA, with the DNA strands containing between one and seven consecutive modified dimers. Some of the five-atom-linked dimers lead to significantly higher RNA-binding affinities compared with that of native DNA. Interestingly, the linkers with opposite stereochemistry at the chiral center stabilize duplexes between the modified DNA and RNA to different degrees. CD spectroscopy in solution and a crystal structure of an RNA-DNA duplex with a single amide-linked dimer demonstrate that the longer amide backbones do not disrupt the duplex geometry. These observations provide further evidence that stable cross-pairing between two different types of nucleic acids does not require the numbers of atoms linking their individual residues to match.

Amides↗
PubMed1998

UV resonance Raman-selective amide vibrational enhancement: quantitative methodology for determining protein secondary structure.

We have directly determined the amide band resonance Raman spectra of the "average" pure alpha-helix, beta-sheet, and unordered secondary structures by exciting within the amide pi-->pi* transitions at 206.5 nm. The Raman spectra are dominated by the amide bands of the peptide backbone. We have empirically determined the average pure alpha-helix, beta-sheet, and unordered resonance Raman spectra from the amide resonance Raman spectra of 13 proteins with well-known X-ray crystal structures. We demonstrate that we can simultaneously utilize the amide I, II, and III bands and the Calpha-H amide bending vibrations of these average secondary structure spectra to directly determine protein secondary structure. The UV Raman method appears to be complementary, and in some cases superior, to the existing methods, such as CD, VCD, and absorption spectroscopy. In addition, the spectra are immune to the light-scattering artifacts that plague CD, VCD, and IR absorption measurements. Thus, it will be possible to examine proteins in micelles and other scattering media.

Amides↗
PubMed1997

Monitoring structural stability of trypsin inhibitor at the submolecular level by amide-proton exchange using Fourier transform infrared spectroscopy: a test case for more general application.

Combining the information on the secondary structure content as present in the shape of a protein amide I infrared band with the approach of monitoring amide-proton exchange using infrared spectroscopy, we have been able to investigate the structural stability of different components present in a protein, which are shown to be correlated to the different classes of secondary structures. For this purpose, the changes in intensity in different regions of the amide I have been detected upon exposure of the protein to a 2H2O environment, revealing four separate classes of exchanging components. As a test case for the approach described in this work, the amide-proton exchange of hydrated protein films of bovine pancreatic trypsin inhibitor has been studied using infrared spectroscopy, and is compared to literature data obtained by other techniques. A slow amide-proton exchange is observed for a class correlated to the beta-strands present in the protein, with protection of amide-protons for more than 19 h. Another class, which has been assigned to mainly helical residues, shows much less protection from exchange. The distribution function of the exchange rates of a class linked to the beta-turns displays five times faster exchange rates compared to those found for the majority of the helical residues, but they are still ten times slower compared to a class which we defined to represent the nonstructured parts of the protein.

Absorption↗
PubMed1998

Amide modes of the alpha-helix: Raman spectroscopy of filamentous virus fd containing peptide 13C and 2H labels in coat protein subunits.

The filamentous virus fd consists of a single-stranded DNA genome sheathed by 2700 copies of a 50-residue alpha-helical subunit (protein pVIII) and serves as a model assembly of alpha-helices. To advance vibrational assignments for the alpha-helix, we have investigated Raman spectra of fd virions containing 13C and 2H (deuterium) labels at various main-chain sites of the pVIII subunits. 13C was introduced at specific peptide carbonyls, while deuterium was introduced at selected alpha-carbon (Calpha) and amide nitrogen positions. Interpretation of the Raman spectra reveals a previously unrecognized alpha-helix band in the spectral interval 730-745 cm-1, tentatively assigned to a carbonyl in-plane bending mode (amide IV). Experimental evidence has also been obtained for a distinctive alpha-helix marker near 1345 cm-1, assigned to a coupled Calpha-H bending and Calpha-C stretching mode. The fd virions containing 13C-labeled carbonyls exhibit unexpectedly complex amide I profiles, consisting of multiple band components. Amide I splitting resulting from 13C substitution of carbonyls is attributed to decoupling of transition-dipole interactions normally occurring in the extended pVIII helix. The present study identifies novel conformation-dependent Raman bands in a native alpha-helix assembly, confirms amide I and amide III assignments proposed previously for filamentous viruses, and facilitates new Raman assignments for the packaged ssDNA. The alpha-helix markers identified here should also be useful in conformation analyses of other proteins by Raman spectroscopy.

Amides↗
PubMed2001

The anomalous infrared amide I intensity distribution in (13)C isotopically labeled peptide beta-sheets comes from extended, multiple-stranded structures: an ab initio study.

Ab initio based calculations of force fields and atomic polar tensors are used to simulate amide I infrared absorption spectra for a series of isotopically substituted (Ac-A(12)-NH-CH(3))(n)() peptides clustered in an antiparallel beta-sheet conformation having a varying number of strands, n = 2-5. The results demonstrate that the anomalous intensity previously reported for the isotopically shifted amide I in (13)C labeled peptides is due to formation of multistranded beta-sheet structures in this conformation. Computations show that the characteristic widely split amide I mode for beta-sheet polypeptides as well as this anomalous intensity enhancement in isotopically substituted beta-sheet peptides grows with increasing sheet size. For sheets of five strands, qualitative and near quantitative agreement with experimental amide I intensity patterns is obtained for both labeled and unlabeled peptides. The strongest transitions primarily represent in-phase coupled modes of the (13)C labeled, next nearest neighbor amides on the inner strands of the multistranded beta-sheet. Long-range transition dipole coupling interactions do not promote the (13)C amide I intensity enhancement. Understanding of the IR intensity mechanisms with this level of detail for the isotopically labeled peptides permits design of site-specific probes of beta-sheet folding and unfolding dynamics.

Alanine↗
PubMed2005

Recognition of RNA by amide modified backbone nucleic acids: molecular dynamics simulations of DNA-RNA hybrids in aqueous solution.

Thermodynamic and structural properties of a chemically modified DNA-RNA hybrid in which a phosphodiester linkage is replaced by a neutral amide-3 linkage (3'-CH(2)-CONH-5') were investigated using UV melting experiments, molecular dynamics simulations in explicit water, and continuum solvent models. van't Hoff analysis of the experimental UV melting curves suggests that the significant increase of the thermodynamic stability of a 15-mer DNA-RNA with seven alternated amide-3 modifications (+11 degrees C) is mainly due to an increased binding enthalpy. To further evaluate the origin in the observed affinities differences, the electrostatic contribution to the binding free energy was calculated by solving the Poisson-Boltzmann equation numerically. The nonelectrostatic contribution was estimated as the product of a hydrophobic surface tension coefficient and the surface area that is buried upon double strand formation. Structures were taken from 10 ns molecular dynamics simulations computed in a consistent fashion using explicit solvent, counterions, and the particle-mesh Ewald procedure. The present preliminary thermodynamic study suggests that the favorable binding free energy of the amide-3 DNA single strand to the complementary RNA is equally driven by electrostatic and nonpolar contributions to the binding compared to their natural analogues. In addition, molecular dynamics simulations in explicit water were performed on an amide-3 DNA single strand and the corresponding natural DNA. Results from the conformations cluster analysis of the simulated amide-3 DNA single strand ensembles suggest that the 25% of the population sampled within 10 ns has a pre-organized conformation where the sugar C3' endo pucker is favored at the 3'-flanking nucleotides. These structural and thermodynamic features contribute to the understanding of the observed increased affinities of the amide-3 DNA-RNA hybrids at the microscopic level.

Amides↗
PubMed1992

alpha-Keto amide inhibitors of aminopeptidases.

The design and synthesis of 3-amino-2-oxo-4-phenylbutanoic acid amides (alpha-keto amides), a new class of aminopeptidase inhibitor, are described. These compounds, illustrated by the Phe-Leu analogue 2, are effective inhibitors of arginyl aminopeptidase (Ki = 1.5 microM), cytosol aminopeptidase (Ki = 1.0 microM), and microsomal aminopeptidase (Ki = 2.5 microM). The ketone carbonyl of the alpha-keto amide was found to hydrate readily in an aqueous DMSO solution, due to the electron-withdrawing effect of the neighboring amide group. A mechanism of inhibition is proposed for the alpha-keto amides that is similar to that proposed for the structurally related aminopeptidase inhibitor bestatin and its analogues, wherein the inhibitor may interact with the S1'-S2' subsite of the enzyme rather than the S1-S1' subsite. Like bestatin, the alpha-keto amides are slow-binding inhibitors of all three enzymes.

Amides↗
PubMed1992

Nonpeptide renin inhibitors employing a novel 3-aza(or oxa)-2,4-dialkyl glutaric acid moiety as a P2/P3 amide bond replacement.

A new series of renin inhibitors has been developed. The inhibitors feature a novel replacement for the P2/P3 dipeptide moiety normally associated with renin inhibitors. The dipeptide replacement was a (2S,4S)-3-aza(or oxa)-2,4-dialkylglutaric acid amide. Extensive structure-activity relationship studies determined that optimum potency was achieved when inhibitors employed a benzyl and butyl group at the C(4) and C(2) carbon position, respectively. In addition, maximum in vitro potency was obtained when the N-terminus was functionalized by incorporating a 4-(1,3-dioxabutyl)piperidine amide. SAR data suggested that the 1,3-dioxabutyl group (methoxymethyl ether) interacted by hydrogen bonding to groups in the S4 domain of renin. This hypothesis was strengthened when a 4-butylpiperidine amide was substituted and inhibitor potency decreased dramatically. Inhibitors employing this novel dipeptide mimic were prepared by coupling the glutaric acid amides with either the transition-state mimic (2S,3R,4S)-2-amino-1-cyclohexyl-3,4-dihydroxy-6- methylheptane (18) or the hydroxyethylene dipeptide isostere. The glutaric acid amides were prepared by two general procedures. The first procedure involved the reductive amination of alpha-amino acid esters with alpha-keto esters. The second procedure involved the displacement reaction of alpha-bromo esters or acids with alpha-amino acid amides.

Amides↗
PubMed1980

Thrombin inhibitors. 2. Amide derivatives of N alpha-substituted L-arginine.

A series of N alpha-(arylsulfonyl)-L-arginine amide derivatives with substituted or unsubstituted naphthalene and heterocyclic compounds as the N alpha-substituent was prepared and tested as inhibitors of the clotting activity of thrombin. N-n-Butyl and N-n-butyl-N-methyl derivatives of N alpha-dansyl-L-arginine amide were the most inhibitory of N-alkyl and N,N-dialkyl derivatives of N alpha-dansyl-L-arginine amide. Their inhibitory effect was as potent as that of N alpha-dansyl-L-arginine-n-butyl ester with an I50 of 2 X 10(-6) M. N alpha-Substituted naphtalenesulfonyl-L-arginine amide derivatives of 4-methyl- and 4-ethylpiperidine also showed a potent inhibition with an I50 of 10(-7) to 10(-6) M. The most potent inhibitior in this study was 1-[N alpha-(4,6-dimethoxynaphthalene-2-sulfonyl)-arginyl]-4-methylpiperidine, with an I50 of 7.5 X 10(-8) M. Arginine amide derivatives of 4-methyl- or 4-ethylpiperidine with tetralin or an oxygen-containing heterocyclic compound as a N alpha-substituent showed an inhibition with an I50 less than 10(-5) M. N-Monosubstituted derivatives of N alpha-dansyl-L-arginine amide were not hydrolyzed at all by thrombin and were hydrolyzed very slowly by trypsin, and N,N-disubstituted derivatives were not hydrolyzed at all by both enzymes.

Amides↗
PubMed2005

Unusually high pyramidal geometry of the bicyclic amide nitrogen in a complex 7-azabicyclo[2.2.1]heptane derivative: Theoretical analysis using a bottom-up strategy.

The high pyramidalization of the bicyclic amide nitrogen found in the crystal structure of a dipeptide incorporating (1S,2S,4R)-N-benzoyl-2-phenyl-7-azabicyclo[2.2.1]heptane-1-carboxylic acid has been investigated using quantum mechanical calculations. More specifically, a bottom-up strategy based on the study of model molecules of progressive complexity has been used. First, an appropriate quantum mechanical method has been selected by examining the distortion of the amide bond in three simple model molecules. Next, the amide distortion induced by the norbornane ring has been evaluated by considering three different 7-azabicyclo[2.2.1]heptane amides. After this, the suitability of quantum mechanical calculations to predict the effect of the substituents on the pyramidalization of the bicyclic amide nitrogen has been investigated by comparing experimental and theoretical parameters for a number of compounds. Finally, the factors responsible for amide distortion in the (1S,2S,4R)-N-benzoyl-2-phenyl-7-azabicyclo[2.2.1]heptane-1-carboxylic acid derivative have been elucidated using a hierarchical approach. For this purpose, several derivatives were generated by removing or modifying the substituents attached to the 7-azanorbornane system. Results have been discussed in terms of intramolecular specific interactions.

Amides↗
PubMed2003

Zinc(II) complexes with intramolecular amide oxygen coordination as models of metalloamidases.

Polydentate ligands (6-R1-2-pyridylmethyl)-R2(R1= NHCOtBu, R2= bis(2-pyridylmethyl)amine L1, bis(2-(methylthio)ethyl)amine L2 and N(CH2CH2)2S L3) form mononuclear zinc(II) complexes with intramolecular amide oxygen coordination and a range of coordination environments. Thus, the reaction of Zn(ClO4)2.6H2O with L1-3 in acetonitrile affords [(L)Zn](ClO4)2(L=L1, 1; L2, 2) and [(L3)Zn(H2O)(NCCH3)](ClO4)2 3. The simultaneous amide/water binding in resembles the motif that has been proposed to be involved in the double substrate/nucleophile Lewis acidic activation and positioning mechanism of amide bond hydrolysis in metallopeptidases. X-ray diffraction, 1H and 13C NMR and IR data suggests that the strength of amide oxygen coordination follows the trend 1>2 >3. L1-3 and undergo cleavage of the tert-butylamide upon addition of Me4NOH.5H2O (1 equiv.) in methanol at 50(1)degrees C. The rate of amide cleavage follows the order 1> 2>> 3, L1-3. The extent by which the amide cleavage reaction is accelerated in 1-3 relative to the free ligands, L1-3, is correlated with the strength of amide oxygen binding and Lewis acidity of the zinc(II) centre in deduced from the X-ray, NMR and IR studies.

Amides↗
PubMed2006

Hyaluronan: the absence of amide-carboxylate hydrogen bonds and the chain conformation in aqueous solution are incompatible with stable secondary and tertiary structure models.

Contradictory descriptions for the aqueous solution conformation of the glycosaminoglycan hyaluronan (HA) exist in the literature. According to hydrodynamic and simulation data, HA molecules are stiffened by a rapidly interchanging network of transient hydrogen bonds at the local level and do not significantly associate at the global level. In marked contrast, models derived from NMR data suggest that the secondary structure involves persistent hydrogen bonds and that strong associations between chains can occur to form vast stable tertiary structures. These models require an extended 2-fold helical conformation of the HA chain and specific hydrogen bonds between amide and carboxylate groups. To test these descriptions, we have used 15N-labelled oligosaccharides and high-field NMR to measure pertinent properties of the acetamido group. The amide proton chemical shift perturbation and carboxylate group pK(a) value are inconsistent with a highly populated hydrogen bond between the amide and carboxylate groups. Amide proton temperature coefficients and chemical exchange rates confirm this conclusion. Comparison of oligomer properties with polymeric HA indicates that there is no discernible difference in amide proton environment between the centre of octasaccharides and the polymer, inconsistent with the formation of tertiary structures. A [1H-1H-15N] NOESY-HSQC (heteronuclear single-quantum correlation) spectrum recorded on an HA octasaccharide revealed that amide groups in the centre are in a trans orientation and that the average solution conformation is not an extended 2-fold helix. Therefore the two key aspects of the secondary and tertiary structure models are unlikely to be correct. Rather, these new NMR data agree with descriptions from hydrodynamic and simulations data.

Amides↗
PubMed2006

A second fatty acid amide hydrolase with variable distribution among placental mammals.

Fatty acid amides constitute a large and diverse class of lipid transmitters that includes the endogenous cannabinoid anandamide and the sleep-inducing substance oleamide. The magnitude and duration of fatty acid amide signaling are controlled by enzymatic hydrolysis in vivo. Fatty acid amide hydrolase (FAAH) activity in mammals has been primarily attributed to a single integral membrane enzyme of the amidase signature (AS) family. Here, we report the functional proteomic discovery of a second membrane-associated AS enzyme in humans that displays FAAH activity. The gene that encodes this second FAAH enzyme was found in multiple primate genomes, marsupials, and more distantly related vertebrates, but, remarkably, not in a number of lower placental mammals, including mouse and rat. The two human FAAH enzymes, which share 20% sequence identity and are referred to hereafter as FAAH-1 and FAAH-2, hydrolyzed primary fatty acid amide substrates (e.g. oleamide) at equivalent rates, whereas FAAH-1 exhibited much greater activity with N-acyl ethanolamines (e.g. anandamide) and N-acyl taurines. Both enzymes were sensitive to the principal classes of FAAH inhibitors synthesized to date, including O-aryl carbamates and alpha-keto heterocycles. These data coupled with the overlapping, but distinct tissue distributions of FAAH-1 and FAAH-2 suggest that these proteins may collaborate to control fatty acid amide catabolism in primates. The apparent loss of the FAAH-2 gene in some lower mammals should be taken into consideration when extrapolating genetic or pharmacological findings on the fatty acid amide signaling system across species.

Amidohydrolases↗
PubMed1978

The effects of replacing ester by amide on the biological properties of compounds related to acetylcholine.

1 Replacement of ester by amine in series of derivatives of diphenylacetic acid reduces the affinity for muscarine-sensitive acetylcholine receptors of the guinea-pig ileum from 40- to 100-fold. With similar series of phenylacetic acid the reduction is only 2- to 4-fold. In both series changes in the composition of the onium group produce similar changes in the affinity of amides and esters and it appears that the stiffness of the amide bond reduces the binding of the phenyl groups at the far end of the molecule from the onium atom. 2 Replacement of ester by amide in similar series of acetyl compounds reduces activity on the guinea-pig ileum over 1000-fold and on the frog rectus over 50-fold. Compounds with larger onium groups are antagonists on both preparations with log affinity constant around 3. The amides have similar affinity for electric eel acetylcholinesterase. 3 The amides are slightly bigger than the esters in solution and slightly more hydrophilic. 4 Replacement of ester by amide in acetylcholine reduces the proportion of gauche conformer about the C--C--bond from 100% to 39%. 5 The ability of acetylcholine to activate receptors is thought to depend on some degree of flexibility in the --CO--0--bond, though the hydration of the bond may also be important.

Acetylcholine↗
PubMed1962

Metabolism of phenylalanine-containing peptide amides in Escherichia coli.

Simmonds, Sofia (Yale University, New Haven, Conn.) and David D. Griffith. Metabolism of phenylalanine-containing peptide amides in Escherichia coli. J. Bacteriol. 83:256-263. 1962.-A study was made of the hydrolysis, by fresh and lyophilized cells of a phenylalanine auxotroph of Escherichia coli, of l-phenylalanyl-glycinamide and glycyl-l-phenylalaninamide. The former dipeptide amide is hydrolyzed to yield phenylalanine and glycinamide, the latter to yield glycine and phenylalaninamide. The slow degradation of phenylalaninamide yields the free amino acid. In spite of the presence of intracellular activity toward the two dipeptide amides, neither permits as much growth as that observed with an equimolar concentration of free phenylalanine. The relatively small extent of growth on the dipeptide amides, and also on l-phenylalaninamide, appears to result from the inability of the cells in a growing culture to take up all of the exogenous amide added to the growth medium. This, in turn, probably reflects the presence in the medium of the amides as a mixture of charged and uncharged forms, of which only the latter are readily taken into the cells.

Amides↗