Effect of an aliphatic spacer group on the adsorption mechanism on the colloidal silver surface of L‐proline phosphonodipeptides

A comparative study of molecular structures of five L ‐proline (L ‐Pro) phosphonodipeptides: L ‐Pro‐NH‐C(Me,Me)‐PO₃H₂ (P1), L ‐Pro‐NH‐C(Me,iPr)‐PO₃H₂ (P2), L ‐Pro‐L ‐NH‐CH(iBu)‐PO₃H₂ (P3), L ‐Pro‐L ‐NH‐CH(PA)‐PO₃H₂ (P4) and L ‐Pro‐L ‐NH‐CH(BA)‐PO₃H₂ (P5) has been carried out using Raman and absorption infrared techniques of molecular spectroscopy. The interpretation of the obtained spectra has been supported by density functional theory calculations (DFT) at the B3LYP; 6–31 + + G** level using Gaussian 2003 software. The surface‐enhanced Raman scattering (SERS) on Ag‐sol in aqueous solutions of these phosphonopeptides has also been investigated. The surface geometry of these molecules on a silver colloidal surface has been determined by observing the position and relative intensity changes of the Pro ring, amide, phosphonate and so‐called spacer (−R) groups vibrations of the enhanced bands in their SERS spectra. Results show that P4 and P5 adsorb onto the silver as anionic molecules mainly via the amide bond (∼1630, ∼1533, ∼1248, ∼800 and ∼565 cm⁻¹), Pro ring (∼956, ∼907 and ∼876 cm⁻¹) and carboxylate group (∼1395 and ∼909 cm⁻¹). Coadsorption of the imine nitrogen atom and PO group with the silver surface, possibly by formation of a weaker interaction with the metal, is also suggested by the enhancement of the bands at 1158 and 1248 cm⁻¹. P1, P2 and P3 show two orientations of their main chain on the silver surface resulting from different interactions of the  C CH₃,  NH and  CONH fragments with this surface. Bonding to the Ag surface occurs mainly through the imino atom (1166 cm⁻¹) for P2, while for P1 and P3 it occurs via the methyl group(s) (1194–1208 cm⁻¹). The amide group functionality (CONH) is practically not involved in the adsorption process for P1 and P2, whereas the C_s P bonds do assist in the adsorption. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of an aliphatic spacer group on the adsorption mechanism of phosphonodipeptides containing N-terminal glycine on the colloidal silver surface

This study presents the complete solid-state vibrational assignments for a series of five zwitterionic phosphonodipeptides containing an N-terminal glycine: L -Gly-L -CH(Me)-PO₃H₂ (G1), L -Gly-C(Me,Me)-PO₃H₂ (G2), L -Gly-L -CH(Et)-PO₃H₂ (G3), L -Gly-C(Me,Et)-PO₃H₂ (G4), and L -Gly-L -CH(iBu)-PO₃H₂ (G5). The assignments are based primarily on Fourier-transform Raman spectra (FT-RS) and Fourier-transform infrared spectra (FT-IR) spectra, as well as density functional theory (DFT) calculations at the B3LYP; 6-31 + + G** level of theory. Existing literature data are also taken into consideration. The surface geometry of these molecules on a colloidal silver surface was also determined by observing the wavenumber, width, and relative intensity changes of enhanced bands in their surface-enhanced Raman scattering spectra. It is proposed that G1 mainly adsorbs onto the colloidal silver particles through the phosphonate terminus, whereas the PO bond in G3 and G5 assists in the interaction of these molecules with the silver surface. G3 interacts with Ag mainly via α-methlyalanine and the amide bond. It is also shown that the amide bond and glycine backbone are involved in the adsorption of G3 on the silver nanoparticles. In addition, the differences recorded for G4 and G5 SERS spectra are mainly due to interactions between the silver surface and the amine group and N- and P-terminus, respectively, and are manifestations of the characteristic vibrations of these groups. Copyright © 2008 John Wiley & Sons, Ltd.     

Vibrational characterization of L‐valine phosphonate dipeptides: FT‐IR,FT‐RS,and SERS spectroscopy studies and DFT calculations

Four L ‐valine (L ‐Val) phosphonate dipeptides that are potent inhibitors of zinc metalloproteases, namely, L ‐Val‐C(Me)₂‐PO₃H₂ (V1), L ‐Val‐CH(iP)‐PO₃H₂ (V2), L ‐Val‐CH(iB)‐PO₃H₂ (V3), and L ‐Val‐C(Me)(iP)‐PO₃H₂ (V4), are studied by Fourier‐transform infrared (FT‐IR) spectroscopy, Fourier‐transform Raman spectroscopy (FT‐RS), and surface‐enhanced Raman scattering (SERS). The band assignment (wavenumbers and intensities) is made based on (B3LYP/6‐311 + + G**) calculations. Comparison of theoretical FT‐IR and FT‐RS spectra with those of SERS allows to obtain information on the orientation of these dipeptides as well as specific‐competitive interactions of their functionalities with the silver substrate. More specifically, V1 and V4 appear to interact with the silver substrate mainly via a  C_sgCH₃ moiety localized at the  N_amideC_sg(CH₃)P molecular fragment. In addition, the  POH and isopropyl units of V4 assist in the adsorption process of this molecule. In contrast, the  C_αNH₂ and  PO₃H⁻ groups of V2 and V3 interact with the silver nanoparticles, whereas their isopropyl and isobutyl fragments seem to be repelled by the silver substrate (except for the  CH₂  of V3), similar to the  C_β(CH₃)₂ fragment of L ‐Val for all L ‐Val phosphonate dipeptides investigated in this work. The adsorption mechanism of these molecules onto the colloidal silver surface is also affected by amide bond behavior. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of amino acid modifications on the molecular structure of adsorbed and nonadsorbed bombesin 6–14 fragments on an electrochemically roughened silver surface

This work used infrared absorption and Raman spectroscopy to determine the structure of seven modified fragments (residues 6–14 of the polypeptide chain) of bombesin (BN^6–14). The peptides studied are cyclo[D ‐Phe⁶, His⁷, Leu¹⁴]BN^6–14, [D ‐Phe⁶, Leu‐NHEt¹³, des‐Met¹⁴]BN^6–14, [D ‐Phe⁶, Leu¹³‐®‐p‐chloro‐Phe¹⁴]BN^6–14, [D ‐Phe⁶, β‐Ala¹¹, Phe¹³, Nle¹⁴]BN^6–14, [D ‐Tyr⁶, β‐Ala¹¹, Phe¹³, Nle¹⁴]BN^6–14, [D ‐Tyr⁶, β‐Phe¹¹, Phe¹³, Nle¹⁴OH]BN^6–14 and [D ‐Cys⁶, Asn⁷, D ‐Ala¹¹, Cys¹⁴]BN^6–14. These peptides are potent bombesin agonists useful in the treatment of tumors. Surface‐enhanced Raman scattering (SERS) spectroscopy was also used to examine the behavior of these molecules on an electrochemically roughened silver surface. The SERS spectra reveal that substituting native amino acids in these molecules with synthetic ones changes their adsorption state slightly on an electrochemically roughened surface of silver. The peptides [D ‐Tyr⁶, β‐Ala¹¹, Phe¹³, Nle¹⁴]BN^6–14 and [D ‐Tyr⁶, β‐Phe¹¹, Phe¹³, Nle¹⁴OH]BN^6–14 tend to adsorb strongly on this surface via C fragment (∼1400 cm⁻¹). The observed medium enhancement of the Trp⁸ residue and amide bond Raman signals indicate further interactions between these fragments and the surface. [D ‐Phe⁶, Leu‐NHEt¹³, des‐Met¹⁴]BN^6–14 and [D ‐Cys⁶, Asn⁷, D ‐Ala¹¹, Cys¹⁴]BN^6–14 are shown to be coordinated to the silver through  CONH , CO, and the indole ring. The strongest SERS bands (∼1506, ∼1275, ∼1149, and ∼1007 cm⁻¹) of [D ‐Phe⁶, Leu¹³‐®‐p‐chloro‐Phe¹⁴]BN^6–14 and [D ‐Phe⁶, β‐Ala¹¹, Phe¹³, Nle¹⁴]BN^6–14 suggest that these two peptides bind to the silver via Trp⁸ and  CONH . In the case of cyclo[D ‐Phe⁶, His⁷, Leu¹⁴]BN^6–14, the formation of a peptide/Ag complex is confirmed by the strong SERS bands involving Trp⁸ and  CONH vibrations, which are accompanied by a SERS signal due to the CO vibrations. For these analogs, the relative potency for inhibition of binding of ¹²⁵I‐[Tyr⁴]BN to rat pancreas acini cells was correlated with the behavior of the amide bond on the silver surface, while the contribution of the structural components to the ability to interact with the rGRP‐R was correlated with the SERS patterns. Copyright © 2008 John Wiley & Sons, Ltd.     

Characterization of adsorption mode of new B2 bradykinin receptor antagonists onto colloidal Ag substrate

In this paper, the surface‐enhanced Raman scattering (SERS) spectra of the potent B₂ bradykinin receptor antagonists, [D‐Arg⁰,Hyp³,Thi^5,8,L‐Pip⁷]BK, Aaa[D‐Arg⁰,Hyp³,Thi^5,8,L‐Pip⁷]BK, [D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK, and Aaa[D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK, were measured when immobilized onto a colloidal assembly of apparently randomly adhering Ag spheres with diameters of approximately 20 – 25 nm. The observed SERS bands corresponding to different vibrational modes of the molecule, attached to or near Ag, and the variations in these bands resulting from competitive interactions of the functional groups of the peptides with the SERS‐active Ag surfaces were analyzed in this study. Briefly, it was shown that Pip, in generally in vertical orientation, and Thi, in the edge‐on position, relative to the colloidal Ag surface interacted with this surface through their lone electron pairs on the nitrogen and sulfur atoms, respectively. The imide bond of the X‐Pro peptide linkage and the guanidine group of Arg were involved in the adsorption process. In addition, it was demonstrated that the specific differences in the amino acid sequences slightly influenced the mode of adsorption. For example, Aaa in Aaa[D‐Arg⁰,Hyp³,Thi^5,8,L‐Pip⁷]BK and Aaa[D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK and D‐Phe (vertical with respect to the colloidal Ag surface) in [D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK, and Aaa[D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK assisted in the adsorption of these peptides onto the colloidal Ag particles. To discuss these spectral alterations due to the different surface adsorption mechanisms of these peptides, the spectral changes were analyzed according to the adsorption process and Fourier‐transform‐Raman spectra. Copyright © 2013 John Wiley & Sons, Ltd.     

Adsorption mechanism of physiologically active l-phenylalanine phosphonodipeptide analogues: Comparison of colloidal silver and macroscopic silver substrates

Here we present SERS spectra of several l-phenylalanine (Phe) phosphonodipeptides, i.e., l-Phe-l-Ala-PO₃H₂ (MD1), l-Phe-l-Val-PO₃H₂ (MD2), l-Phe-β-Ala-CH(OH)-PO₃H₂ (MD3), l-Phe-l-Ala-CH(OH)-PO₃H₂ (MD4), l-Ala-(3,4-dimethoxy)-l-Phe-PO₃H₂ (MD5), and l-Ala-(3,4-dimethoxy)-(des-CH₂)-l-Phe-PO₃H₂ (MD6), immobilized on electrochemically roughened silver electrodes. These spectra are analyzed by theoretical calculations using density functional theory (DFT) at the B3LYP level with 6-31++G∗∗ basis set. In addition, these spectra are compared with SERS spectra of these species adsorbed on a colloidal silver surface. We showed that on the macroscopic silver substrate, the Phe aromatic ring of MD3 and MD4 is oriented vertically, while for MD1 it almost “stands up” on this surface. In the other three cases, the Phe ring adopts a tilted orientation in regard to the substrate. We also find that the phosphonate , methyl/methane, or dimethoxy groups of MD1, MD2, MD3, MD5, and MD6 are involved in the interaction of these phosphonodipeptides with the electrochemically roughened surface. This phenomenon is clearly seen for -CH₂-/-CH₃/-OCH₃ moieties as well as for the group that adsorbs on the macroscopic silver substrates mainly via the PO fragment. We also showed that MD4 binds to the macroscopic silver substrate through the hydroxyl, amine, and phosphonate groups, while the methylene/methane moieties are remote from this surface. We found that studied phosphonodipeptides often adsorb differently on the macroscopic silver substrate and on the colloidal silver nanoparticles. For example, MD1 adopts an almost vertical orientation on the electrochemically roughened silver substrate and is tilted or close to flat on the silver nanoparticles.     

Piperidine adsorption on two different silver electrodes: A combined surface enhanced Raman spectroscopy and density functional theory study

The surface enhanced Raman scattering (SERS) spectra of piperidine in silver colloid solution, on roughened silver electrode and on roughened silver electrode modified with silver nanoparticles were studied, and the high-quality SERS spectra of piperidine on roughened silver electrode modified with silver nanoparticles were obtained for the first time. Surface selection rules derived from the EM enhancement model were employed to deduce piperidine orientations on the different surfaces. On the basis of this, two models of piperidine adsorbed on the surface of the silver nanoparticles were built, and DFT-B3PW91/LanL2dz was applied to calculate the Raman frequencies. It proves that, at higher potential values, the piperidine is perpendicularly standing on the roughened silver electrode surface though its lone-electron pair, but in silver colloid solution and on the silver nanoparticles modified silver electrode the piperidine molecular lies flat on the silver surface. In the meantime, the potential dependent SERS of piperidine on the modified electrode were studied.     

Influence of backbone length and synthetic mutations on orientation of neurotensin fragments adsorbed onto a colloidal silver surface: SERS studies

Neurotensin (NT) is a naturally occurring neurotransmitter that mediates the metabotropic seven‐transmembrane G protein‐coupled receptors, namely NTR1s, richly expressed on tumor surface. Therefore, mutated active molecular fragments of NT that possess selective antagonist or weak agonist properties and the high affinity to NTR1 have attracted considerable interest for use in thrombus, inflammation, and imaging/treatment of tumors. In this work, SERS spectra of three N‐terminal fragments of human NT (NT^1‐6, NT^1‐8, and NT^1‐11) and six specifically mutated C‐terminal fragments of human NT, including NT^8‐13, [Dab⁹]NT^8‐13, [Lys⁸,Lys⁹]NT^8‐13, [Lys⁸‐(®)‐Lys⁹]NT^8‐13, [Lys⁹,Trp¹¹,Glu¹²]NT^8‐13, and NT^9‐13, adsorbed onto nanometer‐sized colloidal silver particles in an aqueous solution at pH level of the solution 2 are presented. A comparison was made between the structures of the native and mutated fragments to determine how changes in peptide length and mutations of the structure influenced the NT adsorption properties. Based on the interpretation of the obtained data, we showed that all of the investigated NT fragments, excluding [Lys⁹,Trp¹¹,Glu¹²]NT^8‐13, tended to adsorb on the silver surface mainly through the L‐tyrosine residue and the carboxylate group. The Tyr ring lied more‐or‐less flat on the silver surface. The hydrogen atom from the phenol group dissociated upon binding. On the other hand, [Lys⁹,Trp¹¹,Glu¹²]NT^8‐13 bound to this substrate through the close to vertical co‐pyrrole ring of the indole ring (Trp¹¹) and –COO^‐ . Comparison of the presented data with those obtained earlier for NT allows to suggest that in the case of naturally occurring neurotensin, both Tyr residues together with the carboxylate group play crucial role in the binding to the nanometer‐sized colloidal silver particles. This geometry of binding forces the NT molecule to lay flat on the surface. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering assessment of DNA from leaf tissues adsorbed on silver colloidal nanoparticles

In this work, the surface‐enhanced Raman scattering (SERS) spectra of seven genomic DNAs from leaves of chrysanthemum (Dendranthema grandiflora Ramat.), common sundew (Drosera rotundifolia L.), edelweiss (Leontopodium alpinum Cass), Epilobium hirsutum L., Hypericum richeri ssp. transsilvanicum (Čelak) Ciocârlan, rose (Rosa x hybrida L.) and redwood (Sequoia sempervirens D. Don. Endl.), respectively, have been analyzed in the wavenumber range 200–1800 cm⁻¹. The surface‐enhanced Raman vibrational modes for each of these cases, spectroscopic band assignments and structural interpretations of genomic DNAs are reported. A high molecular structural information content can be found in the SERS spectra of these DNAs from leaf tissues. Based on this work, specific plant DNA–ligand interactions or accurate local structure of DNA might be further investigated using surface‐enhanced Raman spectroscopy. Besides, this study will generate information which is valuable in the development of label‐free DNA detection for chemical probing in living cell. Copyright © 2013 John Wiley & Sons, Ltd.     

Surface-enhanced Raman scattering studies on bombesin, its selected fragments and related peptides adsorbed at the silver colloidal surface

SERS studies presented in this work on BN^8-14, [d-Phe⁶,β-Ala¹¹,Phe¹³,Nle¹⁴]BN^6-14, [d-Tyr⁶,β-Ala¹¹,Phe¹³,Nle¹⁴]BN^6-14, BN and its modified analogues, as well as NMB, NMC, and PG-L show that these molecules at pH 8.3 bind to a colloidal silver surface mainly through Trp⁸ and Met¹⁴ residues. Trp⁸ adsorbs at the surface almost perpendicularly. Met¹⁴ appears on the surface mainly as a P_C-G conformer. His¹², as is evident from the spectra, practically does not take part in the adsorption process. Substitution of l-leucine at the 13 position of amino acid sequence with l-phenylalanine does not change substantially the pattern of the adsorption mechanism; however, substitution of phenylalanine at the 12 position (instead of l-histidine) causes changes in the SERS spectra that show that Phe¹² takes parallel orientation to the surface upon adsorption of [d-Phe¹²]BN, while in the case of [Tyr⁴,d-Phe¹²]BN this residue is perpendicular to the surface and influences the orientation of the bound Trp⁸. On the other hand, substitution of Asn with Tyr in the 6 position in nonapeptide fragment causes changes in the adsorption mechanism. In this case, the discussed fragment binds to the silver colloidal surface by Tyr⁶, Trp⁸, and Met¹⁴. The SERS spectrum of NMC is very similar to that of BN; although it differs by the binding orientation of the amide bond towards the surface. Appearance of Phe¹³ in NMB and PG-L causes that this residue competes successfully with Trp⁸ forcing it to take tilted orientation. As seen from the enhancement of the characteristic Phe vibrations this moiety in NMB and PG-L adsorbs on the silver surface in a tilted fashion. This arrangements cause that the 8-14 peptide chain in all these studied compounds takes almost a parallel orientation to the surface while the 1-5 fragment of the peptide chain is removed from the silver surface vicinity.     

Raman and surface‐enhanced Raman studies of α‐aminophosphinic inhibitors of metalloenzymes

Here, we report the nature of new di‐α‐amino (L¹–L³) and α‐amino‐α‐hydroxyphosphinic (L⁴–L⁶) acids, which are considered potential inhibitors of the aminopeptidase N, adsorbed on a colloidal silver surface by means of surface‐enhanced Raman scattering (SERS) spectroscopy. In order to reveal the adsorption mechanism of these species from their SERS spectra, Fourier‐transform Raman (FT‐RS) spectra of these nonadsorbed molecules were measured. By examining the enhancement, shift in wavenumbers, and changes in breadth of the SERS bands due to the adsorption process, we revealed that the tilted compounds interact with the colloidal silver substrate mainly through the benzene ring, amino group, and phosphinic moiety in the following way. The benzene ring of L² and L³ is ‘standing up’ on the colloidal silver surface, and the C N bond is almost vertical to it, while the tilt angle between the O PO bond and this surface is greater than 45°. On the other hand, for L¹, L⁴, and L⁵, the aromatic ring and C N bond are arranged more or less tilted, and the tilt angle between the O PO bond and the silver substrate is smaller than 45°. The elongation of the bond to the benzene ring, the L⁶ case, produces an almost horizontal orientation of the benzene ring and the O PO bond on the silver nanoparticles. For these ligands, the complement inhibition IC₅₀ tested in vitro using porcine kidney leucine aminopeptidase was correlated mainly with the behavior of the O PO and C CH N fragments on the silver surface. Copyright © 2009 John Wiley & Sons, Ltd.     

Adsorbed states of substituted α‐aminophosphinic acids on a silver electrode surface: comparison with a colloidal silver substrate

We investigated the interfacial structures of various aromatic (each compound contains one or two phenyls) di‐α‐amino ( L¹ – L³ ) and α‐amino‐α‐hydroxyphosphinic ( L⁴ – L⁶ ) acids immobilized onto an electrochemically roughened silver electrode surface in an aqueous solution using surface‐enhanced Raman scattering (SERS). These structures were compared to those on a colloidal silver surface to determine the relationship between adsorption strength and geometry. The presence of an enhanced ν_19a ring band in the SERS spectra of L⁶ , L² , and L³ on the electrode indicated that the benzene rings of those molecules interact with the electrode surface through localized CC bond(s). We observed significant band broadening of the benzene ring modes for all α‐hydroxyphosphinic acids on both substrates, except for L¹ deposited onto the electrode surface. This suggests the possibility of direct interaction between the ring and Ag, although the benzene ring–surface π overlap is weaker for the colloidal silver than for the Ag electrode. The downward shift in wavenumber and alternations in the enhancement of a ν₁₂ ring band indicate a general increase of tilt angle on both silver substrates in the order L³ < L⁴ < L⁵ < L⁶ . The altered enhancement of the bands due to the vibrations of the  NH₂ and O PO fragments, a finding observed on both silver substrates, strongly suggests that the groups interact with different strength and geometry with these substrates. Copyright © 2009 John Wiley & Sons, Ltd.     

The influence of pH and anions on the adsorption mechanism of rifampicin on silver colloids

The influence of pH and anions on the adsorption mechanism of rifampicin on colloidal silver nanoparticles has been analysed by electronic absorption, resonance Raman (RR) and surface‐enhanced resonance Raman spectroscopy (SERRS). Rifampicin is a widely used antibiotic with a zwitterionic nature. SERRS spectra of rifampicin adsorbed on silver sols, prepared using hydroxylamine hydrochloride as reducing agent, undergo dramatic changes upon lowering the pH. The spectral form changes progressively from that characteristic of chemisorbed rifampicin (at pH > 7) to one very similar to the rifampicin RR spectrum (at lower pH), indicative of a modification of the adsorption mechanism on the surface of the Ag nanoparticles. The RR‐type SERRS spectrum is proposed to result from formation of an ion pair between rifampicin and Cl⁻ anions, which, deriving from the colloid preparation, are adsorbed on the Ag surface. The addition of anions to the hydroxylamine hydrochloride sol facilitates conversion from the chemisorbed to ion pair form and leads to an order of magnitude increase in the SERRS signal. Copyright © 2007 John Wiley & Sons, Ltd.     

Structure and conformation of Arg8 vasopressin modified analogs

Fourier‐transform Raman and infrared spectra were acquired for four arginine vasopressin (AVP) analogs containing L ‐diphenylalanine (Dpa): [Dpa²]AVP, [Cpa¹,Dpa²]AVP, [Dpa³]AVP, and [Cpa¹,Dpa³]AVP (where Cpa denotes 1‐mercaptocyclohexaneacetic acid). We compared and analyzed these spectra. In addition, the Raman spectra were compared to the corresponding surface‐enhanced Raman scattering spectra recorded in an aqueous silver colloidal dispersion. Silver colloidal dispersions prepared by the simple borohydride reduction of silver nitrate were used as substrates. The geometry of these molecules etched on the silver surface was deduced from the observed changes in the intensity enhancement, breadth, and shift in wavenumber of the Raman bands in the spectra of the bound versus free species. Based on the obtained data, adsorption mechanisms were proposed for each case, and the suggested adsorbate structures were compared. All the molecules were thought to adsorb onto a silver surface via a phenyl ring, free electron pairs on the sulfur atom, and CO and  CONH‐bonds. However, the orientation of these fragments on the colloidal silver surface and the strength of the interactions with this surface are different. For [Dpa³]AVP and [Cpa¹,Dpa³]AVP, a strong interaction among the—CCN‐peptide fragment and the colloidal silver surface occurs. Copyright © 2011 John Wiley & Sons, Ltd.     

Food additives characterization by infrared,Raman, and surface‐enhanced Raman spectroscopies

Fourier‐transform infrared (FT‐IR), Raman (RS), and surface‐enhanced Raman scattering (SERS) spectra of β‐hydroxy‐β‐methylobutanoic acid (HMB), L ‐carnitine, and N‐methylglycocyamine (creatine) have been measured. The SERS spectra have been taken from species adsorbed on a colloidal silver surface. The respective FT‐IR and RS band assignments (solid‐state samples) based on the literature data have been proposed. The strongest absorptions in the FT‐IR spectrum of creatine are observed at 1398, 1615, and 1699 cm⁻¹, which are due to ν_s(COOH) + ν(CN) + δ(CN), ρ_s(NH₂), and ν(C O) modes, respectively, whereas those of L ‐carnitine (at 1396/1586 cm⁻¹ and 1480 cm⁻¹) and HMB (at 1405/1555/1585 cm⁻¹ and 1437–1473 cm⁻¹) are associated with carboxyl and methyl/methylene group vibrations, respectively. On the other hand, the strongest bands in the RS spectrum of HMB observed at 748/1442/1462 cm⁻¹ and 1408 cm⁻¹ are due to methyl/methylene deformations and carboxyl group vibrations, respectively. The strongest Raman band of creatine at 831 cm⁻¹ (ρ_w(R NH₂)) is accompanied by two weaker bands at 1054 and 1397 cm⁻¹ due to ν(CN) + ν(R NH₂) and ν_s(COOH) + ν(CN) + δ(CN) modes, respectively. In the case of L ‐carnitine, its RS spectrum is dominated by bands at 772 and 1461 cm⁻¹ assigned to ρ_r(CH₂) and δ(CH₃), respectively. The analysis of the SERS spectra shows that HMB interacts with the silver surface mainly through the  COO⁻, hydroxyl, and  CH₂ groups, whereas L ‐carnitine binds to the surface via  COO⁻ and  N⁺(CH₃)₃ which is rarely enhanced at pH = 8.3. On the other hand, it seems that creatine binds weakly to the silver surface mainly by  NH₂, and C O from the  COO⁻ group. Copyright © 2006 John Wiley & Sons, Ltd.     

Influence of aliphatic spacer group on adsorption mechanisms of phosphonate derivatives of l-phenylalanine: Surface-enhanced Raman, Raman, and infrared studies

The nature of phosphonopeptides containing N-terminal l-phenylalanine (l-Phe), namely l-Phe-dl-NH-CH(CH(CH₃)₂)-PO₃H₂ (A), l-Phe-l-NH-CH(CH₃)-PO₃H₂ (B), and l-Phe-dl-NH-CH(CH₂CH₂COOH)-PO₃H₂ (C) (Fig. 1 presents molecular structure of these molecules), adsorbed on electrochemically roughened and colloidal silver surfaces has been explored by surface-enhanced Raman spectroscopy (SERS). To reveal adsorption mechanism of these species on the basis of their SERS spectra at first Fourier-transform Raman (FT-RS) and absorption infrared (FT-IR) spectra of non-adsorbed molecules were measured. Examination of enhancement, frequency shifts, and changes in relative intensities of SERS bands due to adsorption and surface roughens variation reveals that the tilted compounds adsorb on the electrochemically roughened silver substrate in similar way, while they behave differently on the colloidal silver surface. A stronger enhancement of in-plane ring vibrations of the l-Phe ring, i.e., ν₃ and ν_18b (B₂), over these of the A₂ symmetry in all SERS spectra on the electrochemically roughened silver substrate suggests that the ring interacts with this surface adopting slightly deflect orientation from the perpendicular one. Also, enhancement of PO and -CH₂-/-CH₃ fragments vibrations points out that they are involved in adsorption process on this substrate. This conclusion was drawn on the basis of the enhancement of 1274-1279 and 1138-1152 (ν(PO)), 1393-1400 (δ(CH) + ρ_b(CNH₂) + ν(C-C_O) + δ(CH₃)), ∼1455 (δ(CCH₃/CCH₂) + ρ_b(CH₃/CH₂), and 1505-1512 cm⁻¹ (δ(CH₂) + Phe(ν_19a)) bands. Although a relative intensity ratio of these bands in the presented SERS spectra is different. On the other hand, on the colloidal silver nanoparticles, the aromatic ring of all molecules is lying flat or takes almost parallel orientation to this surface. Besides, A interacts also via P-terminal group (568, 765, 827, 1040, and 1150 cm⁻¹), whereas B mainly through NH₂-C-(CO)-CNH-(712 and 1255 cm⁻¹). In the case of C, it adsorbs on the silver colloidal surface mainly through the aromatic ring of l-Phe, while other fragments of the molecule are in close proximity to this surface as comes off the weak enhancement of bands due to the aliphatic vibrations.     

Coadsorption of trimethyl phosphine and thiocyanate on colloidal silver: a SERS study combined with theoretical calculations

The adsorption of trimethyl phosphine (TMP) on colloidal silver has been investigated by means of surface‐enhanced Raman scattering spectroscopy (SERS). On the basis of surface selection rules, it is deduced from the SERS results that TMP adsorbs on silver surface via its P atom. The electron donor effect of TMP can be sensitively probed by the coadsorbed SCN⁻. The Raman wavenumber of νCN of the adsorbed SCN⁻ shifts to lower wavenumbers when TMP is coadsorbed with SCN⁻ and the red shift of C≡N stretching wavenumber is found to increase with increasing surface coverage of TMP. This could be explained in terms of the electron donor effect of TMP. Density functional theory (DFT) calculations further confirm the experimental results that the charge transfer is from TMP to silver surface rather than reversely. Natural bond orbital (NBO) analysis indicates that the red shift of C≡N stretching mode is due the increase of electronic populations of π* orbital of C≡N bond induced by coadsorbed TMP, consequently the C≡N bond is weakened, and the νCN shifts to lower wavenumbers. An NBO analysis also indicates that the conjugated effect between S atom and C≡N bond could easily make the charge transfer from silver surface to C≡N bond. Copyright © 2008 John Wiley & Sons, Ltd.     

哌啶分子在两种不同银电极上吸附行为的实验和理论研究

分别在粗糙银电极和银纳米颗粒修饰银电极上得到了哌啶分子的表面增强拉曼（SERS）光谱。哌啶在银电极与银纳米颗粒修饰的银电极上的SERS谱有很大的区别,分析认为是由于哌啶在不同基底上的吸附方式不同所引起的,据此建立了哌啶吸附在银颗粒表面的两种模型,用DFT-B3PW91-lanl2dz方法计算了两种模型的拉曼频移,通过与实验结果比较说明了哌啶分子主要通过N原子的孤对电子竖直吸附在粗糙银电极表面,而在银纳米颗粒修饰的银电极上则以平行吸附方式为主。     

Raman scattering of L‐tryptophan enhanced by surface plasmon of silver nanoparticles: vibrational assignment and structural determination

Vibrational bands of L ‐tryptophan which was adsorbed on Ag nanoparticles (∼10 nm in diameter) have been investigated in the spectral range of 200–1700 cm⁻¹ using surface‐enhanced Raman scattering (SERS) spectroscopy. Compared with the normal Raman scattering (NRS) of L ‐tryptophan in either 0.5 M aqueous solution (NRS‐AS) or solid powder (NRS‐SP), the intensified signals by SERS have made the SERS investigation at a lower molecular concentration (5 × 10⁻⁴ M ) possible. Ab initio calculations at the B3LYP/6‐311G level have been carried out to predict the optimal structure and vibrational wavenumbers for the zwitterionic form of L ‐tryptophan. Facilitated with the theoretical prediction, the observed vibrational modes of L ‐tryptophan in the NRS‐AS, NRS‐SP, and SERS spectra have been analyzed. In the spectroscopic observations, there are no significant changes for the vibrational bands of the indole ring in either NRS‐AS, NRS‐SP, or SERS. In contrast, spectral intensities involving the vibrations of carboxylate and amino groups are weak in NRS‐AS and NRS‐SP, but strong in SERS. The intensity enhancement in the SERS spectrum can reach 10³–10⁴‐fold magnification. On the basis of spectroscopic analysis, the carboxylate and amino groups of L ‐tryptophan are determined to be the preferential terminal groups to attach onto the surfaces of Ag nanoparticles in the SERS measurement. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman light scattering,infrared absorption and DSC studies of the phase transition and vibrational and reorientational dynamics of H2O ligands and ClO4– anions in [Ba(H2O)3](ClO4)2

[Ba(H₂O)₃](ClO₄)₂ between 90 and 300 K possesses two solid phases. One phase transition of the first‐order type at: = 211.3 K (on heating) and = 204.6 K (on cooling) was determined by differential scanning calorimetry. The entropy change value (ΔS ≈ 15 Jmol^–1 K^–1), associated with the observed phase transition, indicates a moderate degree of molecular dynamical disorder. Both, vibrational and reorientational motions of H₂O ligands and ClO₄^– anions, in the high‐temperature and low‐temperature phases, were investigated by Fourier transform far‐infrared and middle‐infrared and Raman light scattering spectroscopies. The temperature dependences of the full‐width at half‐maximum values of the bands associated with ρ_w(H₂O) mode, in both infrared (~570 cm^–1) and Raman light scattering (~535 cm^–1) spectra, suggest that the observed phase transition is not associated with a sudden change of a speed of the H₂O reorientational motions. Ligands reorient fast, with correlation time of the order of several picoseconds, with a mean activation energy value E_a = 5.1 kJ mol^–1 in both high and low temperature phases. On the other hand, measurements of temperature dependences of full‐width at half‐maximum values of the infrared band at ~460 cm^–1, associated with δ_d(OClO)E mode, and Raman band at ~1105 cm^–1, associated with ν_as(ClO)F₂ mode, revealed the existence of a fast ClO₄^– reorientation in phase I and in phase II, with the E_a(I) and E_a(II) values equal to 8.0 and 6.5 kJ mol^–1, respectively. These reorientational motions of ClO₄^– are slightly distorted at the T_C. Fourier transform far‐infrared and middle‐infrared spectra with decreasing of temperature indicated characteristic changes at the vicinity of PT at T_C, which suggested lowering of the crystal structure symmetry. All these experimental facts suggest that the discovered phase transition is associated with small change of H₂O ligands and somewhat major change of ClO₄^– anions reorientational dynamics, and with insignificant change of the crystal structure, too. Copyright © 2012 John Wiley & Sons, Ltd.