ID Name References
C1

an oxidized electron donor

C2

a reduced electron donor

C3

a deaminated amino group acceptor

C4

an aminated amino group acceptor

C5

Charged-LEU-tRNAs

C6

LEU-tRNAs

C7

a 2-oxo acid

C8

an amino acid

C9

an acyl-CoA

C10

a carboxylate

C11

an oxidized ferredoxin [iron-sulfur] cluster

C12

a reduced ferredoxin [iron-sulfur] cluster

C13

NAD(P)H

C14

NAD(P)+

C15

an L-tyrosyl-[tRNAtyr]

C16

a tRNAtyr

C17

(2Z,4Z)-2-hydroxy-5-carboxymuconate-6-semialdehyde

C18

4α,14α-dimethyl-porifersta-8,25(27)-dienol

C24

2-amino-4,5-dihydroxy-6-oxo-7-(phosphooxy)heptanoate

C26

4-methylcyclohex-1,5-diene-1-carbonyl-CoA

C27

isoshowacene

C29

2-hydroxyhexadecanal

C30

16-sinapoyloxypalmitate

C31

3-methylquinolin-2-ol

C32

(22R)-hydroxycholesterol

C37

N-(2-methylpropyl)hydroxylamine

C38

chloroeremomycin

C39

3-hydroxy-3-methylbutanoate

C40

2',3',4'-O-methyl-rhamnosyl tetracyclic spinosyn pseudoaglycone

C41

2',3'-O-methyl-rhamnosyl tetracyclic spinosyn pseudoaglycone

C42

2'-O-methyl-rhamnosyl tetracyclic spinosyn pseudoaglycone

C43

rhamnosyl tetracyclic spinosyn pseudoaglycone

C44

rhamnosyl tricyclic spinosyn pseudoaglycone

C45

spinosyn tricyclic macrolactone

C46

11-dehydro-15-oxo spinosyn macrolactone

C47

15-oxo spinosyn macrolactone

C48

spinosyn macrolactone

C51

diethylamine N-oxide

C52

triethylamine N-oxide

C53

UDP-2-acetamido-2-deoxy-α-D-ribo-hex-3-uluronate

C55

nitroxyl

C57

trans,octacis-decaprenylphospho-β-D-arabinofuranose

C58

1,3-dihydro-2H-imidazol-2-one

C63

2-phenylethyl β-D-glucopyranoside

C64

formyl hopane

C65

ribosyl hopane

C66

adenosyl hopane

C70

(2Z)-2-hydroxypenta-2,4-dienoate

C72

(2S,4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinate

C73

(2Z)-2,4-dihydroxypent-2-enoate

C75

(2E,7Z,10Z,13Z,16Z,19Z)-docosahexaenoyl-CoA

C76

(3R,5Z,8Z,11Z,14Z,17Z)-3-hydroxydocosapentaenoyl-CoA

C77

(5Z,8Z,11Z,14Z,17Z)-3-oxodocosapentaenoyl-CoA

C79

(+)-3'-hydroxylarreatricin

C80

(+)-larreatricin

C81

2-trans,4-trans-xanthoxin

C928

CO2

C1184

5-phospho-β-D-ribosylamine

C11930

N-acetylglutamyl-phosphate

C6444

oxaloacetate

C891

citrate

C1621

(R)-2,3-dihydroxy-3-methylpentanoate

C6369

(S)-2-aceto-2-hydroxybutanoate

C6359

(S)-2-acetolactate

C11723

L-quinate

C12369

3-dehydroquinate

C12364

isopentenyl diphosphate

C14074

prenyl diphosphate

C12158

(E)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate

C11737

(3R,5S)-3-hydroxy-1-pyrroline-5-carboxylate

C11479

phosphoenolpyruvate

C12048

γ-L-glutamyl 5-phosphate

C11653

shikimate

C12324

autoinducer 2

C11802

4'-phosphopantetheine

C1431

3-dehydroshikimate

C933

L-canavanine

C11543

H2O

C3417

(S)-3-methyl-2-oxopentanoate

C3262

3-methyl-2-oxobutanoate

C12352

(S)-dihydroorotate

C12492

1,6-anhydro-N-acetyl-β-muramate

C585

(2R,4S)-2-methyl-2,4-dihydroxydihydrofuran-3-one

C584

(2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran

C12494

N-acetyl-D-muramate 6-phosphate

C12178

histidinal

C12176

histidinol

C1555

(S)-2,3-dihydrodipicolinate

C11652

shikimate 3-phosphate

C11857

β-nicotinate D-ribonucleotide

C12162

hydrogen peroxide

C1427

5-enolpyruvoyl-shikimate 3-phosphate

C13786

(2S)-2-isopropyl-3-oxosuccinate

C506

3-ethylmalate

C3259

2-oxovalerate

C1353

3-phosphooxypyruvate

C11604

thiamine diphosphate

C1528

2-dehydropantoate

C11710

(R)-4'-phosphopantothenoyl-L-cysteine

C11832

orotidine 5'-phosphate

C12848

N5-carboxyaminoimidazole ribonucleotide

C1171

2-(formamido)-N1-(5-phospho-β-D-ribosyl)acetamidine

C1172

5-amino-1-(5-phospho-β-D-ribosyl)imidazole

C1182

N2-formyl-N1-(5-phospho-β-D-ribosyl)glycinamide

C1180

N1-(5-phospho-β-D-ribosyl)glycinamide

C932

canavaninosuccinate

C12056

L-arginino-succinate

C14003

N-acetyl-L-glutamate 5-semialdehyde

C7865

urea

C11932

N-acetyl-D-galactosamine 6-phosphate

C15282

D-galactosamine-6-phosphate

C1102

O-acetyl-L-serine

C1438

(2S)-2-isopropylmalate

C1530

(2R,3S)-3-isopropylmalate

C13417

2-isopropylmaleate

C13225

N-acetyl-β-D-glucosamine-1,6-anhydro-N-acetyl-β-D-muramate

C9422

nicotinate adenine dinucleotide

C13627

DNA without lesion

C11483

glycolate

C12029

S-ribosyl-L-homocysteine

C13909

CDP-glycerol

C11760

prephenate

C12125

imidazole acetol-phosphate

C12044

L-histidinol phosphate

C12218

α-D-glucosamine 1-phosphate

C12390

D-erythro-imidazole-glycerol-phosphate

C11774

5-amino-1-(5-phospho-D-ribosyl)imidazole-4-carboxylate

C11812

5'-phosphoribosyl-4-(N-succinocarboxamide)-5-aminoimidazole

C13689

(2-carboxy-2,5-dihydro-5-oxofuran-2-yl)-acetate

C1363

(4,5-dihydro-5-oxofuran-2-yl)-acetate

C3295

2-keto-D-gluconate

C1454

2,5-didehydro-D-gluconate

C6614

O-ureido-L-homoserine

C8333

(indol-3-yl)pyruvate

C11693

(indol-3 yl)acetyl-CoA

C11754

propanoyl phosphate

C1108

acetyl phosphate

C1429

3-deoxy-D-arabino-heptulosonate 7-phosphate

C11826

oxalosuccinate

C1020

4-amino-2-methyl-5-(phosphooxymethyl)pyrimidine

C1018

4-amino-2-methyl-5-(diphosphooxymethyl)pyrimidine

C11606

thiamine phosphate

C11800

(R)-pantothenate

C1268

(R)-4'-phosphopantothenate

C3421

formate

C582

(2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran

C352

2-iminoacetate

C1010

ammonia

C12274

enol-phenylpyruvate

C583

(2S,4S)-2-methyl-2,4-dihydroxydihydrofuran-3-one

C9431

1,2-dihydro-β-NADP

C873

coproporphyrinogen III

C11715

quinolinate

C3482

dihydroxyacetone

C11539

xanthine

C3319

4-methyl-2-oxopentanoate

C939

butanoyl phosphate

C10349

glycerone phosphate

C1538

2-C-methyl-D-erythritol 4-phosphate

C1449

2-C-methyl-D-erythritol-2,4-cyclodiphosphate

C2191

carbamoyl phosphate

C14924

carboxyphosphate

C10799

DNA with lesion

C6043

4-methylphenol

C12247

L-homocysteine

C11480

phenylacetate

C6624

protoheme

C7676

protoporphyrin IX

C7658

uroporphyrinogen-III

C7666

protoporphyrinogen IX

C7121

oxygen

C7118

hydrogen sulfide

C6708

2-iminopropanoate

C6599

2-iminobutanoate

C6596

2-iminosuccinate

C11281

2-chloroacetate

C8419

sn-glycerol 3-phosphate

C11476

(R)-glycerate

C6437

3-phospho-D-glyceroyl-phosphate

C6425

2-phospho-D-glycerate

C6438

3-phospho-D-glycerate

C2150

(2R)-2,3-dihydroxy-3-methylbutanoate

C2017

α-D-glucosamine 6-phosphate

C13366

(indole-3-yl)acetonitrile

C13354

preQ0

C7314

5-fluoro-5-deoxy-D-ribose 1-phosphate

C7306

5'-deoxy-5'-fluoroadenosine

C11254

5-chloro-5-deoxyribose 1-phosphate

C5546

5'-chloro-5'-deoxyadenosine

C11510

5-chloro-5-deoxy-D-ribonate

C3337

5-chloro-4-hydroxy-2-oxopentanoate

C4320

H+

C4328

ammonium

C4326

Na+

C11023

Fe2+

C11005

Zn2+

C11006

Co2+

C7030

nitrate

C7036

hydrogencarbonate

C6097

phosphate

C6093

diphosphate

C6094

triphosphate

C7006

chloride

C9077

(2E,6E)-farnesyl diphosphate

C6524

2-aminoprop-2-enoate

C6526

(2Z)-2-aminobut-2-enoate

C3574

spermidine

C3548

putrescine

C3998

methanethiol

C5759

2-(2-carboxy-4-methylthiazol-5-yl)ethyl phosphate

C5744

5-(2-hydroxyethyl)-4-methylthiazole

C5753

4-methyl-5-(2-phosphooxyethyl)thiazole

C5750

2-[(2R,5Z)-2-carboxy-4-methylthiazol-5(2H)-ylidene]ethyl phosphate

C12587

L-cysteate

C12637

3-sulfopyruvate

C5298

acetyl-CoA

C4901

succinyl-CoA

C4913

malonyl-CoA

C5316

(R)-2-hydroxyglutaryl-CoA

C5014

phenylacetyl-CoA

C4937

3'-dephospho-CoA

C4927

(E)-glutaconyl-CoA

C5314

2-methylacetoacetyl-CoA

C1337

crotonyl-CoA

C5635

oleoyl-CoA

C11195

18-hydroxylinoleoyl-CoA

C10774

(3R)-3-hydroxybutanoyl-CoA

C10775

(S)-3-hydroxybutanoyl-CoA

C1177

propanoyl-CoA

C1175

butanoyl-CoA

C5637

cis-vaccenoyl-CoA

C5651

palmitoleoyl-CoA

C5890

myristoyl-CoA

C2008

acetoacetyl-CoA

C2009

3-oxopentanoyl-CoA

C6358

glycocholate

C6332

cholate

C9096

(indol-3-yl)acetate

C10994

thiamine

C10988

riboflavin

C7134

tetrahydropteroyl tri-L-glutamate

C8566

5-methyltetrahydropteroyl tri-L-glutamate

C14624

5,10-methylene-tetrahydromethanopterin

C14603

tetrahydromethanopterin

C4200

pyridoxal

C4208

pyridoxine

C4214

pyridoxine 5'-phosphate

C4188

pyridoxamine 5'-phosphate

C4210

pyridoxal 5'-phosphate

C4216

pyridoxamine

C10049

FMNH2

C10052

FMN

C5394

(R)-NADHX

C5392

(S)-NADHX

C14497

NADPH

C14500

NADH

C4018

NADP+

C4020

NAD+

C7757

ADP

C7844

ATP

C7811

XMP

C7759

ITP

C5489

1-(5-phospho-β-D-ribosyl)-AMP

C5485

1-(5-phospho-β-D-ribosyl)-ATP

C7813

2'-deoxyadenosine

C7848

2'-deoxyguanosine

C7755

2'-deoxyinosine

C7840

dGMP

C7743

dAMP

C10288

adenylo-succinate

C7751

dGTP

C7807

dGDP

C7783

dATP

C7823

dADP

C7652

cyclic-AMP

C10267

pppGpp

C10266

ppGpp

C5552

5'-deoxyadenosine

C7827

GMP

C7741

IMP

C7785

hypoxanthine

C7745

GTP

C7795

GDP

C7809

AMP

C15085

guanosine

C15091

inosine

C15088

adenosine

C15094

xanthosine

C2702

cytidine

C2706

uridine

C2519

thymidine

C2523

2'-deoxycytidine

C2527

2'-deoxyuridine

C1817

cyclic di-3',5'-guanylate

C10280

5-hydroxy-CMP

C10290

5-hydroxy-CTP

C8764

dTTP

C8772

dCTP

C8724

dUTP

C8761

CTP

C8760

UTP

C8755

dCMP

C8770

dUMP

C8744

dTMP

C2245

CMP

C2241

UMP

C8720

dTDP

C8733

dCDP

C8714

UDP

C8732

CDP

C10263

ADP-α-D-glucose

C9540

preQ1

C10601

thymine

C8739

uracil

C9454

adenine

C9455

guanine

C7780

UDP-α-D-glucose

C8666

UDP-α-D-galactose

C7734

UDP-N-acetyl-α-D-glucosamine-enolpyruvate

C7808

UDP-N-acetyl-α-D-muramoyl-L-alanyl-D-glutamate

C7822

UDP-N-acetyl-α-D-muramate

C7742

UDP-N-acetyl-α-D-muramoyl-L-alanine

C7804

UDP-α-D-galactofuranose

C5848

UDP-N-acetyl-α-D-muramoyl-L-alanyl-γ-D-glutamyl-L-lysyl-D-alanine

C5090

UDP-N-acetyl-α-D-muramoyl-L-alanyl-γ-D-glutamyl-L-lysyl-D-alanyl-D-alanine

C5094

UDP-N-acetyl-α-D-muramoyl-L-alanyl-γ-D-glutamyl-meso-2,6-diaminopimeloyl-D-alanyl-D-alanine

C5150

UDP-N-acetyl-α-D-muramoyl-L-alanyl-γ-D-glutamyl-meso-2,6-diaminopimelate

C7444

UDP-N-acetyl-α-D-glucosamine

C10343

2-phospho-4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol

C8423

4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol

C10614

coenzyme A

C12718

molybdopterin

C6006

adenylyl molybdenum cofactor

C6007

MoO2-molybdopterin cofactor

C6000

bis(molybdenum cofactor)

C6009

bis(guanylyl molybdopterin) cofactor

C6003

guanylyl molybdenum cofactor

C4985

(S)-1-pyrroline-5-carboxylate

C15044

reduced riboflavin

C12728

6-(hydroxymethyl)-7,8-dihydropterin

C12680

(7,8-dihydropterin-6-yl)methyl diphosphate

C12726

2-amino-6-[1-hydroxyethyl]-7-methyl-7,8-dihydropterin

C12676

[1-(2-amino-7-methyl-4-oxo-7,8-dihydro-3H-pteridin-6-yl)]ethyl diphosphate

C8741

dipicolinate

C11481

orotate

C8413

indole

C7694

(1S,2R)-1-C-(indol-3-yl)glycerol 3-phosphate

C11421

protocatechuate

C9503

6-methoxy-3-methyl-2-all-trans-decaprenyl-1,4-benzoquinol

C2883

all trans-decaprenyl-2-methoxy-6-1,4-benzoquinol

C2040

diacetyl

C6090

methylglyoxal

C3485

butan-1-al

C3500

acetaldehyde

C6246

(S)-lactaldehyde

C9494

D-glyceraldehyde

C6134

α-L-rhamnopyranose

C6133

β-L-rhamnopyranose

C7087

α-D-glucopyranose

C7085

β-D-glucopyranose

C2108

β-D-galactopyranose

C2109

α-D-galactopyranose

C3449

butan-1-ol

C3731

glycerol

C6569

D-sorbitol

C6571

L-iditol

C10351

D-ribitol 5-phosphate

C10688

maltotriose

C10689

maltotetraose

C10692

maltoheptaose

C10691

maltohexaose

C10690

maltopentaose

C2012

keto-D-fructose

C5458

keto-L-sorbose

C9477

N-acetyl-α-D-muramoyl-L-alanine(1-)

C15288

N-acetylmuramate

C3186

N-acetyl-α-D-glucosamine 1-phosphate

C5286

N-acetyl-β-D-glucosamine

C8835

β-D-fructofuranose

C7089

keto-L-rhamnulose

C11619

decaprenyl-diphospho-N-acetylmuramoyl-pentapeptide

C10374

decaprenyl-pyrophosphoryl-(N-acetylglucosamine)-N-acetylmuramyl-(pentapeptide)

C10381

a peptidoglycan dimer (mycobacteria)

C10372

a peptidoglycan dimer (S. aureus)

C10379

a peptidoglycan dimer (E. faeciums)

C10370

a peptidoglycan dimer (E. faecium, tetrapeptide)

C10362

undecaprenyl-diphospho-N-acetylmuramoyl-L-alanyl-γ-D-glutamyl-L-lysyl- D-alanine

C13226

a peptidoglycan dimer with pentapeptide stems (meso-diaminopimelate containing)

C13222

a peptidoglycan dimer with pentapeptide stem (meso-diaminopimelate containing)

C3132

ditrans,octacis-undecaprenyldiphospho-N-acetyl-(N-acetylglucosaminyl)muramoyl-L-alanyl-γ-D-isoglutaminyl-L-lysyl-(glycyl)5-D-alanyl-D-alanine

C3120

ditrans,octacis-undecaprenyldiphospho-[(N-acetyl-β-D-glucosaminyl)-(1,4)-]-N-acetyl-α-D-muramoyl-L-alanyl-γ-D-glutamyl-meso-2,6-diaminopimeloyl-D-alanyl-D-alanine

C3127

ditrans,octacis-undecaprenyldiphospho-N-acetyl-(N-acetylglucosaminyl)muramoyl-L-alanyl-γ-D-isoglutaminyl-N-(β-D-asparaginyl)-L-lysyl-D-alanyl-D-alanine

C3130

ditrans,octacis-undecaprenyldiphospho-N-acetyl-(N-acetylglucosaminyl)muramoyl-L-alanyl-γ-D-isoglutaminyl-N-(β-D-asparaginyl)-L-lysyl-D-alanine

C5529

GlcNAc-1,6-anhMurNAc-L-Ala-γ-D-Glu-DAP-D-Ala

C3138

undecaprenyl-diphospho-N-acetylmuramoyl-L-alanyl-γ-D-glutamyl-L-lysyl- D-alanyl-D-alanine

C3137

undecaprenyldiphospho-N-acetylmuramoyl-L-alanyl-γ-D-glutamyl-meso-2,6-diaminopimeloyl-D-alanyl-D-alanine

C6671

N,N'-diacetylchitobiose 6'-phosphate

C6669

N-monoacetylchitobiose 6'-phosphate

C8811

α-D-glucopyranose 1-phosphate

C5615

D-glyceraldehyde 3-phosphate

C10338

5-phospho-α-D-ribose 1-diphosphate

C15289

D-sedoheptulose 7-phosphate

C10340

D-erythrose 4-phosphate

C10326

β-D-fructose 1,6-bisphosphate

C2115

α-D-galactose 6-phosphate

C2989

β-D-fructofuranose 6-phosphate

C15303

D-fructose 6-phosphate

C2125

α-D-glucose 6-phosphate

C2123

β-D-glucose 6-phosphate

C2116

β-D-galactose 6-phosphate

C8815

α-D-galactose 1-phosphate

C8704

D-xylulose 5-phosphate

C8708

1-deoxy-D-xylulose 5-phosphate

C10604

D-ribulose 5-phosphate

C2057

α-D-ribose-1-phosphate

C2056

2-deoxy-α-D-ribose 1-phosphate

C8817

L-rhamnulose 1-phosphate

C2129

β-D-glucopyranose 1-phosphate

C2082

β-D-fructofuranose 1-phosphate

C6241

1-oleoyl-sn-glycerol 3-phosphate

C10024

1-(18-hydroxyoleoyl)-sn-glycrol 3-phosphate

C6242

1-cis-vaccenoyl-sn-glycerol-3-phosphate

C6240

1-palmitoyl-sn-glycerol 3-phosphate

C6239

1-myristoyl-sn-glycerol 3-phosphate

C3826

1-[18-hydroxyoeoyl]-2-[18-hydroxy-lioleoyl]-sn-glycerol 3-phosphate

C3816

1,2-dimyristoyl-sn-glycerol 3-phosphate

C3822

1-palmitoyl-2-myristoyl phosphatidate

C3119

1,2-dicis-vaccenoyl-phosphatidate

C3117

dioleoyl phosphatidate

C3512

1-cis-vaccenoyl-2-palmitoleoyl-sn-glycerol-3-phosphate

C3542

1-palmitoyl-2-cis-vaccenoyl phosphatidate

C3514

1-myristoyl-2-palmitoleoyl-sn-glycerol 3-phosphate

C3510

1-palmitoyl-2-palmitoleoyl-sn-glycerol 3-phosphate

C8321

geranyl diphosphate

C9075

mono-trans,octa-cis-decaprenyl diphosphate

C9304

trans,octacis-decaprenyl phosphate

C8322

di-trans,octa-cis-undecaprenyl phosphate

C5378

geranylgeranyl diphosphate

C5382

all-trans-heptaprenyl diphosphate

C1813

di-trans,octa-cis-undecaprenyl diphosphate

C8043

5-amino-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide

C8039

5-formamido-1-(5-phospho-D-ribosyl)-imidazole-4-carboxamide

C8033

5-amino-4-imidazolecarboxyamide

C8025

1-(5-phospho-β-D-ribosyl)-5-[(5-phosphoribosylamino)methylideneamino]imidazole-4-carboxamide

C8014

phosphoribulosylformimino-AICAR-phosphate

C8950

chorismate

C6442

succinate

C6443

fumarate

C5596

L-aspartate

C2802

(S)-malate

C8536

3-carboxy-cis,cis-muconate

C8531

D-threo-isocitrate

C6371

3-oxoadipate

C3345

3-phenyl-2-oxopropanoate

C3257

2-oxobutanoate

C3308

pyruvate

C3351

glyoxylate

C3301

2-oxoglutarate

C3418

butanoate

C6185

(R)-lactate

C2005

sophorosyloxydocosanoate

C1646

13-sophorosyloxydocosanoate 6',6''-diacetate

C15254

13-[O(2')-β-D-glucopyranosyl-β-D-glucopyranosyloxy]docosanoate O(6'')-acetate

C11482

(R)-pantoate

C11517

acetate

C5032

carbamate

C5579

L-serine

C5609

L-glutamine

C5612

L-cysteine

C5573

L-tyrosine

C5577

L-threonine

C12067

glycine

C5605

L-histidine

C5584

L-lysine

C5642

L-arginine

C5607

L-glutamate

C5581

L-proline

C5583

L-methionine

C5575

L-tryptophan

C5603

L-isoleucine

C5588

L-leucine

C5571

L-valine

C5597

L-alanine

C2469

L-phenylalanine

C11441

β-alanine

C8797

D-glutamate

C8799

D-alanine

C12073

L-aspartyl-4-phosphate

C5595

L-cystathionine

C12231

L-ornithine

C5580

L,L-diaminopimelate

C13322

N-carbamoyl-L-aspartate

C15000

N-acetyl-L-ornithine

C3402

3-phospho-L-serine

C14973

N-acetyl-L-glutamate

C10821

trans-4-hydroxy-L-proline

C11955

O-acetyl-L-homoserine

C12303

L-citrulline

C12239

L-aspartate 4-semialdehyde

C12227

O-succinyl-L-homoserine

C12229

O-phospho-L-homoserine

C12245

L-homoserine

C12237

L-canaline

C12233

L-glutamate-5-semialdehyde

C15292

S-adenosyl-L-methionine

C12203

S-adenosyl-L-homocysteine

C5585

meso-diaminopimelate

C2562

L-alanyl-γ-D-glutamyl-meso-2,6-diaminopimeloyl-D-alanine

C2576

L-alanyl-γ-D-glutamyl-meso-diaminopimelate

C1434

D-alanyl-D-alanine

C6530

L-alanyl-γ-D-glutamyl-meso-2,6-diaminopimeloyl-D-alanyl-D-alanine